mirror of https://github.com/python/cpython.git
357 lines
14 KiB
TeX
357 lines
14 KiB
TeX
\section{Built-in Functions}
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The Python interpreter has a number of functions built into it that
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are always available. They are listed here in alphabetical order.
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\renewcommand{\indexsubitem}{(built-in function)}
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\begin{funcdesc}{abs}{x}
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Return the absolute value of a number. The argument may be a plain
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or long integer or a floating point number.
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\end{funcdesc}
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\begin{funcdesc}{apply}{function\, args}
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The \var{function} argument must be a callable object (a user-defined or
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built-in function or method, or a class object) and the \var{args}
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argument must be a tuple. The \var{function} is called with
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\var{args} as argument list; the number of arguments is the the length
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of the tuple. (This is different from just calling
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\code{\var{func}(\var{args})}, since in that case there is always
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exactly one argument.)
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\end{funcdesc}
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\begin{funcdesc}{chr}{i}
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Return a string of one character whose \ASCII{} code is the integer
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\var{i}, e.g., \code{chr(97)} returns the string \code{'a'}. This is the
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inverse of \code{ord()}. The argument must be in the range [0..255],
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inclusive.
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\end{funcdesc}
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\begin{funcdesc}{cmp}{x\, y}
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Compare the two objects \var{x} and \var{y} and return an integer
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according to the outcome. The return value is negative if \code{\var{x}
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< \var{y}}, zero if \code{\var{x} == \var{y}} and strictly positive if
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\code{\var{x} > \var{y}}.
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\end{funcdesc}
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\begin{funcdesc}{coerce}{x\, y}
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Return a tuple consisting of the two numeric arguments converted to
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a common type, using the same rules as used by arithmetic
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operations.
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\end{funcdesc}
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\begin{funcdesc}{compile}{string\, filename\, kind}
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Compile the \var{string} into a code object. Code objects can be
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executed by a \code{exec()} statement or evaluated by a call to
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\code{eval()}. The \var{filename} argument should
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give the file from which the code was read; pass e.g. \code{'<string>'}
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if it wasn't read from a file. The \var{kind} argument specifies
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what kind of code must be compiled; it can be \code{'exec'} if
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\var{string} consists of a sequence of statements, or \code{'eval'}
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if it consists of a single expression.
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\end{funcdesc}
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\begin{funcdesc}{dir}{}
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Without arguments, return the list of names in the current local
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symbol table. With a module, class or class instance object as
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argument (or anything else that has a \code{__dict__} attribute),
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returns the list of names in that object's attribute dictionary.
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The resulting list is sorted. For example:
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\bcode\begin{verbatim}
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>>> import sys
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>>> dir()
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['sys']
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>>> dir(sys)
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['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout']
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>>>
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\end{verbatim}\ecode
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\end{funcdesc}
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\begin{funcdesc}{divmod}{a\, b}
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Take two numbers as arguments and return a pair of integers
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consisting of their integer quotient and remainder. With mixed
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operand types, the rules for binary arithmetic operators apply. For
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plain and long integers, the result is the same as
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\code{(\var{a} / \var{b}, \var{a} \%{} \var{b})}.
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For floating point numbers the result is the same as
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\code{(math.floor(\var{a} / \var{b}), \var{a} \%{} \var{b})}.
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\end{funcdesc}
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\begin{funcdesc}{eval}{s\, globals\, locals}
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The arguments are a string and two optional dictionaries. The
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string argument is parsed and evaluated as a Python expression
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(technically speaking, a condition list) using the dictionaries as
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global and local name space. The string must not contain null bytes
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or newline characters. The return value is the
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result of the expression. If the third argument is omitted it
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defaults to the second. If both dictionaries are omitted, the
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expression is executed in the environment where \code{eval} is
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called. Syntax errors are reported as exceptions. Example:
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\bcode\begin{verbatim}
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>>> x = 1
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>>> print eval('x+1')
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2
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>>>
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\end{verbatim}\ecode
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This function can also be used to execute arbitrary code objects
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(e.g. created by \code{compile()}). In this case pass a code
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object instead of a string. The code object must have been compiled
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passing \code{'eval'} to the \var{kind} argument.
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Note: dynamic execution of statements is supported by the
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\code{exec} statement.
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\end{funcdesc}
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\begin{funcdesc}{filter}{function\, list}
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Construct a list from those elements of \var{list} for which
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\var{function} returns true. If \var{list} is a string or a tuple,
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the result also has that type; otherwise it is always a list. If
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\var{function} is \code{None}, the identity function is assumed,
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i.e. all elements of \var{list} that are false (zero or empty) are
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removed.
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\end{funcdesc}
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\begin{funcdesc}{float}{x}
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Convert a number to floating point. The argument may be a plain or
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long integer or a floating point number.
