906 lines
34 KiB
ReStructuredText
906 lines
34 KiB
ReStructuredText
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.. _simple:
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*****************
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Simple statements
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*****************
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.. index:: pair: simple; statement
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Simple statements are comprised within a single logical line. Several simple
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statements may occur on a single line separated by semicolons. The syntax for
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simple statements is:
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.. productionlist::
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simple_stmt: `expression_stmt`
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: | `assert_stmt`
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: | `assignment_stmt`
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: | `augmented_assignment_stmt`
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: | `pass_stmt`
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: | `del_stmt`
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: | `return_stmt`
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: | `yield_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`
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: | `nonlocal_stmt`
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.. _exprstmts:
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Expression statements
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=====================
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.. index::
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pair: expression; statement
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pair: expression; list
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.. index:: pair: expression; list
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Expression statements are used (mostly interactively) to compute and write a
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value, or (usually) to call a procedure (a function that returns no meaningful
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result; in Python, procedures return the value ``None``). Other uses of
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expression statements are allowed and occasionally useful. The syntax for an
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expression statement is:
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.. productionlist::
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expression_stmt: `expression_list`
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An expression statement evaluates the expression list (which may be a single
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expression).
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.. index::
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builtin: repr
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object: None
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pair: string; conversion
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single: output
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pair: standard; output
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pair: writing; values
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pair: procedure; call
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In interactive mode, if the value is not ``None``, it is converted to a string
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using the built-in :func:`repr` function and the resulting string is written to
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standard output on a line by itself (except if the result is ``None``, so that
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procedure calls do not cause any output.)
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.. _assignment:
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Assignment statements
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=====================
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.. index::
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pair: assignment; statement
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pair: binding; name
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pair: rebinding; name
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object: mutable
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pair: attribute; assignment
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Assignment statements are used to (re)bind names to values and to modify
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attributes or items of mutable objects:
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.. productionlist::
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assignment_stmt: (`target_list` "=")+ (`expression_list` | `yield_expression`)
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target_list: `target` ("," `target`)* [","]
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target: `identifier`
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: | "(" `target_list` ")"
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: | "[" `target_list` "]"
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: | `attributeref`
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: | `subscription`
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: | `slicing`
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: | "*" `target`
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(See section :ref:`primaries` for the syntax definitions for the last three
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symbols.)
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An assignment statement evaluates the expression list (remember that this can be
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a single expression or a comma-separated list, the latter yielding a tuple) and
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assigns the single resulting object to each of the target lists, from left to
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right.
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.. index::
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single: target
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pair: target; list
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Assignment is defined recursively depending on the form of the target (list).
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When a target is part of a mutable object (an attribute reference, subscription
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or slicing), the mutable object must ultimately perform the assignment and
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decide about its validity, and may raise an exception if the assignment is
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unacceptable. The rules observed by various types and the exceptions raised are
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given with the definition of the object types (see section :ref:`types`).
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.. index:: triple: target; list; assignment
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Assignment of an object to a target list, optionally enclosed in parentheses or
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square brackets, is recursively defined as follows.
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* If the target list is a single target: The object is assigned to that target.
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* If the target list is a comma-separated list of targets:
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* If the target list contains one target prefixed with an asterisk, called a
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"starred" target: The object must be a sequence with at least as many items
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as there are targets in the target list, minus one. The first items of the
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sequence are assigned, from left to right, to the targets before the starred
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target. The final items of the sequence are assigned to the targets after
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the starred target. A list of the remaining items in the sequence is then
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assigned to the starred target (the list can be empty).
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* Else: The object must be a sequence with the same number of items as there
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are targets in the target list, and the items are assigned, from left to
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right, to the corresponding targets.
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Assignment of an object to a single target is recursively defined as follows.
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* If the target is an identifier (name):
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* If the name does not occur in a :keyword:`global` or :keyword:`nonlocal`
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statement in the current code block: the name is bound to the object in the
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current local namespace.
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* Otherwise: the name is bound to the object in the global namespace or the
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outer namespace determined by :keyword:`nonlocal`, respectively.
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The name is rebound if it was already bound. This may cause the reference
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count for the object previously bound to the name to reach zero, causing the
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object to be deallocated and its destructor (if it has one) to be called.
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.. index:: single: destructor
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The name is rebound if it was already bound. This may cause the reference count
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for the object previously bound to the name to reach zero, causing the object to
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be deallocated and its destructor (if it has one) to be called.
