632 lines
24 KiB
ReStructuredText
632 lines
24 KiB
ReStructuredText
.. _compound:
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*******************
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Compound statements
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*******************
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.. index:: pair: compound; statement
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Compound statements contain (groups of) other statements; they affect or control
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the execution of those other statements in some way. In general, compound
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statements span multiple lines, although in simple incarnations a whole compound
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statement may be contained in one line.
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The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement
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traditional control flow constructs. :keyword:`try` specifies exception
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handlers and/or cleanup code for a group of statements, while the
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:keyword:`with` statement allows the execution of initialization and
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finalization code around a block of code. Function and class definitions are
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also syntactically compound statements.
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.. index::
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single: clause
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single: suite
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Compound statements consist of one or more 'clauses.' A clause consists of a
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header and a 'suite.' The clause headers of a particular compound statement are
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all at the same indentation level. Each clause header begins with a uniquely
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identifying keyword and ends with a colon. A suite is a group of statements
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controlled by a clause. A suite can be one or more semicolon-separated simple
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statements on the same line as the header, following the header's colon, or it
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can be one or more indented statements on subsequent lines. Only the latter
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form of suite can contain nested compound statements; the following is illegal,
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mostly because it wouldn't be clear to which :keyword:`if` clause a following
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:keyword:`else` clause would belong::
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if test1: if test2: print(x)
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Also note that the semicolon binds tighter than the colon in this context, so
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that in the following example, either all or none of the :func:`print` calls are
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executed::
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if x < y < z: print(x); print(y); print(z)
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Summarizing:
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.. productionlist::
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compound_stmt: `if_stmt`
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: | `while_stmt`
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: | `for_stmt`
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: | `try_stmt`
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: | `with_stmt`
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: | `funcdef`
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: | `classdef`
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suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
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statement: `stmt_list` NEWLINE | `compound_stmt`
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stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
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.. index::
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single: NEWLINE token
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single: DEDENT token
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pair: dangling; else
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Note that statements always end in a ``NEWLINE`` possibly followed by a
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``DEDENT``. Also note that optional continuation clauses always begin with a
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keyword that cannot start a statement, thus there are no ambiguities (the
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'dangling :keyword:`else`' problem is solved in Python by requiring nested
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:keyword:`if` statements to be indented).
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The formatting of the grammar rules in the following sections places each clause
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on a separate line for clarity.
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.. _if:
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.. _elif:
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.. _else:
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The :keyword:`if` statement
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===========================
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.. index::
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statement: if
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keyword: elif
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keyword: else
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keyword: elif
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keyword: else
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The :keyword:`if` statement is used for conditional execution:
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.. productionlist::
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if_stmt: "if" `expression` ":" `suite`
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: ( "elif" `expression` ":" `suite` )*
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: ["else" ":" `suite`]
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It selects exactly one of the suites by evaluating the expressions one by one
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until one is found to be true (see section :ref:`booleans` for the definition of
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true and false); then that suite is executed (and no other part of the
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:keyword:`if` statement is executed or evaluated). If all expressions are
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false, the suite of the :keyword:`else` clause, if present, is executed.
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.. _while:
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The :keyword:`while` statement
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==============================
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.. index::
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statement: while
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keyword: else
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pair: loop; statement
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keyword: else
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The :keyword:`while` statement is used for repeated execution as long as an
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expression is true:
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.. productionlist::
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while_stmt: "while" `expression` ":" `suite`
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: ["else" ":" `suite`]
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This repeatedly tests the expression and, if it is true, executes the first
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suite; if the expression is false (which may be the first time it is tested) the
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suite of the :keyword:`else` clause, if present, is executed and the loop
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terminates.
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.. index::
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statement: break
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statement: continue
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A :keyword:`break` statement executed in the first suite terminates the loop
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without executing the :keyword:`else` clause's suite. A :keyword:`continue`
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statement executed in the first suite skips the rest of the suite and goes back
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to testing the expression.
