1992-08-14 09:17:29 +00:00
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\chapter{Simple statements}
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\indexii{simple}{statement}
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|
|
Simple statements are comprised within a single logical line.
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Several simple statements may occur on a single line separated
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|
|
|
by semicolons. The syntax for simple statements is:
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\begin{verbatim}
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|
simple_stmt: expression_stmt
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| assignment_stmt
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| pass_stmt
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| del_stmt
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| print_stmt
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| return_stmt
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| raise_stmt
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| break_stmt
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| continue_stmt
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| import_stmt
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| global_stmt
|
1993-10-18 17:59:42 +00:00
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| access_stmt
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| exec_stmt
|
1992-08-14 09:17:29 +00:00
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|
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\end{verbatim}
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\section{Expression statements}
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\indexii{expression}{statement}
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Expression statements are used (mostly interactively) to compute and
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write a value, or (usually) to call a procedure (a function that
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returns no meaningful result; in Python, procedures return the value
|
1994-08-01 12:38:14 +00:00
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\verb@None@):
|
1992-08-14 09:17:29 +00:00
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\begin{verbatim}
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expression_stmt: expression_list
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\end{verbatim}
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An expression statement evaluates the expression list (which may be a
|
1994-08-01 12:38:14 +00:00
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|
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single expression). If the value is not \verb@None@, it is converted
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1992-08-14 09:17:29 +00:00
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|
to a string using the rules for string conversions (expressions in
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reverse quotes), and the resulting string is written to standard
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|
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output (see section \ref{print}) on a line by itself.
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\indexii{expression}{list}
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\ttindex{None}
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\indexii{string}{conversion}
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\index{output}
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\indexii{standard}{output}
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\indexii{writing}{values}
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|
1994-08-01 12:38:14 +00:00
|
|
|
(The exception for \verb@None@ is made so that procedure calls, which
|
1992-08-14 09:17:29 +00:00
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|
|
are syntactically equivalent to expressions, do not cause any output.
|
1994-08-01 12:38:14 +00:00
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|
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A tuple with only \verb@None@ items is written normally.)
|
1992-08-14 09:17:29 +00:00
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\indexii{procedure}{call}
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\section{Assignment statements}
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\indexii{assignment}{statement}
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Assignment statements are used to (re)bind names to values and to
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modify attributes or items of mutable objects:
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\indexii{binding}{name}
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\indexii{rebinding}{name}
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\obindex{mutable}
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\indexii{attribute}{assignment}
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\begin{verbatim}
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assignment_stmt: (target_list "=")+ expression_list
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target_list: target ("," target)* [","]
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target: identifier | "(" target_list ")" | "[" target_list "]"
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|
|
| attributeref | subscription | slicing
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\end{verbatim}
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|
(See section \ref{primaries} for the syntax definitions for the last
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three symbols.)
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|
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An assignment statement evaluates the expression list (remember that
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this can be a single expression or a comma-separated list, the latter
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yielding a tuple) and assigns the single resulting object to each of
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the target lists, from left to right.
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|
\indexii{expression}{list}
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|
|
Assignment is defined recursively depending on the form of the target
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(list). When a target is part of a mutable object (an attribute
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reference, subscription or slicing), the mutable object must
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|
ultimately perform the assignment and decide about its validity, and
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|
may raise an exception if the assignment is unacceptable. The rules
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|
observed by various types and the exceptions raised are given with the
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|
|
definition of the object types (see section \ref{types}).
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\index{target}
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\indexii{target}{list}
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Assignment of an object to a target list is recursively defined as
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follows.
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\indexiii{target}{list}{assignment}
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\begin{itemize}
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\item
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If the target list is a single target: the object is assigned to that
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target.
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\item
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If the target list is a comma-separated list of targets: the object
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must be a tuple with the same number of items as the list contains
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|
|
targets, and the items are assigned, from left to right, to the
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|
|
corresponding targets.
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|
|
\end{itemize}
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|
|
Assignment of an object to a single target is recursively defined as
|
|
|
|
follows.
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|
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\begin{itemize} % nested
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\item
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|
|
If the target is an identifier (name):
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|
|
\begin{itemize}
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\item
|
1994-08-01 12:38:14 +00:00
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|
|
If the name does not occur in a \verb@global@ statement in the current
|
1992-08-14 09:17:29 +00:00
|
|
|
code block: the name is bound to the object in the current local name
|
|
|
|
space.