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\end{funcdesc}
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\begin{funcdesc}{getattr}{object\, name}
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The arguments are an object and a string. The string must be the
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name
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of one of the object's attributes. The result is the value of that
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attribute. For example, \code{getattr(\var{x}, '\var{foobar}')} is equivalent to
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\code{\var{x}.\var{foobar}}.
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\end{funcdesc}
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\begin{funcdesc}{hasattr}{object\, name}
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The arguments are an object and a string. The result is 1 if the
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string is the name of one of the object's attributes, 0 if not.
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(This is implemented by calling \code{getattr(object, name)} and
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seeing whether it raises an exception or not.)
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\end{funcdesc}
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\begin{funcdesc}{hash}{object}
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Return the hash value of the object (if it has one). Hash values
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are 32-bit integers. They are used to quickly compare dictionary
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keys during a dictionary lookup. Numeric values that compare equal
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have the same hash value (even if they are of different types, e.g.
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1 and 1.0).
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\end{funcdesc}
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\begin{funcdesc}{hex}{x}
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Convert a number to a hexadecimal string. The result is a valid
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Python expression.
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\end{funcdesc}
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\begin{funcdesc}{id}{object}
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Return the `identity' of an object. This is an integer which is
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guaranteed to be unique and constant for this object during its
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lifetime. (Two objects whose lifetimes are disjunct may have the
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same id() value.) (Implementation note: this is the address of the
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object.)
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\end{funcdesc}
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\begin{funcdesc}{input}{prompt}
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Almost equivalent to \code{eval(raw_input(\var{prompt}))}. As for
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\code{raw_input()}, the prompt argument is optional. The difference is
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that a long input expression may be broken over multiple lines using the
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backslash convention.
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\end{funcdesc}
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\begin{funcdesc}{int}{x}
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Convert a number to a plain integer. The argument may be a plain or
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long integer or a floating point number.
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\end{funcdesc}
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\begin{funcdesc}{len}{s}
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Return the length (the number of items) of an object. The argument
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may be a sequence (string, tuple or list) or a mapping (dictionary).
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\end{funcdesc}
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\begin{funcdesc}{long}{x}
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Convert a number to a long integer. The argument may be a plain or
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long integer or a floating point number.
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\end{funcdesc}
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\begin{funcdesc}{map}{function\, list\, ...}
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Apply \var{function} to every item of \var{list} and return a list
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of the results. If additional \var{list} arguments are passed,
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\var{function} must take that many arguments and is applied to
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the items of all lists in parallel; if a list is shorter than another
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it is assumed to be extended with \code{None} items. If
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\var{function} is \code{None}, the identity function is assumed; if
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there are multiple list arguments, \code{map} returns a list
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consisting of tuples containing the corresponding items from all lists
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(i.e. a kind of transpose operation). The \var{list} arguments may be
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any kind of sequence; the result is always a list.
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\end{funcdesc}
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\begin{funcdesc}{max}{s}
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Return the largest item of a non-empty sequence (string, tuple or
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list).
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\end{funcdesc}
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\begin{funcdesc}{min}{s}
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Return the smallest item of a non-empty sequence (string, tuple or
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list).
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\end{funcdesc}
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\begin{funcdesc}{oct}{x}
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Convert a number to an octal string. The result is a valid Python
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expression.
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\end{funcdesc}
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\begin{funcdesc}{open}{filename\, mode}
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% XXXJH xrefs here to Built-in types?
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Return a new file object (described earlier under Built-in Types).
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The string arguments are the same as for \code{stdio}'s
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\code{fopen()}: \var{filename} is the file name to be opened,
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\var{mode} indicates how the file is to be opened: \code{'r'} for
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reading, \code{'w'} for writing (truncating an existing file), and
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\code{'a'} opens it for appending. Modes \code{'r+'}, \code{'w+'} and
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\code{'a+'} open the file for updating, provided the underlying
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\code{stdio} library understands this. On systems that differentiate
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between binary and text files, \code{'b'} appended to the mode opens
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the file in binary mode. If the file cannot be opened, \code{IOError}
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is raised.
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\end{funcdesc}
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\begin{funcdesc}{ord}{c}
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Return the \ASCII{} value of a string of one character. E.g.,
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\code{ord('a')} returns the integer \code{97}. This is the inverse of
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\code{chr()}.
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\end{funcdesc}
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\begin{funcdesc}{pow}{x\, y}
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Return \var{x} to the power \var{y}. The arguments must have
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numeric types. With mixed operand types, the rules for binary
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arithmetic operators apply. The effective operand type is also the
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type of the result; if the result is not expressible in this type, the
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function raises an exception; e.g., \code{pow(2, -1)} is not allowed.