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* If the target is a target list enclosed in parentheses or in square brackets:
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The object must be a sequence with the same number of items as there are targets
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in the target list, and its items are assigned, from left to right, to the
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corresponding targets.
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.. index:: pair: attribute; assignment
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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 attributes;
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if this is not the case, :exc:`TypeError` is raised. That object is then
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asked to assign the assigned object to the given attribute; if it cannot
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perform the assignment, it raises an exception (usually but not necessarily
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:exc:`AttributeError`).
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.. index::
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pair: subscription; assignment
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object: mutable
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* If the target is a subscription: The primary expression in the reference is
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evaluated. It should yield either a mutable sequence object (such as a list)
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or a mapping object (such as a dictionary). Next, the subscript expression is
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evaluated.
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.. index::
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object: sequence
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object: list
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If the primary is a mutable sequence object (such as a list), the subscript
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must yield an integer. If it is negative, the sequence's length is added to
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it. The resulting value must be a nonnegative integer less than the
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sequence's length, and the sequence is asked to assign the assigned object to
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its item with that index. If the index is out of range, :exc:`IndexError` is
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raised (assignment to a subscripted sequence cannot add new items to a list).
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.. index::
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object: mapping
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object: dictionary
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If the primary is a mapping object (such as a dictionary), the subscript must
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have a type compatible with the mapping's key type, and the mapping is then
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asked to create a key/datum pair which maps the subscript to the assigned
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object. This can either replace an existing key/value pair with the same key
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value, or insert a new key/value pair (if no key with the same value existed).
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For user-defined objects, the :meth:`__setitem__` method is called with
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appropriate arguments.
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.. index:: pair: slicing; assignment
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* If the target is a slicing: The primary expression in the reference is
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evaluated. It should yield a mutable sequence object (such as a list). The
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assigned object should be a sequence object of the same type. Next, the lower
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and upper bound expressions are evaluated, insofar they are present; defaults
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are zero and the sequence's length. The bounds should evaluate to integers.
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If either bound is negative, the sequence's length is added to it. The
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resulting bounds are clipped to lie between zero and the sequence's length,
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inclusive. Finally, the sequence object is asked to replace the slice with
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the items of the assigned sequence. The length of the slice may be different
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from the length of the assigned sequence, thus changing the length of the
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target sequence, if the object allows it.
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(In the current implementation, the syntax for targets is taken to be the same
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as for expressions, and invalid syntax is rejected during the code generation
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phase, causing less detailed error messages.)
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WARNING: Although the definition of assignment implies that overlaps between the
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left-hand side and the right-hand side are 'safe' (for example ``a, b = b, a``
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swaps two variables), overlaps *within* the collection of assigned-to variables
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are not safe! For instance, the following program prints ``[0, 2]``::
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x = [0, 1]
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i = 0
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i, x[i] = 1, 2
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print(x)
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.. seealso::
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:pep:`3132` - Extended Iterable Unpacking
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The specification for the ``*target`` feature.
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.. _augassign:
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Augmented assignment statements
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-------------------------------
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.. index::
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pair: augmented; assignment
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single: statement; assignment, augmented
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Augmented assignment is the combination, in a single statement, of a binary
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operation and an assignment statement:
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.. productionlist::
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augmented_assignment_stmt: `target` `augop` (`expression_list` | `yield_expression`)
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augop: "+=" | "-=" | "*=" | "/=" | "//=" | "%=" | "**="
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: | ">>=" | "<<=" | "&=" | "^=" | "|="
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(See section :ref:`primaries` for the syntax definitions for the last three
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symbols.)
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An augmented assignment evaluates the target (which, unlike normal assignment
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statements, cannot be an unpacking) and the expression list, performs the binary
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operation specific to the type of assignment on the two operands, and assigns
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the result to the original target. The target is only evaluated once.
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An augmented assignment expression like ``x += 1`` can be rewritten as ``x = x +
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1`` to achieve a similar, but not exactly equal effect. In the augmented
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version, ``x`` is only evaluated once. Also, when possible, the actual operation
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is performed *in-place*, meaning that rather than creating a new object and
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assigning that to the target, the old object is modified instead.
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With the exception of assigning to tuples and multiple targets in a single
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statement, the assignment done by augmented assignment statements is handled the
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same way as normal assignments. Similarly, with the exception of the possible
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*in-place* behavior, the binary operation performed by augmented assignment is
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the same as the normal binary operations.