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|
|
\stindex{global}
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\item
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|
Otherwise: the name is bound to the object in the current global name
|
|
|
|
space.
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|
|
\end{itemize} % nested
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|
|
The name is rebound if it was already bound.
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\item
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|
If the target is a target list enclosed in parentheses: the object is
|
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|
|
assigned to that target list as described above.
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|
\item
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|
|
If the target is a target list enclosed in square brackets: the object
|
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|
|
must be a list with the same number of items as the target list
|
|
|
|
contains targets, and its items are assigned, from left to right, to
|
|
|
|
the corresponding targets.
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|
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|
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|
|
\item
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|
|
If the target is an attribute reference: The primary expression in the
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|
|
|
reference is evaluated. It should yield an object with assignable
|
1994-08-01 12:38:14 +00:00
|
|
|
attributes; if this is not the case, \verb@TypeError@ is raised. That
|
1992-08-14 09:17:29 +00:00
|
|
|
object is then asked to assign the assigned object to the given
|
|
|
|
attribute; if it cannot perform the assignment, it raises an exception
|
1994-08-01 12:38:14 +00:00
|
|
|
(usually but not necessarily \verb@AttributeError@).
|
1992-08-14 09:17:29 +00:00
|
|
|
\indexii{attribute}{assignment}
|
|
|
|
|
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|
|
\item
|
|
|
|
If the target is a subscription: The primary expression in the
|
|
|
|
reference is evaluated. It should yield either a mutable sequence
|
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|
|
(list) object or a mapping (dictionary) object. Next, the subscript
|
|
|
|
expression is evaluated.
|
|
|
|
\indexii{subscription}{assignment}
|
|
|
|
\obindex{mutable}
|
|
|
|
|
|
|
|
If the primary is a mutable sequence object (a list), the subscript
|
|
|
|
must yield a plain integer. If it is negative, the sequence's length
|
|
|
|
is added to it. The resulting value must be a nonnegative integer
|
|
|
|
less than the sequence's length, and the sequence is asked to assign
|
|
|
|
the assigned object to its item with that index. If the index is out
|
1994-08-01 12:38:14 +00:00
|
|
|
of range, \verb@IndexError@ is raised (assignment to a subscripted
|
1992-08-14 09:17:29 +00:00
|
|
|
sequence cannot add new items to a list).
|
|
|
|
\obindex{sequence}
|
|
|
|
\obindex{list}
|
|
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|
|
|
|
If the primary is a mapping (dictionary) object, the subscript must
|
|
|
|
have a type compatible with the mapping's key type, and the mapping is
|
1995-01-04 19:17:34 +00:00
|
|
|
then asked to create a key/datum pair which maps the subscript to
|
1992-08-14 09:17:29 +00:00
|
|
|
the assigned object. This can either replace an existing key/value
|
|
|
|
pair with the same key value, or insert a new key/value pair (if no
|
|
|
|
key with the same value existed).
|
|
|
|
\obindex{mapping}
|
|
|
|
\obindex{dictionary}
|
|
|
|
|
|
|
|
\item
|
|
|
|
If the target is a slicing: The primary expression in the reference is
|
1994-08-01 12:38:14 +00:00
|
|
|
evaluated. It should yield a mutable sequence object (e.g. a list). The
|
1992-08-14 09:17:29 +00:00
|
|
|
assigned object should be a sequence object of the same type. Next,
|
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|
|
the lower and upper bound expressions are evaluated, insofar they are
|
|
|
|
present; defaults are zero and the sequence's length. The bounds
|
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|
|
should evaluate to (small) integers. If either bound is negative, the
|
|
|
|
sequence's length is added to it. The resulting bounds are clipped to
|
|
|
|
lie between zero and the sequence's length, inclusive. Finally, the
|
1994-08-01 12:38:14 +00:00
|
|
|
sequence object is asked to replace the slice with the items of the
|
|
|
|
assigned sequence. The length of the slice may be different from the
|
|
|
|
length of the assigned sequence, thus changing the length of the
|
|
|
|
target sequence, if the object allows it.
