mirror of https://github.com/python/cpython.git
1331 lines
46 KiB
TeX
1331 lines
46 KiB
TeX
% Format this file with latex.
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\documentstyle[myformat]{report}
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\title{\bf
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Python Reference Manual \\
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{\em Incomplete Draft}
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}
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\author{
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Guido van Rossum \\
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Dept. CST, CWI, Kruislaan 413 \\
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1098 SJ Amsterdam, The Netherlands \\
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E-mail: {\tt guido@cwi.nl}
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}
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\begin{document}
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\pagenumbering{roman}
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\maketitle
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\begin{abstract}
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\noindent
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Python is a simple, yet powerful programming language that bridges the
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gap between C and shell programming, and is thus ideally suited for
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``throw-away programming''
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and rapid prototyping. Its syntax is put
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together from constructs borrowed from a variety of other languages;
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most prominent are influences from ABC, C, Modula-3 and Icon.
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The Python interpreter is easily extended with new functions and data
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types implemented in C. Python is also suitable as an extension
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language for highly customizable C applications such as editors or
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window managers.
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Python is available for various operating systems, amongst which
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several flavors of {\UNIX}, Amoeba, the Apple Macintosh O.S.,
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and MS-DOS.
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This reference manual describes the syntax and ``core semantics'' of
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the language. It is terse, but exact and complete. The semantics of
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non-essential built-in object types and of the built-in functions and
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modules are described in the {\em Python Library Reference}. For an
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informal introduction to the language, see the {\em Python Tutorial}.
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\end{abstract}
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\pagebreak
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{
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\parskip = 0mm
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\tableofcontents
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}
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\pagebreak
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\pagenumbering{arabic}
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\chapter{Introduction}
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This reference manual describes the Python programming language.
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It is not intended as a tutorial.
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While I am trying to be as precise as possible, I chose to use English
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rather than formal specifications for everything except syntax and
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lexical analysis. This should make the document better understandable
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to the average reader, but will leave room for ambiguities.
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Consequently, if you were coming from Mars and tried to re-implement
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Python from this document alone, you might have to guess things and in
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fact you would be implementing quite a different language.
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On the other hand, if you are using
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Python and wonder what the precise rules about a particular area of
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the language are, you should definitely be able to find it here.
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It is dangerous to add too many implementation details to a language
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reference document -- the implementation may change, and other
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implementations of the same language may work differently. On the
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other hand, there is currently only one Python implementation, and
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its particular quirks are sometimes worth being mentioned, especially
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where the implementation imposes additional limitations.
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Every Python implementation comes with a number of built-in and
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standard modules. These are not documented here, but in the separate
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{\em Python Library Reference} document. A few built-in modules are
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mentioned when they interact in a significant way with the language
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definition.
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\section{Warning}
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This version of the manual is incomplete. Sections that still need to
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be written or need considerable work are marked with ``XXX''.
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\section{Notation}
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The descriptions of lexical analysis and syntax use a modified BNF
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grammar notation. This uses the following style of definition:
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\begin{verbatim}
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name: lcletter (lcletter | "_")*
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lcletter: "a"..."z"
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\end{verbatim}
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The first line says that a \verb\name\ is an \verb\lcletter\ followed by
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a sequence of zero or more \verb\lcletter\s and underscores. An
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\verb\lcletter\ in turn is any of the single characters `a' through `z'.
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(This rule is actually adhered to for the names defined in syntax and
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grammar rules in this document.)
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Each rule begins with a name (which is the name defined by the rule)
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and a colon, and is wholly contained on one line. A vertical bar
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(\verb\|\) is used to separate alternatives; it is the least binding
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operator in this notation. A star (\verb\*\) means zero or more
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repetitions of the preceding item; likewise, a plus (\verb\+\) means
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one or more repetitions, and a question mark (\verb\?\) zero or one
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(in other words, the preceding item is optional). These three
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operators bind as tightly as possible; parentheses are used for
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grouping. Literal strings are enclosed in double quotes. White space
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is only meaningful to separate tokens.
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In lexical definitions (as the example above), two more conventions
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are used: Two literal characters separated by three dots mean a choice
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of any single character in the given (inclusive) range of ASCII
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characters. A phrase between angular brackets (\verb\<...>\) gives an
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informal description of the symbol defined; e.g., this could be used
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to describe the notion of `control character' if needed.
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Even though the notation used is almost the same, there is a big
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difference between the meaning of lexical and syntactic definitions:
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a lexical definition operates on the individual characters of the
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input source, while a syntax definition operates on the stream of
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tokens generated by the lexical analysis.
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\chapter{Lexical analysis}
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A Python program is read by a {\em parser}. Input to the parser is a
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stream of {\em tokens}, generated by the {\em lexical analyzer}. This
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chapter describes how the lexical analyzer breaks a file into tokens.
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\section{Line structure}
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A Python program is divided in a number of logical lines. The end of
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a logical line is represented by the token NEWLINE. Statements cannot
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cross logical line boundaries except where NEWLINE is allowed by the
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syntax (e.g., between statements in compound statements).
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\subsection{Comments}
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A comment starts with a hash character (\verb\#\) that is not part of
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a string literal, and ends at the end of the physical line. A comment
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always signifies the end of the logical line. Comments are ignored by
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the syntax.
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\subsection{Line joining}
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Two or more physical lines may be joined into logical lines using
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backslash characters (\verb/\/), as follows: when a physical line ends
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in a backslash that is not part of a string literal or comment, it is
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joined with the following forming a single logical line, deleting the
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backslash and the following end-of-line character. For example:
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%
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\begin{verbatim}
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samplingrates = (48000, AL.RATE_48000), \
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(44100, AL.RATE_44100), \
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(32000, AL.RATE_32000), \
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(22050, AL.RATE_22050), \
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(16000, AL.RATE_16000), \
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(11025, AL.RATE_11025), \
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( 8000, AL.RATE_8000)
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\end{verbatim}
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\subsection{Blank lines}
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A logical line that contains only spaces, tabs, and possibly a
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comment, is ignored (i.e., no NEWLINE token is generated), except that
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during interactive input of statements, an entirely blank logical line
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terminates a multi-line statement.
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\subsection{Indentation}
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Leading whitespace (spaces and tabs) at the beginning of a logical
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line is used to compute the indentation level of the line, which in
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turn is used to determine the grouping of statements.