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.. _for:
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The :keyword:`for` statement
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============================
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.. index::
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statement: for
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keyword: in
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keyword: else
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pair: target; list
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pair: loop; statement
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keyword: in
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keyword: else
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pair: target; list
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object: sequence
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The :keyword:`for` statement is used to iterate over the elements of a sequence
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(such as a string, tuple or list) or other iterable object:
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.. productionlist::
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for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`
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: ["else" ":" `suite`]
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The expression list is evaluated once; it should yield an iterable object. An
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iterator is created for the result of the ``expression_list``. The suite is
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then executed once for each item provided by the iterator, in the order of
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ascending indices. Each item in turn is assigned to the target list using the
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standard rules for assignments (see :ref:`assignment`), and then the suite is
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executed. When the items are exhausted (which is immediately when the sequence
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is empty or an iterator raises a :exc:`StopIteration` exception), the suite in
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the :keyword:`else` clause, if present, is executed, and the loop terminates.
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.. index::
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statement: break
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statement: continue
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A :keyword:`break` statement executed in the first suite terminates the loop
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without executing the :keyword:`else` clause's suite. A :keyword:`continue`
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statement executed in the first suite skips the rest of the suite and continues
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with the next item, or with the :keyword:`else` clause if there was no next
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item.
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The suite may assign to the variable(s) in the target list; this does not affect
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the next item assigned to it.
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.. index::
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builtin: range
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Names in the target list are not deleted when the loop is finished, but if the
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sequence is empty, it will not have been assigned to at all by the loop. Hint:
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the built-in function :func:`range` returns an iterator of integers suitable to
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emulate the effect of Pascal's ``for i := a to b do``; e.g., ``list(range(3))``
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returns the list ``[0, 1, 2]``.
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.. note::
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.. index::
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single: loop; over mutable sequence
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single: mutable sequence; loop over
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There is a subtlety when the sequence is being modified by the loop (this can
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only occur for mutable sequences, i.e. lists). An internal counter is used
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to keep track of which item is used next, and this is incremented on each
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iteration. When this counter has reached the length of the sequence the loop
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terminates. This means that if the suite deletes the current (or a previous)
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item from the sequence, the next item will be skipped (since it gets the
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index of the current item which has already been treated). Likewise, if the
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suite inserts an item in the sequence before the current item, the current
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item will be treated again the next time through the loop. This can lead to
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nasty bugs that can be avoided by making a temporary copy using a slice of
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the whole sequence, e.g., ::
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for x in a[:]:
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if x < 0: a.remove(x)
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.. _try:
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.. _except:
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.. _finally:
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The :keyword:`try` statement
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============================
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.. index::
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statement: try
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keyword: except
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keyword: finally
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.. index:: keyword: except
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The :keyword:`try` statement specifies exception handlers and/or cleanup code
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for a group of statements:
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.. productionlist::
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try_stmt: try1_stmt | try2_stmt
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try1_stmt: "try" ":" `suite`
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: ("except" [`expression` ["as" `target`]] ":" `suite`)+
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: ["else" ":" `suite`]
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: ["finally" ":" `suite`]
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try2_stmt: "try" ":" `suite`
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: "finally" ":" `suite`
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The :keyword:`except` clause(s) specify one or more exception handlers. When no
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exception occurs in the :keyword:`try` clause, no exception handler is executed.
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When an exception occurs in the :keyword:`try` suite, a search for an exception
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handler is started. This search inspects the except clauses in turn until one
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is found that matches the exception. An expression-less except clause, if
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present, must be last; it matches any exception. For an except clause with an
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expression, that expression is evaluated, and the clause matches the exception
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if the resulting object is "compatible" with the exception. An object is
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compatible with an exception if it is the class or a base class of the exception
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object or a tuple containing an item compatible with the exception.
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If no except clause matches the exception, the search for an exception handler
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continues in the surrounding code and on the invocation stack. [#]_
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If the evaluation of an expression in the header of an except clause raises an
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exception, the original search for a handler is canceled and a search starts for
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the new exception in the surrounding code and on the call stack (it is treated
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as if the entire :keyword:`try` statement raised the exception).
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When a matching except clause is found, the exception is assigned to the target
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specified after the :keyword:`as` keyword in that except clause, if present, and
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the except clause's suite is executed. All except clauses must have an
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executable block. When the end of this block is reached, execution continues
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normally after the entire try statement. (This means that if two nested
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handlers exist for the same exception, and the exception occurs in the try
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clause of the inner handler, the outer handler will not handle the exception.)