|
1992-08-14 09:17:29 +00:00
|
|
|
\indexii{slicing}{assignment}
|
|
|
|
|
|
|
|
\end{itemize}
|
|
|
|
|
1994-08-23 13:26:22 +00:00
|
|
|
(In the current implementation, the syntax for targets is taken
|
1992-08-14 09:17:29 +00:00
|
|
|
to be the same as for expressions, and invalid syntax is rejected
|
|
|
|
during the code generation phase, causing less detailed error
|
|
|
|
messages.)
|
|
|
|
|
1994-08-23 13:26:22 +00:00
|
|
|
WARNING: Although the definition of assignment implies that overlaps
|
|
|
|
between the left-hand side and the right-hand side are `safe' (e.g.
|
|
|
|
\verb@a, b = b, a@ swaps two variables), overlaps within the
|
|
|
|
collection of assigned-to variables are not safe! For instance, the
|
|
|
|
following program prints \code@[0, 2]@:
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
x = [0, 1]
|
|
|
|
i = 0
|
|
|
|
i, x[i] = 1, 2
|
|
|
|
print x
|
|
|
|
\end{verbatim}
|
|
|
|
|
|
|
|
|
1992-08-14 09:17:29 +00:00
|
|
|
\section{The {\tt pass} statement}
|
|
|
|
\stindex{pass}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
pass_stmt: "pass"
|
|
|
|
\end{verbatim}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
\verb@pass@ is a null operation --- when it is executed, nothing
|
1992-08-14 09:17:29 +00:00
|
|
|
happens. It is useful as a placeholder when a statement is
|
|
|
|
required syntactically, but no code needs to be executed, for example:
|
|
|
|
\indexii{null}{operation}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
def f(arg): pass # a function that does nothing (yet)
|
|
|
|
|
|
|
|
class C: pass # an class with no methods (yet)
|
|
|
|
\end{verbatim}
|
|
|
|
|
|
|
|
\section{The {\tt del} statement}
|
|
|
|
\stindex{del}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
del_stmt: "del" target_list
|
|
|
|
\end{verbatim}
|
|
|
|
|
|
|
|
Deletion is recursively defined very similar to the way assignment is
|
|
|
|
defined. Rather that spelling it out in full details, here are some
|
|
|
|
hints.
|
|
|
|
\indexii{deletion}{target}
|
|
|
|
\indexiii{deletion}{target}{list}
|
|
|
|
|
|
|
|
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 or global name space, depending on whether the name
|
1994-08-01 12:38:14 +00:00
|
|
|
occurs in a \verb@global@ statement in the same code block.
|
1992-08-14 09:17:29 +00:00
|
|
|
\stindex{global}
|
|
|
|
\indexii{unbinding}{name}
|
|
|
|
|
|
|
|
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).
|
|
|
|
\indexii{attribute}{deletion}
|
|
|
|
|
|
|
|
\section{The {\tt print} statement} \label{print}
|
|
|
|
\stindex{print}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
print_stmt: "print" [ condition ("," condition)* [","] ]
|
|
|
|
\end{verbatim}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
\verb@print@ evaluates each condition in turn and writes the resulting
|
1992-08-14 09:17:29 +00:00
|
|
|
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 yet been written
|
|
|
|
to standard output; or (2) when the last character written to standard
|
|
|
|
output is \verb/\n/; or (3) when the last write operation on standard
|
1994-08-01 12:38:14 +00:00
|
|
|
output was not a \verb@print@ statement. (In some cases it may be
|
1992-08-14 09:17:29 +00:00
|
|
|
functional to write an empty string to standard output for this
|
|
|
|
reason.)
|
|
|
|
\index{output}
|
|
|
|
\indexii{writing}{values}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
A \verb/"\n"/ character is written at the end, unless the \verb@print@
|
1992-08-14 09:17:29 +00:00
|
|
|
statement ends with a comma. This is the only action if the statement
|
1994-08-01 12:38:14 +00:00
|
|
|
contains just the keyword \verb@print@.
|
1992-08-14 09:17:29 +00:00
|
|
|
\indexii{trailing}{comma}
|
|
|
|
\indexii{newline}{suppression}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
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 \verb@RuntimeError@ exception is raised.
|
1992-08-14 09:17:29 +00:00
|
|
|
(The original implementation attempts to write to the system's original
|
|
|
|
standard output instead, but this is not safe, and should be fixed.)