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First, tabs are replaced (from left to right) by one to eight spaces
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such that the total number of characters up to there is a multiple of
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eight (this is intended to be the same rule as used by UNIX). The
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total number of spaces preceding the first non-blank character then
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determines the line's indentation. Indentation cannot be split over
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multiple physical lines using backslashes.
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The indentation levels of consecutive lines are used to generate
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INDENT and DEDENT tokens, using a stack, as follows.
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Before the first line of the file is read, a single zero is pushed on
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the stack; this will never be popped off again. The numbers pushed on
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the stack will always be strictly increasing from bottom to top. At
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the beginning of each logical line, the line's indentation level is
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compared to the top of the stack. If it is equal, nothing happens.
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If it larger, it is pushed on the stack, and one INDENT token is
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generated. If it is smaller, it {\em must} be one of the numbers
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occurring on the stack; all numbers on the stack that are larger are
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popped off, and for each number popped off a DEDENT token is
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generated. At the end of the file, a DEDENT token is generated for
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each number remaining on the stack that is larger than zero.
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Here is an example of a correctly (though confusingly) indented piece
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of Python code:
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\begin{verbatim}
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def perm(l):
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# Compute the list of all permutations of l
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if len(l) <= 1:
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return [l]
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r = []
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for i in range(len(l)):
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s = l[:i] + l[i+1:]
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p = perm(s)
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for x in p:
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r.append(l[i:i+1] + x)
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return r
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\end{verbatim}
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The following example shows various indentation errors:
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\begin{verbatim}
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def perm(l): # error: first line indented
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for i in range(len(l)): # error: not indented
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s = l[:i] + l[i+1:]
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p = perm(l[:i] + l[i+1:]) # error: unexpected indent
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for x in p:
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r.append(l[i:i+1] + x)
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return r # error: inconsistent indent
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\end{verbatim}
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(Actually, the first three errors are detected by the parser; only the
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last error is found by the lexical analyzer -- the indentation of
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\verb\return r\ does not match a level popped off the stack.)
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\section{Other tokens}
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Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
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exist: identifiers, keywords, literals, operators, and delimiters.
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Spaces and tabs are not tokens, but serve to delimit tokens. Where
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ambiguity exists, a token comprises the longest possible string that
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forms a legal token, when read from left to right.
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\section{Identifiers}
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Identifiers are described by the following regular expressions:
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\begin{verbatim}
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identifier: (letter|"_") (letter|digit|"_")*
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letter: lowercase | uppercase
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lowercase: "a"..."z"
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uppercase: "A"..."Z"
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digit: "0"..."9"
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\end{verbatim}
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Identifiers are unlimited in length. Case is significant.
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\subsection{Keywords}
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The following identifiers are used as reserved words, or {\em
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keywords} of the language, and cannot be used as ordinary
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identifiers. They must be spelled exactly as written here:
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\begin{verbatim}
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and del for in print
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break elif from is raise
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class else global not return
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continue except if or try
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def finally import pass while
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\end{verbatim}
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% # This Python program sorts and formats the above table
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% import string
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% l = []
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% try:
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% while 1:
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% l = l + string.split(raw_input())
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% except EOFError:
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% pass
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% l.sort()
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% for i in range((len(l)+4)/5):
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% for j in range(i, len(l), 5):
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% print string.ljust(l[j], 10),
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% print
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\section{Literals}
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\subsection{String literals}
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String literals are described by the following regular expressions:
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\begin{verbatim}
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stringliteral: "'" stringitem* "'"
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stringitem: stringchar | escapeseq
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stringchar: <any ASCII character except newline or "\" or "'">
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escapeseq: "'" <any ASCII character except newline>
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\end{verbatim}
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String literals cannot span physical line boundaries. Escape
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sequences in strings are actually interpreted according to rules
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simular to those used by Standard C. The recognized escape sequences
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are:
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\begin{center}
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\begin{tabular}{|l|l|}
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\hline
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\verb/\\/ & Backslash (\verb/\/) \\
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\verb/\'/ & Single quote (\verb/'/) \\
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\verb/\a/ & ASCII Bell (BEL) \\
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\verb/\b/ & ASCII Backspace (BS) \\
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%\verb/\E/ & ASCII Escape (ESC) \\
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\verb/\f/ & ASCII Formfeed (FF) \\
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\verb/\n/ & ASCII Linefeed (LF) \\
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\verb/\r/ & ASCII Carriage Return (CR) \\
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\verb/\t/ & ASCII Horizontal Tab (TAB) \\
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\verb/\v/ & ASCII Vertical Tab (VT) \\
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\verb/\/{\em ooo} & ASCII character with octal value {\em ooo} \\
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\verb/\x/{em xx...} & ASCII character with hex value {\em xx...} \\
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\hline
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\end{tabular}
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\end{center}
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In strict compatibility with in Standard C, up to three octal digits are
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accepted, but an unlimited number of hex digits is taken to be part of
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the hex escape (and then the lower 8 bits of the resulting hex number
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are used in all current implementations...).
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All unrecognized escape sequences are left in the string unchanged,
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i.e., {\em the backslash is left in the string.} (This rule is
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useful when debugging: if an escape sequence is mistyped, the
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resulting output is more easily recognized as broken. It also helps a
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great deal for string literals used as regular expressions or
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otherwise passed to other modules that do their own escape handling --
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but you may end up quadrupling backslashes that must appear literally.)
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\subsection{Numeric literals}
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There are three types of numeric literals: plain integers, long
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integers, and floating point numbers.
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Integers and long integers are described by the following regular expressions:
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\begin{verbatim}
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longinteger: integer ("l"|"L")
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integer: decimalinteger | octinteger | hexinteger
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decimalinteger: nonzerodigit digit* | "0"
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octinteger: "0" octdigit+
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hexinteger: "0" ("x"|"X") hexdigit+
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nonzerodigit: "1"..."9"
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octdigit: "0"..."7"
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hexdigit: digit|"a"..."f"|"A"..."F"
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\end{verbatim}
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Although both lower case `l'and upper case `L' are allowed as suffix
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for long integers, it is strongly recommended to always use `L', since
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the letter `l' looks too much like the digit `1'.