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When an exception has been assigned using ``as target``, it is cleared at the
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end of the except clause. This is as if ::
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except E as N:
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foo
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was translated to ::
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except E as N:
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try:
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foo
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finally:
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del N
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This means the exception must be assigned to a different name to be able to
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refer to it after the except clause. Exceptions are cleared because with the
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traceback attached to them, they form a reference cycle with the stack frame,
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keeping all locals in that frame alive until the next garbage collection occurs.
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.. index::
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module: sys
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object: traceback
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Before an except clause's suite is executed, details about the exception are
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stored in the :mod:`sys` module and can be access via :func:`sys.exc_info`.
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:func:`sys.exc_info` returns a 3-tuple consisting of the exception class, the
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exception instance and a traceback object (see section :ref:`types`) identifying
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the point in the program where the exception occurred. :func:`sys.exc_info`
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values are restored to their previous values (before the call) when returning
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from a function that handled an exception.
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.. index::
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keyword: else
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statement: return
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statement: break
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statement: continue
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The optional :keyword:`else` clause is executed if and when control flows off
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the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
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clause are not handled by the preceding :keyword:`except` clauses.
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.. index:: keyword: finally
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If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
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:keyword:`try` clause is executed, including any :keyword:`except` and
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:keyword:`else` clauses. If an exception occurs in any of the clauses and is
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not handled, the exception is temporarily saved. The :keyword:`finally` clause
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is executed. If there is a saved exception, it is re-raised at the end of the
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:keyword:`finally` clause. If the :keyword:`finally` clause raises another
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exception or executes a :keyword:`return` or :keyword:`break` statement, the
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saved exception is lost. The exception information is not available to the
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program during execution of the :keyword:`finally` clause.
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.. index::
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statement: return
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statement: break
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statement: continue
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When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
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executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
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statement, the :keyword:`finally` clause is also executed 'on the way out.' A
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:keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
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reason is a problem with the current implementation --- this restriction may be
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lifted in the future).
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Additional information on exceptions can be found in section :ref:`exceptions`,
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and information on using the :keyword:`raise` statement to generate exceptions
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may be found in section :ref:`raise`.
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.. _with:
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.. _as:
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The :keyword:`with` statement
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=============================
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.. index:: statement: with
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The :keyword:`with` statement is used to wrap the execution of a block with
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methods defined by a context manager (see section :ref:`context-managers`).
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This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally`
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usage patterns to be encapsulated for convenient reuse.
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.. productionlist::
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with_stmt: "with" with_item ("," with_item)* ":" `suite`
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with_item: `expression` ["as" `target`]
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The execution of the :keyword:`with` statement with one "item" proceeds as follows:
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#. The context expression (the expression given in the :token:`with_item`) is
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evaluated to obtain a context manager.
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#. The context manager's :meth:`__exit__` is loaded for later use.
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#. The context manager's :meth:`__enter__` method is invoked.
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#. If a target was included in the :keyword:`with` statement, the return value
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from :meth:`__enter__` is assigned to it.
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.. note::
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The :keyword:`with` statement guarantees that if the :meth:`__enter__`
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method returns without an error, then :meth:`__exit__` will always be
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called. Thus, if an error occurs during the assignment to the target list,
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it will be treated the same as an error occurring within the suite would
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be. See step 6 below.
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#. The suite is executed.
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#. The context manager's :meth:`__exit__` method is invoked. If an exception
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caused the suite to be exited, its type, value, and traceback are passed as
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arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
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supplied.
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If the suite was exited due to an exception, and the return value from the
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:meth:`__exit__` method was false, the exception is reraised. If the return
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value was true, the exception is suppressed, and execution continues with the
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statement following the :keyword:`with` statement.
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If the suite was exited for any reason other than an exception, the return
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value from :meth:`__exit__` is ignored, and execution proceeds at the normal
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location for the kind of exit that was taken.
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With more than one item, the context managers are processed as if multiple
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:keyword:`with` statements were nested::
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with A() as a, B() as b:
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suite
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is equivalent to ::
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with A() as a:
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with B() as b:
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suite
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.. versionchanged:: 3.1
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Support for multiple context expressions.