|
|
|
|
\indexii{standard}{output}
|
|
|
|
\bimodindex{sys}
|
|
|
|
\ttindex{stdout}
|
|
|
|
\exindex{RuntimeError}
|
|
|
|
|
|
|
|
\section{The {\tt return} statement}
|
|
|
|
\stindex{return}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
return_stmt: "return" [condition_list]
|
|
|
|
\end{verbatim}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
\verb@return@ may only occur syntactically nested in a function
|
1992-08-14 09:17:29 +00:00
|
|
|
definition, not within a nested class definition.
|
|
|
|
\indexii{function}{definition}
|
|
|
|
\indexii{class}{definition}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
If a condition list is present, it is evaluated, else \verb@None@
|
1992-08-14 09:17:29 +00:00
|
|
|
is substituted.
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
\verb@return@ leaves the current function call with the condition
|
|
|
|
list (or \verb@None@) as return value.
|
1992-08-14 09:17:29 +00:00
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
When \verb@return@ passes control out of a \verb@try@ statement
|
|
|
|
with a \verb@finally@ clause, that finally clause is executed
|
1992-08-14 09:17:29 +00:00
|
|
|
before really leaving the function.
|
|
|
|
\kwindex{finally}
|
|
|
|
|
|
|
|
\section{The {\tt raise} statement}
|
|
|
|
\stindex{raise}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
raise_stmt: "raise" condition ["," condition]
|
|
|
|
\end{verbatim}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
\verb@raise@ evaluates its first condition, which must yield
|
1995-02-07 14:37:17 +00:00
|
|
|
a string, class, or instance object. If there is a second condition,
|
|
|
|
this is evaluated, else \verb@None@ is substituted. If the first
|
|
|
|
condition is a class object, then the second condition must be an
|
|
|
|
instance of that class or one of its derivatives. If the first
|
|
|
|
condition is an instance object, the second condition must be
|
|
|
|
\verb@None@.
|
1992-08-14 09:17:29 +00:00
|
|
|
\index{exception}
|
|
|
|
\indexii{raising}{exception}
|
|
|
|
|
1995-02-07 14:37:17 +00:00
|
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If the first object is a class or string, it then raises the exception
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identified by the first object, with the second one (or \verb@None@)
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as its parameter. If the first object is an instance, it raises the
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exception identified by the class of the object, with the instance as
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its parameter.
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1992-08-14 09:17:29 +00:00
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\section{The {\tt break} statement}
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\stindex{break}
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\begin{verbatim}
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break_stmt: "break"
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\end{verbatim}
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1994-08-01 12:38:14 +00:00
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\verb@break@ may only occur syntactically nested in a \verb@for@
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or \verb@while@ loop, but not nested in a function or class definition
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within that loop.
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1992-08-14 09:17:29 +00:00
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\stindex{for}
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\stindex{while}
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\indexii{loop}{statement}
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1994-08-01 12:38:14 +00:00
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It terminates the nearest enclosing loop, skipping the optional
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\verb@else@ clause if the loop has one.
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1992-08-14 09:17:29 +00:00
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\kwindex{else}
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1994-08-01 12:38:14 +00:00
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If a \verb@for@ loop is terminated by \verb@break@, the loop control
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1992-08-14 09:17:29 +00:00
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target keeps its current value.
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\indexii{loop control}{target}
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1994-08-01 12:38:14 +00:00
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When \verb@break@ passes control out of a \verb@try@ statement
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with a \verb@finally@ clause, that finally clause is executed
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1992-08-14 09:17:29 +00:00
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before really leaving the loop.
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\kwindex{finally}
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\section{The {\tt continue} statement}
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\stindex{continue}
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\begin{verbatim}
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continue_stmt: "continue"
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\end{verbatim}
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1994-08-01 12:38:14 +00:00
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\verb@continue@ may only occur syntactically nested in a \verb@for@ or
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\verb@while@ loop, but not nested in a function or class definition or
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\verb@try@ statement within that loop.\footnote{Except that it may
|
1995-01-04 19:17:34 +00:00
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currently occur within an {\tt except} clause.}
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1992-08-14 09:17:29 +00:00
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\stindex{for}
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\stindex{while}
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\indexii{loop}{statement}
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\kwindex{finally}
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It continues with the next cycle of the nearest enclosing loop.