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(Plain) integer decimal literals must be at most $2^{31} - 1$ (i.e., the
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largest positive integer, assuming 32-bit arithmetic); octal and
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hexadecimal literals may be as large as $2^{32} - 1$. There is no limit
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for long integer literals.
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Some examples of (plain and long) integer literals:
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\begin{verbatim}
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7 2147483647 0177 0x80000000
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3L 79228162514264337593543950336L 0377L 0100000000L
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\end{verbatim}
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Floating point numbers are described by the following regular expressions:
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\begin{verbatim}
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floatnumber: pointfloat | exponentfloat
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pointfloat: [intpart] fraction | intpart "."
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exponentfloat: (intpart | pointfloat) exponent
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intpart: digit+
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fraction: "." digit+
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exponent: ("e"|"E") ["+"|"-"] digit+
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\end{verbatim}
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The range of floating point literals is implementation-dependent.
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Some examples of floating point literals:
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\begin{verbatim}
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3.14 10. .001 1e100 3.14e-10
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\end{verbatim}
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Note that numeric literals do not include a sign; a phrase like
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\verb\-1\ is actually an expression composed of the operator
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\verb\-\ and the literal \verb\1\.
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\section{Operators}
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The following tokens are operators:
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\begin{verbatim}
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+ - * / %
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<< >> & | ^ ~
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< == > <= <> != >=
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\end{verbatim}
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The comparison operators \verb\<>\ and \verb\!=\ are alternate
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spellings of the same operator.
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\section{Delimiters}
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The following tokens serve as delimiters or otherwise have a special
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meaning:
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\begin{verbatim}
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( ) [ ] { }
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; , : . ` =
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\end{verbatim}
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The following printing ASCII characters are not used in Python (except
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in string literals and in comments). Their occurrence is an
|
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unconditional error:
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\begin{verbatim}
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! @ $ " ?
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\end{verbatim}
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They may be used by future versions of the language though!
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\chapter{Execution model}
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\section{Objects, values and types}
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I won't try to define rigorously here what an object is, but I'll give
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some properties of objects that are important to know about.
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Every object has an identity, a type and a value. An object's {\em
|
|
identity} never changes once it has been created; think of it as the
|
|
object's (permanent) address. An object's {\em type} determines the
|
|
operations that an object supports (e.g., does it have a length?) and
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|
also defines the ``meaning'' of the object's value. The type also
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|
never changes. The {\em value} of some objects can change; whether
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this is possible is a property of its type.
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|
|
Objects are never explicitly destroyed; however, when they become
|
|
unreachable they may be garbage-collected. An implementation is
|
|
allowed to delay garbage collection or omit it altogether -- it is a
|
|
matter of implementation quality how garbage collection is
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|
implemented, as long as no objects are collected that are still
|
|
reachable. (Implementation note: the current implementation uses a
|
|
reference-counting scheme which collects most objects as soon as they
|
|
become onreachable, but never collects garbage containing circular
|
|
references.)
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|
Note that the use of the implementation's tracing or debugging
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|
facilities may keep objects alive that would normally be collectable.
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|
(Some objects contain references to ``external'' resources such as
|
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open files. It is understood that these resources are freed when the
|
|
object is garbage-collected, but since garbage collection is not
|
|
guaranteed, such objects also provide an explicit way to release the
|
|
external resource (e.g., a \verb\close\ method). Programs are strongly
|
|
recommended to use this.)
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|
|
Some objects contain references to other objects. These references
|
|
are part of the object's value; in most cases, when such a
|
|
``container'' object is compared to another (of the same type), the
|
|
comparison applies to the {\em values} of the referenced objects (not
|
|
their identities).
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|
Types affect almost all aspects of objects.
|
|
Even object identity is affected in some sense: for immutable
|
|
types, operations that compute new values may actually return a
|
|
reference to any existing object with the same type and value, while
|
|
for mutable objects this is not allowed. E.g., after
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|
\begin{verbatim}
|
|
a = 1; b = 1; c = []; d = []
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|
\end{verbatim}
|
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|
|
\verb\a\ and \verb\b\ may or may not refer to the same object, but
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|
\verb\c\ and \verb\d\ are guaranteed to refer to two different, unique,
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|
newly created lists.
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|
\section{The standard type hierarchy}
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|
XXX None, sequences, numbers, mappings, ...
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\section{Execution frames, name spaces, and scopes}
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XXX code blocks, scopes, name spaces, name binding, exceptions
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|
\chapter{Expressions and conditions}
|
|
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|
From now on, extended BNF notation will be used to describe syntax,
|
|
not lexical analysis.
|
|
|
|
This chapter explains the meaning of the elements of expressions and
|
|
conditions. Conditions are a superset of expressions, and a condition
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|
may be used wherever an expression is required by enclosing it in
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|
parentheses. The only places where expressions are used in the syntax
|
|
instead of conditions is in expression statements and on the
|
|
right-hand side of assignments; this catches some nasty bugs like
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|
accedentally writing \verb\x == 1\ instead of \verb\x = 1\.
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|
|
The comma has several roles in Python's syntax. It is usually an
|
|
operator with a lower precedence than all others, but occasionally
|
|
serves other purposes as well; e.g., it separates function arguments,
|
|
is used in list and dictionary constructors, and has special semantics
|
|
in \verb\print\ statements.
|
|
|
|
When (one alternative of) a syntax rule has the form
|
|
|
|
\begin{verbatim}
|
|
name: othername
|
|
\end{verbatim}
|
|
|
|
and no semantics are given, the semantics of this form of \verb\name\
|
|
are the same as for \verb\othername\.
|
|
|
|
\section{Arithmetic conversions}
|
|
|
|
When a description of an arithmetic operator below uses the phrase
|
|
``the numeric arguments are converted to a common type'',
|
|
this both means that if either argument is not a number, a
|
|
{\tt TypeError} exception is raised, and that otherwise
|
|
the following conversions are applied:
|
|
|
|
\begin{itemize}
|
|
\item First, if either argument is a floating point number,
|
|
the other is converted to floating point;
|
|
\item else, if either argument is a long integer,
|
|
the other is converted to long integer;
|
|
\item otherwise, both must be plain integers and no conversion
|
|
is necessary.
|
|
\end{itemize}
|
|
|
|
(Note: ``plain integers'' in Python are at least 32 bits in size;
|
|
``long integers'' are arbitrary precision integers.)