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For targets which are attribute references, the initial value is retrieved with
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a :meth:`getattr` and the result is assigned with a :meth:`setattr`. Notice
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that the two methods do not necessarily refer to the same variable. When
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:meth:`getattr` refers to a class variable, :meth:`setattr` still writes to an
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instance variable. For example::
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class A:
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x = 3 # class variable
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a = A()
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a.x += 1 # writes a.x as 4 leaving A.x as 3
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.. _assert:
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The :keyword:`assert` statement
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===============================
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.. index::
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statement: assert
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pair: debugging; assertions
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Assert statements are a convenient way to insert debugging assertions into a
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program:
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.. productionlist::
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assert_stmt: "assert" `expression` ["," `expression`]
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The simple form, ``assert expression``, is equivalent to ::
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if __debug__:
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if not expression: raise AssertionError
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The extended form, ``assert expression1, expression2``, is equivalent to ::
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if __debug__:
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if not expression1: raise AssertionError(expression2)
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.. index::
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single: __debug__
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exception: AssertionError
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These equivalences assume that :const:`__debug__` and :exc:`AssertionError` refer to
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the built-in variables with those names. In the current implementation, the
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built-in variable :const:`__debug__` is ``True`` under normal circumstances,
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``False`` when optimization is requested (command line option -O). The current
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code generator emits no code for an assert statement when optimization is
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requested at compile time. Note that it is unnecessary to include the source
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code for the expression that failed in the error message; it will be displayed
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as part of the stack trace.
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Assignments to :const:`__debug__` are illegal. The value for the built-in variable
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is determined when the interpreter starts.
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.. _pass:
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The :keyword:`pass` statement
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=============================
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.. index::
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statement: pass
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pair: null; operation
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pair: null; operation
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.. productionlist::
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pass_stmt: "pass"
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:keyword:`pass` is a null operation --- when it is executed, nothing happens.
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It is useful as a placeholder when a statement is required syntactically, but no
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code needs to be executed, for example::
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def f(arg): pass # a function that does nothing (yet)
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class C: pass # a class with no methods (yet)
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.. _del:
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The :keyword:`del` statement
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============================
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.. index::
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statement: del
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pair: deletion; target
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triple: deletion; target; list
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.. productionlist::
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del_stmt: "del" `target_list`
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Deletion is recursively defined very similar to the way assignment is defined.
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Rather that spelling it out in full details, here are some hints.
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Deletion of a target list recursively deletes each target, from left to right.
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.. index::
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statement: global
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pair: unbinding; name
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Deletion of a name removes the binding of that name from the local or global
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namespace, depending on whether the name occurs in a :keyword:`global` statement
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in the same code block. If the name is unbound, a :exc:`NameError` exception
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will be raised.
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.. index:: pair: free; variable
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It is illegal to delete a name from the local namespace if it occurs as a free
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variable in a nested block.
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.. index:: pair: attribute; deletion
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Deletion of attribute references, subscriptions and slicings is passed to the
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primary object involved; deletion of a slicing is in general equivalent to
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assignment of an empty slice of the right type (but even this is determined by
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the sliced object).
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.. _return:
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The :keyword:`return` statement
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===============================
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.. index::
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statement: return
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pair: function; definition
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pair: class; definition
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.. productionlist::
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return_stmt: "return" [`expression_list`]
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:keyword:`return` may only occur syntactically nested in a function definition,
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not within a nested class definition.
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If an expression list is present, it is evaluated, else ``None`` is substituted.
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:keyword:`return` leaves the current function call with the expression list (or
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``None``) as return value.
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.. index:: keyword: finally
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When :keyword:`return` passes control out of a :keyword:`try` statement with a
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:keyword:`finally` clause, that :keyword:`finally` clause is executed before
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really leaving the function.
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In a generator function, the :keyword:`return` statement is not allowed to
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include an :token:`expression_list`. In that context, a bare :keyword:`return`
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indicates that the generator is done and will cause :exc:`StopIteration` to be
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raised.
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.. _yield:
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The :keyword:`yield` statement
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==============================
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.. index::
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statement: yield
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single: generator; function
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single: generator; iterator
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single: function; generator
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exception: StopIteration
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.. productionlist::
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yield_stmt: `yield_expression`
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The :keyword:`yield` statement is only used when defining a generator function,
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and is only used in the body of the generator function. Using a :keyword:`yield`
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statement in a function definition is sufficient to cause that definition to
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create a generator function instead of a normal function.