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\section{The {\tt import} statement} \label{import}
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\stindex{import}
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\begin{verbatim}
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import_stmt: "import" identifier ("," identifier)*
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| "from" identifier "import" identifier ("," identifier)*
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| "from" identifier "import" "*"
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\end{verbatim}
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Import statements are executed in two steps: (1) find a module, and
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initialize it if necessary; (2) define a name or names in the local
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1994-08-01 12:38:14 +00:00
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name space (of the scope where the \verb@import@ statement occurs).
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The first form (without \verb@from@) repeats these steps for each
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identifier in the list, the \verb@from@ form performs them once, with
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1992-08-14 09:17:29 +00:00
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the first identifier specifying the module name.
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\indexii{importing}{module}
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\indexii{name}{binding}
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\kwindex{from}
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|
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The system maintains a table of modules that have been initialized,
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|
|
indexed by module name. (The current implementation makes this table
|
1994-08-01 12:38:14 +00:00
|
|
|
accessible as \verb@sys.modules@.) When a module name is found in
|
1992-08-14 09:17:29 +00:00
|
|
|
this table, step (1) is finished. If not, a search for a module
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|
|
definition is started. This first looks for a built-in module
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|
|
definition, and if no built-in module if the given name is found, it
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|
searches a user-specified list of directories for a file whose name is
|
1994-08-01 12:38:14 +00:00
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|
|
the module name with extension \verb@".py"@. (The current
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|
|
implementation uses the list of strings \verb@sys.path@ as the search
|
1992-08-14 09:17:29 +00:00
|
|
|
path; it is initialized from the shell environment variable
|
1994-08-01 12:38:14 +00:00
|
|
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\verb@$PYTHONPATH@, with an installation-dependent default.)
|
1992-08-14 09:17:29 +00:00
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|
|
\ttindex{modules}
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\ttindex{sys.modules}
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\indexii{module}{name}
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\indexii{built-in}{module}
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\indexii{user-defined}{module}
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\bimodindex{sys}
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\ttindex{path}
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\ttindex{sys.path}
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\indexii{filename}{extension}
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If a built-in module is found, its built-in initialization code is
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|
|
executed and step (1) is finished. If no matching file is found,
|
1994-08-01 12:38:14 +00:00
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|
|
\verb@ImportError@ is raised. If a file is found, it is parsed,
|
1992-08-14 09:17:29 +00:00
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|
|
yielding an executable code block. If a syntax error occurs,
|
1994-08-01 12:38:14 +00:00
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|
|
\verb@SyntaxError@ is raised. Otherwise, an empty module of the given
|
1992-08-14 09:17:29 +00:00
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|
|
name is created and inserted in the module table, and then the code
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|
|
block is executed in the context of this module. Exceptions during
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|
|
this execution terminate step (1).
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|
\indexii{module}{initialization}
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|
\exindex{SyntaxError}
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|
\exindex{ImportError}
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|
|
\index{code block}
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|
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|
|
When step (1) finishes without raising an exception, step (2) can
|
|
|
|
begin.
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|
|
1994-08-01 12:38:14 +00:00
|
|
|
The first form of \verb@import@ statement binds the module name in the
|
1992-08-14 09:17:29 +00:00
|
|
|
local name space to the module object, and then goes on to import the
|
1994-08-01 12:38:14 +00:00
|
|
|
next identifier, if any. The \verb@from@ from does not bind the
|
1992-08-14 09:17:29 +00:00
|
|
|
module name: it goes through the list of identifiers, looks each one
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|
|
of them up in the module found in step (1), and binds the name in the
|
|
|
|
local name space to the object thus found. If a name is not found,
|
1994-08-01 12:38:14 +00:00
|
|
|
\verb@ImportError@ is raised. If the list of identifiers is replaced
|
|
|
|
by a star (\verb@*@), all names defined in the module are bound,
|
|
|
|
except those beginning with an underscore(\verb@_@).
|
1992-08-14 09:17:29 +00:00
|
|
|
\indexii{name}{binding}
|
|
|
|
\exindex{ImportError}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
Names bound by import statements may not occur in \verb@global@
|
1992-08-14 09:17:29 +00:00
|
|
|
statements in the same scope.