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.. seealso::
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:pep:`0343` - The "with" statement
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The specification, background, and examples for the Python :keyword:`with`
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statement.
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.. _function:
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.. _def:
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Function definitions
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====================
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.. index::
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statement: def
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pair: function; definition
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pair: function; name
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pair: name; binding
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object: user-defined function
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object: function
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pair: function; name
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pair: name; binding
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A function definition defines a user-defined function object (see section
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:ref:`types`):
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.. productionlist::
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funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`] ":" `suite`
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decorators: `decorator`+
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decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE
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dotted_name: `identifier` ("." `identifier`)*
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parameter_list: (`defparameter` ",")*
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: ( "*" [`parameter`] ("," `defparameter`)*
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: [, "**" `parameter`]
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: | "**" `parameter`
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: | `defparameter` [","] )
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parameter: `identifier` [":" `expression`]
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defparameter: `parameter` ["=" `expression`]
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funcname: `identifier`
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A function definition is an executable statement. Its execution binds the
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function name in the current local namespace to a function object (a wrapper
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around the executable code for the function). This function object contains a
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reference to the current global namespace as the global namespace to be used
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when the function is called.
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The function definition does not execute the function body; this gets executed
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only when the function is called. [#]_
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.. index::
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statement: @
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A function definition may be wrapped by one or more :term:`decorator` expressions.
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Decorator expressions are evaluated when the function is defined, in the scope
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that contains the function definition. The result must be a callable, which is
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invoked with the function object as the only argument. The returned value is
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bound to the function name instead of the function object. Multiple decorators
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are applied in nested fashion. For example, the following code ::
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@f1(arg)
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@f2
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def func(): pass
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is equivalent to ::
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def func(): pass
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func = f1(arg)(f2(func))
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.. index:: triple: default; parameter; value
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When one or more parameters have the form *parameter* ``=`` *expression*, the
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function is said to have "default parameter values." For a parameter with a
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default value, the corresponding argument may be omitted from a call, in which
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case the parameter's default value is substituted. If a parameter has a default
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value, all following parameters up until the "``*``" must also have a default
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value --- this is a syntactic restriction that is not expressed by the grammar.
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**Default parameter values are evaluated when the function definition is
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executed.** This means that the expression is evaluated once, when the function
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is defined, and that that same "pre-computed" value is used for each call. This
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is especially important to understand when a default parameter is a mutable
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object, such as a list or a dictionary: if the function modifies the object
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(e.g. by appending an item to a list), the default value is in effect modified.
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This is generally not what was intended. A way around this is to use ``None``
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as the default, and explicitly test for it in the body of the function, e.g.::
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def whats_on_the_telly(penguin=None):
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if penguin is None:
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penguin = []
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penguin.append("property of the zoo")
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return penguin
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.. index::
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statement: *
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statement: **
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Function call semantics are described in more detail in section :ref:`calls`. A
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function call always assigns values to all parameters mentioned in the parameter
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list, either from position arguments, from keyword arguments, or from default
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values. If the form "``*identifier``" is present, it is initialized to a tuple
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receiving any excess positional parameters, defaulting to the empty tuple. If
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the form "``**identifier``" is present, it is initialized to a new dictionary
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receiving any excess keyword arguments, defaulting to a new empty dictionary.
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Parameters after "``*``" or "``*identifier``" are keyword-only parameters and
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may only be passed used keyword arguments.
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.. index:: pair: function; annotations
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Parameters may have annotations of the form "``: expression``" following the
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parameter name. Any parameter may have an annotation even those of the form
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``*identifier`` or ``**identifier``. Functions may have "return" annotation of
|
|
the form "``-> expression``" after the parameter list. These annotations can be
|
|
any valid Python expression and are evaluated when the function definition is
|
|
executed. Annotations may be evaluated in a different order than they appear in
|
|
the source code. The presence of annotations does not change the semantics of a
|
|
function. The annotation values are available as values of a dictionary keyed
|
|
by the parameters' names in the :attr:`__annotations__` attribute of the
|
|
function object.