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When a generator function is called, it returns an iterator known as a generator
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iterator, or more commonly, a generator. The body of the generator function is
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executed by calling the generator's :meth:`next` method repeatedly until it
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raises an exception.
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When a :keyword:`yield` statement is executed, the state of the generator is
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frozen and the value of :token:`expression_list` is returned to :meth:`next`'s
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caller. By "frozen" we mean that all local state is retained, including the
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current bindings of local variables, the instruction pointer, and the internal
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evaluation stack: enough information is saved so that the next time :meth:`next`
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is invoked, the function can proceed exactly as if the :keyword:`yield`
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statement were just another external call.
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The :keyword:`yield` statement is allowed in the :keyword:`try` clause of a
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:keyword:`try` ... :keyword:`finally` construct. If the generator is not
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resumed before it is finalized (by reaching a zero reference count or by being
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garbage collected), the generator-iterator's :meth:`close` method will be
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called, allowing any pending :keyword:`finally` clauses to execute.
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.. seealso::
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:pep:`0255` - Simple Generators
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The proposal for adding generators and the :keyword:`yield` statement to Python.
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:pep:`0342` - Coroutines via Enhanced Generators
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The proposal that, among other generator enhancements, proposed allowing
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:keyword:`yield` to appear inside a :keyword:`try` ... :keyword:`finally` block.
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.. _raise:
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The :keyword:`raise` statement
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==============================
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.. index::
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statement: raise
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single: exception
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pair: raising; exception
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single: __traceback__ (exception attribute)
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.. productionlist::
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raise_stmt: "raise" [`expression` ["from" `expression`]]
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If no expressions are present, :keyword:`raise` re-raises the last exception
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that was active in the current scope. If no exception is active in the current
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scope, a :exc:`TypeError` exception is raised indicating that this is an error
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(if running under IDLE, a :exc:`queue.Empty` exception is raised instead).
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Otherwise, :keyword:`raise` evaluates the first expression as the exception
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object. It must be either a subclass or an instance of :class:`BaseException`.
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If it is a class, the exception instance will be obtained when needed by
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instantiating the class with no arguments.
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The :dfn:`type` of the exception is the exception instance's class, the
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:dfn:`value` is the instance itself.
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.. index:: object: traceback
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A traceback object is normally created automatically when an exception is raised
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and attached to it as the :attr:`__traceback__` attribute, which is writable.
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You can create an exception and set your own traceback in one step using the
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:meth:`with_traceback` exception method (which returns the same exception
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instance, with its traceback set to its argument), like so::
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raise RuntimeError("foo occurred").with_traceback(tracebackobj)
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.. index:: pair: exception; chaining
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__cause__ (exception attribute)
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__context__ (exception attribute)
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The ``from`` clause is used for exception chaining: if given, the second
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*expression* must be another exception class or instance, which will then be
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attached to the raised exception as the :attr:`__cause__` attribute (which is
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writable). If the raised exception is not handled, both exceptions will be
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printed::
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>>> try:
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... print(1 / 0)
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... except Exception as exc:
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... raise RuntimeError("Something bad happened") from exc
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...
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Traceback (most recent call last):
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File "<stdin>", line 2, in <module>
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ZeroDivisionError: int division or modulo by zero
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The above exception was the direct cause of the following exception:
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Traceback (most recent call last):
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File "<stdin>", line 4, in <module>
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RuntimeError: Something bad happened
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A similar mechanism works implicitly if an exception is raised inside an
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exception handler: the previous exception is then attached as the new
|
|
exception's :attr:`__context__` attribute::
|
|
|
|
>>> try:
|
|
... print(1 / 0)
|
|
... except:
|
|
... raise RuntimeError("Something bad happened")
|
|
...