|
|
|
|
\section{Atoms}
|
|
|
|
Atoms are the most basic elements of expressions. Forms enclosed in
|
|
reverse quotes or in parentheses, brackets or braces are also
|
|
categorized syntactically as atoms. The syntax for atoms is:
|
|
|
|
\begin{verbatim}
|
|
atom: identifier | literal | enclosure
|
|
enclosure: parenth_form | list_display | dict_display | string_conversion
|
|
\end{verbatim}
|
|
|
|
\subsection{Identifiers (Names)}
|
|
|
|
An identifier occurring as an atom is a reference to a local, global
|
|
or built-in name binding. If a name can be assigned to anywhere in a
|
|
code block, and is not mentioned in a \verb\global\ statement in that
|
|
code block, it refers to a local name throughout that code block.
|
|
Otherwise, it refers to a global name if one exists, else to a
|
|
built-in name.
|
|
|
|
When the name is bound to an object, evaluation of the atom yields
|
|
that object. When a name is not bound, an attempt to evaluate it
|
|
raises a {\tt NameError} exception.
|
|
|
|
\subsection{Literals}
|
|
|
|
Python knows string and numeric literals:
|
|
|
|
\begin{verbatim}
|
|
literal: stringliteral | integer | longinteger | floatnumber
|
|
\end{verbatim}
|
|
|
|
Evaluation of a literal yields an object of the given type
|
|
(string, integer, long integer, floating point number)
|
|
with the given value.
|
|
The value may be approximated in the case of floating point literals.
|
|
|
|
All literals correspond to immutable data types, and hence the
|
|
object's identity is less important than its value. Multiple
|
|
evaluations of literals with the same value (either the same
|
|
occurrence in the program text or a different occurrence) may obtain
|
|
the same object or a different object with the same value.
|
|
|
|
(In the original implementation, all literals in the same code block
|
|
with the same type and value yield the same object.)
|
|
|
|
\subsection{Parenthesized form}
|
|
|
|
A parenthesized form is an optional condition list enclosed in
|
|
parentheses:
|
|
|
|
\begin{verbatim}
|
|
parenth_form: "(" [condition_list] ")"
|
|
\end{verbatim}
|
|
|
|
A parenthesized condition list yields whatever that condition list
|
|
yields.
|
|
|
|
An empty pair of parentheses yields an empty tuple object (since
|
|
tuples are immutable, the rules for literals apply here).
|
|
|
|
(Note that tuples are not formed by the parentheses, but rather by use
|
|
of the comma operator. The exception is the empty tuple, for which
|
|
parentheses {\em are} required -- allowing unparenthesized ``nothing''
|
|
in expressions would causes ambiguities and allow common typos to
|
|
pass uncaught.)
|
|
|
|
\subsection{List displays}
|
|
|
|
A list display is a possibly empty series of conditions enclosed in
|
|
square brackets:
|
|
|
|
\begin{verbatim}
|
|
list_display: "[" [condition_list] "]"
|
|
\end{verbatim}
|
|
|
|
A list display yields a new list object.
|
|
|
|
If it has no condition list, the list object has no items.
|
|
Otherwise, the elements of the condition list are evaluated
|
|
from left to right and inserted in the list object in that order.
|
|
|
|
\subsection{Dictionary displays}
|
|
|
|
A dictionary display is a possibly empty series of key/datum pairs
|
|
enclosed in curly braces:
|
|
|
|
\begin{verbatim}
|
|
dict_display: "{" [key_datum_list] "}"
|
|
key_datum_list: [key_datum ("," key_datum)* [","]
|
|
key_datum: condition ":" condition
|
|
\end{verbatim}
|
|
|
|
A dictionary display yields a new dictionary object.
|
|
|
|
The key/datum pairs are evaluated from left to right to define the
|
|
entries of the dictionary: each key object is used as a key into the
|
|
dictionary to store the corresponding datum.
|
|
|
|
Keys must be strings, otherwise a {\tt TypeError} exception is raised.%
|
|
\footnote{
|
|
This restriction may be lifted in a future version of the language.
|
|
}
|
|
Clashes between duplicate keys are not detected; the last datum
|
|
(textually rightmost in the display) stored for a given key value
|
|
prevails.
|
|
|
|
\subsection{String conversions}
|
|
|
|
A string conversion is a condition list enclosed in {\em reverse} (or
|
|
backward) quotes:
|
|
|
|
\begin{verbatim}
|
|
string_conversion: "`" condition_list "`"
|
|
\end{verbatim}
|
|
|
|
A string conversion evaluates the contained condition list and converts the
|
|
resulting object into a string according to rules specific to its type.
|
|
|
|
If the object is a string, a number, \verb\None\, or a tuple, list or
|
|
dictionary containing only objects whose type is one of these, the
|
|
resulting string is a valid Python expression which can be passed to
|
|
the built-in function \verb\eval()\ to yield an expression with the
|
|
same value (or an approximation, if floating point numbers are
|
|
involved).
|
|
|
|
(In particular, converting a string adds quotes around it and converts
|
|
``funny'' characters to escape sequences that are safe to print.)
|
|
|
|
It is illegal to attempt to convert recursive objects (e.g., lists or
|
|
dictionaries that contain a reference to themselves, directly or
|
|
indirectly.)
|
|
|
|
\section{Primaries}
|
|
|
|
Primaries represent the most tightly bound operations of the language.
|
|
Their syntax is:
|
|
|
|
\begin{verbatim}
|
|
primary: atom | attributeref | subscription | slicing | call
|
|
\end{verbatim}
|
|
|
|
\subsection{Attribute references}
|
|
|
|
An attribute reference is a primary followed by a period and a name:
|
|
|
|
\begin{verbatim}
|
|
attributeref: primary "." identifier
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to an object of a type that supports
|
|
attribute references, e.g., a module or a list. This object is then
|
|
asked to produce the attribute whose name is the identifier. If this
|
|
attribute is not available, the exception \verb\AttributeError\ is
|
|
raised. Otherwise, the type and value of the object produced is
|
|
determined by the object. Multiple evaluations of the same attribute
|
|
reference may yield different objects.