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\end{funcdesc}
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\begin{funcdesc}{range}{start\, end\, step}
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This is a versatile function to create lists containing arithmetic
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progressions. It is most often used in \code{for} loops. The
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arguments must be plain integers. If the \var{step} argument is
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omitted, it defaults to \code{1}. If the \var{start} argument is
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omitted, it defaults to \code{0}. The full form returns a list of
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plain integers \code{[\var{start}, \var{start} + \var{step},
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\var{start} + 2 * \var{step}, \ldots]}. If \var{step} is positive,
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the last element is the largest \code{\var{start} + \var{i} *
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\var{step}} less than \var{end}; if \var{step} is negative, the last
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element is the largest \code{\var{start} + \var{i} * \var{step}}
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greater than \var{end}. \var{step} must not be zero. Example:
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\bcode\begin{verbatim}
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>>> range(10)
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[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
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>>> range(1, 11)
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[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
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>>> range(0, 30, 5)
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[0, 5, 10, 15, 20, 25]
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>>> range(0, 10, 3)
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[0, 3, 6, 9]
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>>> range(0, -10, -1)
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[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
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>>> range(0)
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[]
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>>> range(1, 0)
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[]
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>>>
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\end{verbatim}\ecode
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\end{funcdesc}
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\begin{funcdesc}{raw_input}{prompt}
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The string argument is optional; if present, it is written to
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standard
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output without a trailing newline. The function then reads a line
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from input, converts it to a string (stripping a trailing newline),
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and returns that. When \EOF{} is read, \code{EOFError} is raised.
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Example:
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\bcode\begin{verbatim}
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>>> s = raw_input('--> ')
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--> Monty Python's Flying Circus
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>>> s
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'Monty Python\'s Flying Circus'
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>>>
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\end{verbatim}\ecode
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\end{funcdesc}
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\begin{funcdesc}{reduce}{function\, list\, initializer}
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Apply the binary \var{function} to the items of \var{list} so as to
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reduce the list to a single value. E.g.,
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\code{reduce(lambda x, y: x*y, \var{list}, 1)} returns the product of
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the elements of \var{list}. The optional \var{initializer} can be
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thought of as being prepended to \var{list} so as to allow reduction
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of an empty \var{list}. The \var{list} arguments may be any kind of
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sequence.
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\end{funcdesc}
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\begin{funcdesc}{reload}{module}
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Re-parse and re-initialize an already imported \var{module}. The
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argument must be a module object, so it must have been successfully
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imported before. This is useful if you have edited the module source
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file using an external editor and want to try out the new version
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without leaving the Python interpreter. Note that if a module is
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syntactically correct but its initialization fails, the first
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\code{import} statement for it does not import the name, but does
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create a (partially initialized) module object; to reload the module
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you must first \code{import} it again (this will just make the
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partially initialized module object available) before you can
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\code{reload()} it.
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\end{funcdesc}
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\begin{funcdesc}{repr}{object}
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Return a string containing a printable representation of an object.
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This is the same value yielded by conversions (reverse quotes).
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It is sometimes useful to be able to access this operation as an
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ordinary function. For many types, this function makes an attempt
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to return a string that would yield an object with the same value
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when passed to \code{eval()}.
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\end{funcdesc}
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\begin{funcdesc}{round}{x\, n}
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Return the floating point value \var{x} rounded to \var{n} digits
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after the decimal point. If \var{n} is omitted, it defaults to zero.
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The result is a floating point number. Values are rounded to the
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closest multiple of 10 to the power minus \var{n}; if two multiples
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are equally close, rounding is done away from 0 (so e.g.
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\code{round(0.5)} is \code{1.0} and \code{round(-0.5)} is \code{-1.0}).
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\end{funcdesc}
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\begin{funcdesc}{setattr}{object\, name\, value}
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This is the counterpart of \code{getattr}. The arguments are an
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object, a string and an arbitrary value. The string must be the name
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of one of the object's attributes. The function assigns the value to
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the attribute, provided the object allows it. For example,
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\code{setattr(\var{x}, '\var{foobar}', 123)} is equivalent to
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\code{\var{x}.\var{foobar} = 123}.
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\end{funcdesc}
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\begin{funcdesc}{str}{object}
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Return a string containing a nicely printable representation of an
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object. For strings, this returns the string itself. The difference
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with \code{repr(\var{object}} is that \code{str(\var{object}} does not
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always attempt to return a string that is acceptable to \code{eval()};
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its goal is to return a printable string.
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\end{funcdesc}
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\begin{funcdesc}{type}{object}
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% XXXJH xref to buil-in objects here?
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Return the type of an \var{object}. The return value is a type
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object. There is not much you can do with type objects except compare
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them to other type objects; e.g., the following checks if a variable
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is a string:
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\bcode\begin{verbatim}
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>>> if type(x) == type(''): print 'It is a string'
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\end{verbatim}\ecode
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\end{funcdesc}
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