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 2, in <module>
|
|
ZeroDivisionError: int division or modulo by zero
|
|
|
|
During handling of the above exception, another exception occurred:
|
|
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 4, in <module>
|
|
RuntimeError: Something bad happened
|
|
|
|
Additional information on exceptions can be found in section :ref:`exceptions`,
|
|
and information about handling exceptions is in section :ref:`try`.
|
|
|
|
|
|
.. _break:
|
|
|
|
The :keyword:`break` statement
|
|
==============================
|
|
|
|
.. index::
|
|
statement: break
|
|
statement: for
|
|
statement: while
|
|
pair: loop; statement
|
|
|
|
.. productionlist::
|
|
break_stmt: "break"
|
|
|
|
:keyword:`break` may only occur syntactically nested in a :keyword:`for` or
|
|
:keyword:`while` loop, but not nested in a function or class definition within
|
|
that loop.
|
|
|
|
.. index:: keyword: else
|
|
pair: loop control; target
|
|
|
|
It terminates the nearest enclosing loop, skipping the optional :keyword:`else`
|
|
clause if the loop has one.
|
|
|
|
If a :keyword:`for` loop is terminated by :keyword:`break`, the loop control
|
|
target keeps its current value.
|
|
|
|
.. index:: keyword: finally
|
|
|
|
When :keyword:`break` passes control out of a :keyword:`try` statement with a
|
|
:keyword:`finally` clause, that :keyword:`finally` clause is executed before
|
|
really leaving the loop.
|
|
|
|
|
|
.. _continue:
|
|
|
|
The :keyword:`continue` statement
|
|
=================================
|
|
|
|
.. index::
|
|
statement: continue
|
|
statement: for
|
|
statement: while
|
|
pair: loop; statement
|
|
keyword: finally
|
|
|
|
.. productionlist::
|
|
continue_stmt: "continue"
|
|
|
|
:keyword:`continue` may only occur syntactically nested in a :keyword:`for` or
|
|
:keyword:`while` loop, but not nested in a function or class definition or
|
|
:keyword:`finally` clause within that loop. It continues with the next
|
|
cycle of the nearest enclosing loop.
|
|
|
|
When :keyword:`continue` passes control out of a :keyword:`try` statement with a
|
|
:keyword:`finally` clause, that :keyword:`finally` clause is executed before
|
|
really starting the next loop cycle.
|
|
|
|
|
|
.. _import:
|
|
.. _from:
|
|
|
|
The :keyword:`import` statement
|
|
===============================
|
|
|
|
.. index::
|
|
statement: import
|
|
single: module; importing
|
|
pair: name; binding
|
|
keyword: from
|
|
|
|
.. productionlist::
|
|
import_stmt: "import" `module` ["as" `name`] ( "," `module` ["as" `name`] )*
|
|
: | "from" `relative_module` "import" `identifier` ["as" `name`]
|
|
: ( "," `identifier` ["as" `name`] )*
|
|
: | "from" `relative_module` "import" "(" `identifier` ["as" `name`]
|
|
: ( "," `identifier` ["as" `name`] )* [","] ")"
|
|
: | "from" `module` "import" "*"
|
|
module: (`identifier` ".")* `identifier`
|
|
relative_module: "."* `module` | "."+
|
|
name: `identifier`
|
|
|
|
Import statements are executed in two steps: (1) find a module, and initialize
|
|
it if necessary; (2) define a name or names in the local namespace (of the scope
|
|
where the :keyword:`import` statement occurs). The first form (without
|
|
:keyword:`from`) repeats these steps for each identifier in the list. The form
|
|
with :keyword:`from` performs step (1) once, and then performs step (2)
|
|
repeatedly.
|
|
|
|
In this context, to "initialize" a built-in or extension module means to call an
|
|
initialization function that the module must provide for the purpose (in the
|
|
reference implementation, the function's name is obtained by prepending string
|
|
"init" to the module's name); to "initialize" a Python-coded module means to
|
|
execute the module's body.
|
|
|
|
.. index::
|
|
single: modules (in module sys)
|
|
single: sys.modules
|
|
pair: module; name
|
|
pair: built-in; module
|
|
pair: user-defined; module
|
|
pair: filename; extension
|
|
triple: module; search; path
|
|
module: sys
|
|
|
|
The system maintains a table of modules that have been or are being initialized,
|
|
indexed by module name. This table is accessible as ``sys.modules``. When a
|
|
module name is found in this table, step (1) is finished. If not, a search for
|
|
a module definition is started. When a module is found, it is loaded. Details
|
|
of the module searching and loading process are implementation and platform
|
|
specific. It generally involves searching for a "built-in" module with the
|
|
given name and then searching a list of locations given as ``sys.path``.