|
|
|
|
\stindex{global}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
The \verb@from@ form with \verb@*@ may only occur in a module scope.
|
1992-08-14 09:17:29 +00:00
|
|
|
\kwindex{from}
|
|
|
|
\ttindex{from ... import *}
|
|
|
|
|
|
|
|
(The current implementation does not enforce the latter two
|
|
|
|
restrictions, but programs should not abuse this freedom, as future
|
|
|
|
implementations may enforce them or silently change the meaning of the
|
|
|
|
program.)
|
|
|
|
|
|
|
|
\section{The {\tt global} statement} \label{global}
|
|
|
|
\stindex{global}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
global_stmt: "global" identifier ("," identifier)*
|
|
|
|
\end{verbatim}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
The \verb@global@ statement is a declaration which holds for the
|
1992-08-14 09:17:29 +00:00
|
|
|
entire current scope. It means that the listed identifiers are to be
|
|
|
|
interpreted as globals. While {\em using} global names is automatic
|
|
|
|
if they are not defined in the local scope, {\em assigning} to global
|
1994-08-01 12:38:14 +00:00
|
|
|
names would be impossible without \verb@global@.
|
1992-08-14 09:17:29 +00:00
|
|
|
\indexiii{global}{name}{binding}
|
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
Names listed in a \verb@global@ statement must not be used in the same
|
|
|
|
scope before that \verb@global@ statement is executed.
|
1992-08-14 09:17:29 +00:00
|
|
|
|
1994-08-01 12:38:14 +00:00
|
|
|
Names listed in a \verb@global@ statement must not be defined as formal
|
|
|
|
parameters or in a \verb@for@ loop control target, \verb@class@
|
|
|
|
definition, function definition, or \verb@import@ statement.
|
1992-08-14 09:17:29 +00:00
|
|
|
|
|
|
|
(The current implementation does not enforce the latter two
|
|
|
|
restrictions, but programs should not abuse this freedom, as future
|
|
|
|
implementations may enforce them or silently change the meaning of the
|
|
|
|
program.)
|
1993-10-18 17:59:42 +00:00
|
|
|
|
|
|
|
\section{The {\tt access} statement} \label{access}
|
|
|
|
\stindex{access}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
access_stmt: "access" ...
|
|
|
|
\end{verbatim}
|
|
|
|
|
|
|
|
This statement will be used in the future to control access to
|
|
|
|
instance and class variables. Currently its syntax and effects are
|
1994-08-01 12:38:14 +00:00
|
|
|
undefined; however the keyword \verb@access@ is a reserved word for
|
1993-10-18 17:59:42 +00:00
|
|
|
the parser.
|
|
|
|
|
|
|
|
\section{The {\tt exec} statement} \label{exec}
|
|
|
|
\stindex{exec}
|
|
|
|
|
|
|
|
\begin{verbatim}
|
|
|
|
exec_stmt: "exec" expression ["in" expression ["," expression]]
|
|
|
|
\end{verbatim}
|
|
|
|
|
|
|
|
This statement supports dynamic execution of Python code. The first
|
|
|
|
expression should evaluate to either a string, an open file object, or
|
|
|
|
a code object. If it is a string, the string is parsed as a suite of
|
|
|
|
Python statements which is then executed (unless a syntax error
|
|
|
|
occurs). If it is an open file, the file is parsed until EOF and
|
|
|
|
executed. If it is a code object, it is simply executed.
|
|
|
|
|
|
|
|
In all cases, if the optional parts are omitted, the code is executed
|
1994-08-01 12:38:14 +00:00
|
|
|
in the current scope. If only the first expression after \verb@in@ is
|
1993-10-18 17:59:42 +00:00
|
|
|
specified, it should be a dictionary, which will be used for both the
|
|
|
|
global and the local variables. If two expressions are given, both
|
|
|
|
must be dictionaries and they are used for the global and local
|
|
|
|
variables, respectively.
|
|
|
|
|
1995-03-07 10:09:34 +00:00
|
|
|
Hints: dynamic evaluation of expressions is supported by the built-in
|
|
|
|
function \verb@eval()@. The built-in function \verb@vars()@ returns
|
|
|
|
the current local dictionary, which may be useful to pass around for
|
|
|
|
use by \verb@exec@.
|