|
|
|
|
\subsection{Subscriptions}
|
|
|
|
A subscription selects an item of a sequence or mapping object:
|
|
|
|
\begin{verbatim}
|
|
subscription: primary "[" condition "]"
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to an object of a sequence or mapping type.
|
|
|
|
If it is a mapping, the condition must evaluate to an object whose
|
|
value is one of the keys of the mapping, and the subscription selects
|
|
the value in the mapping that corresponds to that key.
|
|
|
|
If it is a sequence, the condition must evaluate to a nonnegative
|
|
plain integer smaller than the number of items in the sequence, and
|
|
the subscription selects the item whose index is that value (counting
|
|
from zero).
|
|
|
|
A string's items are characters. A character is not a separate data
|
|
type but a string of exactly one character.
|
|
|
|
\subsection{Slicings}
|
|
|
|
A slicing selects a range of items in a sequence object:
|
|
|
|
\begin{verbatim}
|
|
slicing: primary "[" [condition] ":" [condition] "]"
|
|
\end{verbatim}
|
|
|
|
XXX
|
|
|
|
\subsection{Calls}
|
|
|
|
A call calls a function with a possibly empty series of arguments:
|
|
|
|
\begin{verbatim}
|
|
call: primary "(" [condition_list] ")"
|
|
\end{verbatim}
|
|
|
|
The primary must evaluate to a callable object. Callable objects are
|
|
user-defined functions, built-in functions, methods of built-in
|
|
objects (``built-in methods''), class objects, and methods of class
|
|
instances (``user-defined methods''). If it is a class, the argument
|
|
list must be empty.
|
|
|
|
XXX explain what happens on function call
|
|
|
|
\section{Factors}
|
|
|
|
Factors represent the unary numeric operators.
|
|
Their syntax is:
|
|
|
|
\begin{verbatim}
|
|
factor: primary | "-" factor | "+" factor | "~" factor
|
|
\end{verbatim}
|
|
|
|
The unary \verb\-\ operator yields the negative of its numeric argument.
|
|
|
|
The unary \verb\+\ operator yields its numeric argument unchanged.
|
|
|
|
The unary \verb\~\ operator yields the bit-wise negation of its
|
|
(plain or long) integral numerical argument, using 2's complement.
|
|
|
|
In all three cases, if the argument does not have the proper type,
|
|
a {\tt TypeError} exception is raised.
|
|
|
|
\section{Terms}
|
|
|
|
Terms represent the most tightly binding binary operators:
|
|
|
|
\begin{verbatim}
|
|
term: factor | term "*" factor | term "/" factor | term "%" factor
|
|
\end{verbatim}
|
|
|
|
The \verb\*\ (multiplication) operator yields the product of its
|
|
arguments. The arguments must either both be numbers, or one argument
|
|
must be a plain integer and the other must be a sequence. In the
|
|
former case, the numbers are converted to a common type and then
|
|
multiplied together. In the latter case, sequence repetition is
|
|
performed; a negative repetition factor yields the empty string.
|
|
|
|
The \verb|"/"| (division) operator yields the quotient of its
|
|
arguments. The numeric arguments are first converted to a common
|
|
type. (Plain or long) integer division yields an integer of the same
|
|
type; the result is that of mathematical division with the {\em floor}
|
|
operator applied to the result, to match the modulo operator.
|
|
Division by zero raises a {\tt RuntimeError} exception.
|
|
|
|
The \verb|"%"| (modulo) operator yields the remainder from the
|
|
division of the first argument by the second. The numeric arguments
|
|
are first converted to a common type. A zero right argument raises a
|
|
{\tt RuntimeError} exception. The arguments may be floating point
|
|
numbers, e.g., $3.14 \% 0.7$ equals $0.34$. The modulo operator
|
|
always yields a result with the same sign as its second operand (or
|
|
zero); the absolute value of the result is strictly smaller than the
|
|
second operand.
|
|
|
|
The integer division and modulo operators are connected by the
|
|
following identity: $x = (x/y)*y + (x\%y)$.
|
|
|
|
\section{Arithmetic expressions}
|
|
|
|
\begin{verbatim}
|
|
arith_expr: term | arith_expr "+" term | arith_expr "-" term
|
|
\end{verbatim}
|
|
|
|
HIRO
|
|
|
|
The \verb|"+"| operator yields the sum of its arguments. The
|
|
arguments must either both be numbers, or both sequences. In the
|
|
former case, the numbers are converted to a common type and then added
|
|
together. In the latter case, the sequences are concatenated
|
|
directly.
|
|
|
|
The \verb|"-"| operator yields the difference of its arguments.
|
|
The numeric arguments are first converted to a common type.
|
|
|
|
\section{Shift expressions}
|
|
|
|
\begin{verbatim}
|
|
shift_expr: arith_expr | shift_expr "<<" arith_expr | shift_expr ">>" arith_expr
|
|
\end{verbatim}
|
|
|
|
These operators accept (plain) integers as arguments only.
|
|
They shift their left argument to the left or right by the number of bits
|
|
given by the right argument. Shifts are ``logical"", e.g., bits shifted
|
|
out on one end are lost, and bits shifted in are zero;
|
|
negative numbers are shifted as if they were unsigned in C.
|
|
Negative shift counts and shift counts greater than {\em or equal to}
|
|
the word size yield undefined results.
|
|
|
|
\section{Bitwise AND expressions}
|
|
|
|
\begin{verbatim}
|
|
and_expr: shift_expr | and_expr "&" shift_expr
|
|
\end{verbatim}
|
|
|
|
This operator yields the bitwise AND of its arguments,
|
|
which must be (plain) integers.
|
|
|
|
\section{Bitwise XOR expressions}
|
|
|
|
\begin{verbatim}
|
|
xor_expr: and_expr | xor_expr "^" and_expr
|
|
\end{verbatim}
|
|
|
|
This operator yields the bitwise exclusive OR of its arguments,
|
|
which must be (plain) integers.
|
|
|
|
\section{Bitwise OR expressions}
|
|
|
|
\begin{verbatim}
|
|
or_expr: xor_expr | or_expr "|" xor_expr
|
|
\end{verbatim}
|
|
|
|
This operator yields the bitwise OR of its arguments,
|
|
which must be (plain) integers.