|
|
|
|
.. index:: pair: lambda; form
|
|
|
|
It is also possible to create anonymous functions (functions not bound to a
|
|
name), for immediate use in expressions. This uses lambda forms, described in
|
|
section :ref:`lambda`. Note that the lambda form is merely a shorthand for a
|
|
simplified function definition; a function defined in a ":keyword:`def`"
|
|
statement can be passed around or assigned to another name just like a function
|
|
defined by a lambda form. The ":keyword:`def`" form is actually more powerful
|
|
since it allows the execution of multiple statements and annotations.
|
|
|
|
**Programmer's note:** Functions are first-class objects. A "``def``" form
|
|
executed inside a function definition defines a local function that can be
|
|
returned or passed around. Free variables used in the nested function can
|
|
access the local variables of the function containing the def. See section
|
|
:ref:`naming` for details.
|
|
|
|
|
|
.. _class:
|
|
|
|
Class definitions
|
|
=================
|
|
|
|
.. index::
|
|
object: class
|
|
statement: class
|
|
pair: class; definition
|
|
pair: class; name
|
|
pair: name; binding
|
|
pair: execution; frame
|
|
single: inheritance
|
|
single: docstring
|
|
|
|
A class definition defines a class object (see section :ref:`types`):
|
|
|
|
.. productionlist::
|
|
classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite`
|
|
inheritance: "(" [`argument_list` [","] | `comprehension`] ")"
|
|
classname: `identifier`
|
|
|
|
A class definition is an executable statement. The inheritance list usually
|
|
gives a list of base classes (see :ref:`metaclasses` for more advanced uses), so
|
|
each item in the list should evaluate to a class object which allows
|
|
subclassing. Classes without an inheritance list inherit, by default, from the
|
|
base class :class:`object`; hence, ::
|
|
|
|
class Foo:
|
|
pass
|
|
|
|
is equivalent to ::
|
|
|
|
class Foo(object):
|
|
pass
|
|
|
|
The class's suite is then executed in a new execution frame (see :ref:`naming`),
|
|
using a newly created local namespace and the original global namespace.
|
|
(Usually, the suite contains mostly function definitions.) When the class's
|
|
suite finishes execution, its execution frame is discarded but its local
|
|
namespace is saved. [#]_ A class object is then created using the inheritance
|
|
list for the base classes and the saved local namespace for the attribute
|
|
dictionary. The class name is bound to this class object in the original local
|
|
namespace.
|
|
|
|
Class creation can be customized heavily using :ref:`metaclasses <metaclasses>`.
|
|
|
|
Classes can also be decorated: just like when decorating functions, ::
|
|
|
|
@f1(arg)
|
|
@f2
|
|
class Foo: pass
|
|
|
|
is equivalent to ::
|
|
|
|
class Foo: pass
|
|
Foo = f1(arg)(f2(Foo))
|
|
|
|
The evaluation rules for the decorator expressions are the same as for function
|
|
decorators. The result must be a class object, which is then bound to the class
|
|
name.
|
|
|
|
**Programmer's note:** Variables defined in the class definition are class
|
|
attributes; they are shared by instances. Instance attributes can be set in a
|
|
method with ``self.name = value``. Both class and instance attributes are
|
|
accessible through the notation "``self.name``", and an instance attribute hides
|
|
a class attribute with the same name when accessed in this way. Class
|
|
attributes can be used as defaults for instance attributes, but using mutable
|
|
values there can lead to unexpected results. :ref:`Descriptors <descriptors>`
|
|
can be used to create instance variables with different implementation details.
|
|
|
|
|
|
.. seealso::
|
|
|
|
:pep:`3115` - Metaclasses in Python 3
|
|
:pep:`3129` - Class Decorators
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] The exception is propagated to the invocation stack only if there is no
|
|
:keyword:`finally` clause that negates the exception.
|
|
|
|
.. [#] Currently, control "flows off the end" except in the case of an exception
|
|
or the execution of a :keyword:`return`, :keyword:`continue`, or
|
|
:keyword:`break` statement.
|
|
|
|
.. [#] A string literal appearing as the first statement in the function body is
|
|
transformed into the function's ``__doc__`` attribute and therefore the
|
|
function's :term:`docstring`.
|
|
|
|
.. [#] A string literal appearing as the first statement in the class body is
|
|
transformed into the namespace's ``__doc__`` item and therefore the class's
|
|
:term:`docstring`.
|