|
|
|
|
\section{Expressions and expression lists}
|
|
|
|
\begin{verbatim}
|
|
expression: or_expression
|
|
expr_list: expression ("," expression)* [","]
|
|
\end{verbatim}
|
|
|
|
An expression list containing at least one comma yields a new tuple.
|
|
The length of the tuple is the number of expressions in the list.
|
|
The expressions are evaluated from left to right.
|
|
|
|
The trailing comma is required only to create a single tuple;
|
|
it is optional in all other cases (a single expression without
|
|
a trailing comma doesn't create a tuple, but rather yields the
|
|
value of that expression).
|
|
|
|
To create an empty tuple, use an empty pair of parentheses: \verb\()\.
|
|
|
|
\section{Comparisons}
|
|
|
|
\begin{verbatim}
|
|
comparison: expression (comp_operator expression)*
|
|
comp_operator: "<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"
|
|
\end{verbatim}
|
|
|
|
Comparisons yield integer value: 1 for true, 0 for false.
|
|
|
|
Comparisons can be chained arbitrarily,
|
|
e.g., $x < y <= z$ is equivalent to
|
|
$x < y$ {\tt and} $y <= z$, except that $y$ is evaluated only once
|
|
(but in both cases $z$ is not evaluated at all when $x < y$ is
|
|
found to be false).
|
|
|
|
Formally, $e_0 op_1 e_1 op_2 e_2 ...e_{n-1} op_n e_n$ is equivalent to
|
|
$e_0 op_1 e_1$ {\tt and} $e_1 op_2 e_2$ {\tt and} ... {\tt and}
|
|
$e_{n-1} op_n e_n$, except that each expression is evaluated at most once.
|
|
|
|
Note that $e_0 op_1 e_1 op_2 e_2$ does not imply any kind of comparison
|
|
between $e_0$ and $e_2$, e.g., $x < y > z$ is perfectly legal.
|
|
|
|
The forms \verb\<>\ and \verb\!=\ are equivalent.
|
|
|
|
The operators {\tt "<", ">", "==", ">=", "<="}, and {\tt "<>"} compare
|
|
the values of two objects. The objects needn't have the same type.
|
|
If both are numbers, they are compared to a common type.
|
|
Otherwise, objects of different types {\em always} compare unequal,
|
|
and are ordered consistently but arbitrarily, except that
|
|
the value \verb\None\ compares smaller than the values of any other type.
|
|
|
|
(This unusual
|
|
definition of comparison is done to simplify the definition of
|
|
operations like sorting and the \verb\in\ and \verb\not in\ operators.)
|
|
|
|
Comparison of objects of the same type depends on the type:
|
|
|
|
\begin{itemize}
|
|
\item Numbers are compared arithmetically.
|
|
\item Strings are compared lexicographically using the numeric
|
|
equivalents (the result of the built-in function ord())
|
|
of their characters.
|
|
\item Tuples and lists are compared lexicographically
|
|
using comparison of corresponding items.
|
|
\item Dictionaries compare unequal unless they are the same object;
|
|
the choice whether one dictionary object is considered smaller
|
|
or larger than another one is made arbitrarily but
|
|
consistently within one execution of a program.
|
|
\item The latter rule is also used for most other built-in types.
|
|
\end{itemize}
|
|
|
|
The operators \verb\in\ and \verb\not in\ test for sequence membership:
|
|
if $y$ is a sequence, $x {\tt in} y$ is true if and only if there exists
|
|
an index $i$ such that $x = y_i$.
|
|
$x {\tt not in} y$ yields the inverse truth value.
|
|
The exception {\tt TypeError} is raised when $y$ is not a sequence,
|
|
or when $y$ is a string and $x$ is not a string of length one.
|
|
|
|
The operators \verb\is\ and \verb\is not\ compare object identity:
|
|
$x {\tt is} y$ is true if and only if $x$ and $y$ are the same object.
|
|
$x {\tt is not} y$ yields the inverse truth value.
|
|
|
|
\section{Boolean operators}
|
|
|
|
\begin{verbatim}
|
|
condition: or_test
|
|
or_test: and_test | or_test "or" and_test
|
|
and_test: not_test | and_test "and" not_test
|
|
not_test: comparison | "not" not_test
|
|
\end{verbatim}
|
|
|
|
In the context of Boolean operators, and also when conditions are
|
|
used by control flow statements, the following values are interpreted
|
|
as false: None, numeric zero of all types, empty sequences (strings,
|
|
tuples and lists), and empty mappings (dictionaries).
|
|
All other values are interpreted as true.
|
|
|
|
The operator \verb\not\ yields 1 if its argument is false, 0 otherwise.
|
|
|
|
The condition $x {\tt and} y$ first evaluates $x$; if $x$ is false,
|
|
$x$ is returned; otherwise, $y$ is evaluated and returned.
|
|
|
|
The condition $x {\tt or} y$ first evaluates $x$; if $x$ is true,
|
|
$x$ is returned; otherwise, $y$ is evaluated and returned.
|
|
|
|
(Note that \verb\and\ and \verb\or\ do not restrict the value and type
|
|
they return to 0 and 1, but rather return the last evaluated argument.
|
|
This is sometimes useful, e.g., if $s$ is a string, which should be
|
|
replaced by a default value if it is empty, $s {\tt or} 'foo'$
|
|
returns the desired value. Because \verb\not\ has to invent a value
|
|
anyway, it does not bother to return a value of the same type as its
|
|
argument, so \verb\not 'foo'\ yields $0$, not $''$.)
|
|
|
|
\chapter{Simple statements}
|
|
|
|
Simple statements are comprised within a single logical line.
|
|
Several simple statements may occor on a single line separated
|
|
by semicolons. The syntax for simple statements is:
|
|
|
|
\begin{verbatim}
|
|
stmt_list: simple_stmt (";" simple_stmt)* [";"]
|
|
simple_stmt: expression_stmt
|
|
| assignment
|
|
| pass_stmt
|
|
| del_stmt
|
|
| print_stmt
|
|
| return_stmt
|
|
| raise_stmt
|
|
| break_stmt
|
|
| continue_stmt
|
|
| import_stmt
|
|
| global_stmt
|
|
\end{verbatim}
|
|
|
|
\section{Expression statements}
|
|
|
|
\begin{verbatim}
|
|
expression_stmt: expression_list
|
|
\end{verbatim}
|
|
|
|
An expression statement evaluates the expression list (which may
|
|
be a single expression).
|
|
If the value is not \verb\None\, it is converted to a string
|
|
using the rules for string conversions, and the resulting string
|
|
is written to standard output on a line by itself.
|
|
|
|
(The exception for \verb\None\ is made so that procedure calls,
|
|
which are syntactically equivalent to expressions,
|
|
do not cause any output.)
|
|
|
|
\section{Assignments}
|
|
|
|
\begin{verbatim}
|
|
assignment: target_list ("=" target_list)* "=" expression_list
|
|
target_list: target ("," target)* [","]
|
|
target: identifier | "(" target_list ")" | "[" target_list "]"
|
|
| attributeref | subscription | slicing
|
|
\end{verbatim}
|
|
|
|
(See the section on primaries for the definition of the last
|
|
three symbols.)
|
|
|
|
An assignment evaluates the expression list (remember that this can
|
|
be a single expression or a comma-separated list,
|
|
the latter yielding a tuple)
|
|
and assigns the single resulting object to each of the target lists,
|
|
from left to right.
|
|
|
|
Assignment is defined recursively depending on the type of the
|
|
target. Where assignment is to part of a mutable object
|
|
(through an attribute reference, subscription or slicing),
|
|
the mutable object must ultimately perform the
|
|
assignment and decide about its validity, raising an exception
|
|
if the assignment is unacceptable. The rules observed by
|
|
various types and the exceptions raised are given with the
|
|
definition of the object types (some of which are defined
|
|
in the library reference).
|
|
|
|
Assignment of an object to a target list is recursively
|
|
defined as follows.
|
|
|
|
\begin{itemize}
|
|
\item
|
|
If the target list contains no commas (except in nested constructs):
|
|
the object is assigned to the single target contained in the list.
|
|
|
|
\item
|
|
If the target list contains commas (that are not in nested constructs):
|
|
the object must be a tuple with as many items
|
|
as the list contains targets, and the items are assigned, from left
|
|
to right, to the corresponding targets.
|
|
|
|
\end{itemize}
|
|
|
|
Assignment of an object to a (non-list)
|
|
target is recursively defined as follows.
|
|
|
|
\begin{itemize}
|
|
|
|
\item
|
|
If the target is an identifier (name):
|
|
the object is bound to that name
|
|
in the current local scope. Any previous binding of the same name
|
|
is undone.
|
|
|
|
\item
|
|
If the target is a target list enclosed in parentheses:
|
|
the object is assigned to that target list.
|
|
|
|
\item
|
|
If the target is a target list enclosed in square brackets:
|
|
the object must be a list with as many items
|
|
as the target list contains targets,
|
|
and the list's items are assigned, from left to right,
|
|
to the corresponding targets.
|
|
|
|
\item
|
|
If the target is an attribute reference:
|
|
The primary expression in the reference is evaluated.
|
|
It should yield an object with assignable attributes;
|
|
if this is not the case, a {\tt TypeError} exception is raised.
|
|
That object is then asked to assign the assigned object
|
|
to the given attribute; if it cannot perform the assignment,
|
|
it raises an exception.
|
|
|
|
\item
|
|
If the target is a subscription:
|
|
The primary expression in the reference is evaluated.
|
|
It should yield either a mutable sequence object or a mapping
|
|
(dictionary) object.
|
|
Next, the subscript expression is evaluated.
|
|
|
|
If the primary is a sequence object, the subscript must yield a
|
|
nonnegative integer smaller than the sequence's length,
|
|
and the sequence is asked to assign the assigned object
|
|
to its item with that index.
|
|
|
|
If the primary is a mapping object, the subscript must have a
|
|
type compatible with the mapping's key type,
|
|
and the mapping is then asked to to create a key/datum pair
|
|
which maps the subscript to the assigned object.
|
|
|
|
Various exceptions can be raised.
|
|
|
|
\item
|
|
If the target is a slicing:
|
|
The primary expression in the reference is evaluated.
|
|
It should yield a mutable sequence object (currently only lists).
|
|
The assigned object should be a sequence object of the same type.
|
|
Next, the lower and upper bound expressions are evaluated,
|
|
insofar they are present; defaults are zero and the sequence's length.
|
|
The bounds should evaluate to (small) integers.
|
|
If either bound is negative, the sequence's length is added to it (once).
|
|
The resulting bounds are clipped to lie between zero
|
|
and the sequence's length, inclusive.
|
|
(XXX Shouldn't this description be with expressions?)
|
|
Finally, the sequence object is asked to replace the items
|
|
indicated by the slice with the items of the assigned sequence.
|
|
This may change the sequence's length, if it allows it.
|
|
|
|
\end{itemize}
|
|
|
|
(In the original implementation, the syntax for targets is taken
|
|
to be the same as for expressions, and invalid syntax is rejected
|
|
during the code generation phase, causing less detailed error
|
|
messages.)
|
|
|
|
\section{The {\tt pass} statement}
|
|
|
|
\begin{verbatim}
|
|
pass_stmt: "pass"
|
|
\end{verbatim}
|
|
|
|
{\tt pass} is a null operation -- when it is executed,
|
|
nothing happens.
|
|
|
|
\section{The {\tt del} statement}
|
|
|
|
\begin{verbatim}
|
|
del_stmt: "del" target_list
|
|
\end{verbatim}
|
|
|
|
Deletion is recursively defined similar to assignment.
|
|
|
|
(XXX Rather that spelling it out in full details,
|
|
here are some hints.)
|
|
|
|
Deletion of a target list recursively deletes each target,
|
|
from left to right.
|
|
|
|
Deletion of a name removes the binding of that name (which must exist)
|
|
from the local scope.
|
|
|
|
Deletion of attribute references, subscriptions and slicings
|
|
is passed to the primary object involved; deletion of a slicing
|
|
is in general equivalent to assignment of an empty slice of the
|
|
right type (but even this is determined by the sliced object).
|
|
|
|
\section{The {\tt print} statement}
|
|
|
|
\begin{verbatim}
|
|
print_stmt: "print" [ condition ("," condition)* [","] ]
|
|
\end{verbatim}
|
|
|
|
{\tt print} evaluates each condition in turn and writes the resulting
|
|
object to standard output (see below).
|
|
If an object is not a string, it is first converted to
|
|
a string using the rules for string conversions.
|
|
The (resulting or original) string is then written.
|
|
A space is written before each object is (converted and) written,
|
|
unless the output system believes it is positioned at the beginning
|
|
of a line. This is the case: (1) when no characters have been written
|
|
to standard output; or (2) when the last character written to
|
|
standard output is \verb/\n/;
|
|
or (3) when the last I/O operation
|
|
on standard output was not a \verb\print\ statement.
|
|
|
|
Finally,
|
|
a \verb/\n/ character is written at the end,
|
|
unless the \verb\print\ statement ends with a comma.
|
|
This is the only action if the statement contains just the keyword
|
|
\verb\print\.
|
|
|
|
Standard output is defined as the file object named \verb\stdout\
|
|
in the built-in module \verb\sys\. If no such object exists,
|
|
or if it is not a writable file, a {\tt RuntimeError} exception is raised.
|
|
(The original implementation attempts to write to the system's original
|
|
standard output instead, but this is not safe, and should be fixed.)
|
|
|
|
\section{The {\tt return} statement}
|
|
|
|
\begin{verbatim}
|
|
return_stmt: "return" [condition_list]
|
|
\end{verbatim}
|
|
|
|
\verb\return\ may only occur syntactically nested in a function
|
|
definition, not within a nested class definition.
|
|
|
|
If a condition list is present, it is evaluated, else \verb\None\
|
|
is substituted.
|
|
|
|
\verb\return\ leaves the current function call with the condition
|
|
list (or \verb\None\) as return value.
|
|
|
|
When \verb\return\ passes control out of a \verb\try\ statement
|
|
with a \verb\finally\ clause, that finally clause is executed
|
|
before really leaving the function.
|
|
(XXX This should be made more exact, a la Modula-3.)
|
|
|
|
\section{The {\tt raise} statement}
|
|
|
|
\begin{verbatim}
|
|
raise_stmt: "raise" condition ["," condition]
|
|
\end{verbatim}
|
|
|
|
\verb\raise\ evaluates its first condition, which must yield
|
|
a string object. If there is a second condition, this is evaluated,
|
|
else \verb\None\ is substituted.
|
|
|
|
It then raises the exception identified by the first object,
|
|
with the second one (or \verb\None\) as its parameter.
|
|
|
|
\section{The {\tt break} statement}
|
|
|
|
\begin{verbatim}
|
|
break_stmt: "break"
|
|
\end{verbatim}
|
|
|
|
\verb\break\ may only occur syntactically nested in a \verb\for\
|
|
or \verb\while\ loop, not nested in a function or class definition.
|
|
|
|
It terminates the neares enclosing loop, skipping the optional
|
|
\verb\else\ clause if the loop has one.
|
|
|
|
If a \verb\for\ loop is terminated by \verb\break\, the loop control
|
|
target (list) keeps its current value.
|
|
|
|
When \verb\break\ passes control out of a \verb\try\ statement
|
|
with a \verb\finally\ clause, that finally clause is executed
|
|
before really leaving the loop.
|
|
|
|
\section{The {\tt continue} statement}
|
|
|
|
\begin{verbatim}
|
|
continue_stmt: "continue"
|
|
\end{verbatim}
|
|
|
|
\verb\continue\ may only occur syntactically nested in a \verb\for\
|
|
or \verb\while\ loop, not nested in a function or class definition,
|
|
and {\em not nested in a \verb\try\ statement with a \verb\finally\
|
|
clause}.
|
|
|
|
It continues with the next cycle of the nearest enclosing loop.
|
|
|
|
\section{The {\tt import} statement}
|
|
|
|
\begin{verbatim}
|
|
import_stmt: "import" identifier ("," identifier)*
|
|
| "from" identifier "import" identifier ("," identifier)*
|
|
| "from" identifier "import" "*"
|
|
\end{verbatim}
|
|
|
|
(XXX To be done.)
|
|
|
|
\section{The {\tt global} statement}
|
|
|
|
\begin{verbatim}
|
|
global_stmt: "global" identifier ("," identifier)*
|
|
\end{verbatim}
|
|
|
|
(XXX To be done.)
|
|
|
|
\chapter{Compound statements}
|
|
|
|
(XXX The semantic definitions of this chapter are still to be done.)
|
|
|
|
\begin{verbatim}
|
|
statement: stmt_list NEWLINE | compound_stmt
|
|
compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | funcdef | classdef
|
|
suite: statement | NEWLINE INDENT statement+ DEDENT
|
|
\end{verbatim}
|
|
|
|
\section{The {\tt if} statement}
|
|
|
|
\begin{verbatim}
|
|
if_stmt: "if" condition ":" suite
|
|
("elif" condition ":" suite)*
|
|
["else" ":" suite]
|
|
\end{verbatim}
|
|
|
|
\section{The {\tt while} statement}
|
|
|
|
\begin{verbatim}
|
|
while_stmt: "while" condition ":" suite ["else" ":" suite]
|
|
\end{verbatim}
|
|
|
|
\section{The {\tt for} statement}
|
|
|
|
\begin{verbatim}
|
|
for_stmt: "for" target_list "in" condition_list ":" suite
|
|
["else" ":" suite]
|
|
\end{verbatim}
|
|
|
|
\section{The {\tt try} statement}
|
|
|
|
\begin{verbatim}
|
|
try_stmt: "try" ":" suite
|
|
("except" condition ["," condition] ":" suite)*
|
|
["finally" ":" suite]
|
|
\end{verbatim}
|
|
|
|
\section{Function definitions}
|
|
|
|
\begin{verbatim}
|
|
funcdef: "def" identifier "(" [parameter_list] ")" ":" suite
|
|
parameter_list: parameter ("," parameter)*
|
|
parameter: identifier | "(" parameter_list ")"
|
|
\end{verbatim}
|
|
|
|
\section{Class definitions}
|
|
|
|
\begin{verbatim}
|
|
classdef: "class" identifier [inheritance] ":" suite
|
|
inheritance: "(" expression ("," expression)* ")"
|
|
\end{verbatim}
|
|
|
|
XXX Syntax for scripts, modules
|
|
XXX Syntax for interactive input, eval, exec, input
|
|
XXX New definition of expressions (as conditions)
|
|
|
|
\end{document}
|