|
|
|
|
.. index::
|
|
pair: module; initialization
|
|
exception: ImportError
|
|
single: code block
|
|
exception: SyntaxError
|
|
|
|
If a built-in module is found, its built-in initialization code is executed and
|
|
step (1) is finished. If no matching file is found, :exc:`ImportError` is
|
|
raised. If a file is found, it is parsed, yielding an executable code block. If
|
|
a syntax error occurs, :exc:`SyntaxError` is raised. Otherwise, an empty module
|
|
of the given name is created and inserted in the module table, and then the code
|
|
block is executed in the context of this module. Exceptions during this
|
|
execution terminate step (1).
|
|
|
|
When step (1) finishes without raising an exception, step (2) can begin.
|
|
|
|
The first form of :keyword:`import` statement binds the module name in the local
|
|
namespace to the module object, and then goes on to import the next identifier,
|
|
if any. If the module name is followed by :keyword:`as`, the name following
|
|
:keyword:`as` is used as the local name for the module.
|
|
|
|
.. index::
|
|
pair: name; binding
|
|
exception: ImportError
|
|
|
|
The :keyword:`from` form does not bind the module name: it goes through the list
|
|
of identifiers, looks each one of them up in the module found in step (1), and
|
|
binds the name in the local namespace to the object thus found. As with the
|
|
first form of :keyword:`import`, an alternate local name can be supplied by
|
|
specifying ":keyword:`as` localname". If a name is not found,
|
|
:exc:`ImportError` is raised. If the list of identifiers is replaced by a star
|
|
(``'*'``), all public names defined in the module are bound in the local
|
|
namespace of the :keyword:`import` statement..
|
|
|
|
.. index:: single: __all__ (optional module attribute)
|
|
|
|
The *public names* defined by a module are determined by checking the module's
|
|
namespace for a variable named ``__all__``; if defined, it must be a sequence of
|
|
strings which are names defined or imported by that module. The names given in
|
|
``__all__`` are all considered public and are required to exist. If ``__all__``
|
|
is not defined, the set of public names includes all names found in the module's
|
|
namespace which do not begin with an underscore character (``'_'``).
|
|
``__all__`` should contain the entire public API. It is intended to avoid
|
|
accidentally exporting items that are not part of the API (such as library
|
|
modules which were imported and used within the module).
|
|
|
|
The :keyword:`from` form with ``*`` may only occur in a module scope. If the
|
|
wild card form of import --- ``import *`` --- is used in a function and the
|
|
function contains or is a nested block with free variables, the compiler will
|
|
raise a :exc:`SyntaxError`.
|
|
|
|
.. index::
|
|
keyword: from
|
|
statement: from
|
|
triple: hierarchical; module; names
|
|
single: packages
|
|
single: __init__.py
|
|
|
|
**Hierarchical module names:** when the module names contains one or more dots,
|
|
the module search path is carried out differently. The sequence of identifiers
|
|
up to the last dot is used to find a "package"; the final identifier is then
|
|
searched inside the package. A package is generally a subdirectory of a
|
|
directory on ``sys.path`` that has a file :file:`__init__.py`.
|
|
|
|
..
|
|
[XXX Can't be
|
|
bothered to spell this out right now; see the URL
|
|
http://www.python.org/doc/essays/packages.html for more details, also about how
|
|
the module search works from inside a package.]
|
|
|
|
.. index:: builtin: __import__
|
|
|
|
The built-in function :func:`__import__` is provided to support applications
|
|
that determine which modules need to be loaded dynamically; refer to
|
|
:ref:`built-in-funcs` for additional information.
|
|
|
|
|
|
.. _future:
|
|
|
|
Future statements
|
|
-----------------
|
|
|
|
.. index:: pair: future; statement
|
|
|
|
A :dfn:`future statement` is a directive to the compiler that a particular
|
|
module should be compiled using syntax or semantics that will be available in a
|
|
specified future release of Python. The future statement is intended to ease
|
|
migration to future versions of Python that introduce incompatible changes to
|
|
the language. It allows use of the new features on a per-module basis before
|
|
the release in which the feature becomes standard.
|
|
|
|
.. productionlist:: *
|
|
future_statement: "from" "__future__" "import" feature ["as" name]
|
|
: ("," feature ["as" name])*
|
|
: | "from" "__future__" "import" "(" feature ["as" name]
|
|
: ("," feature ["as" name])* [","] ")"
|
|
feature: identifier
|
|
name: identifier
|
|
|
|
A future statement must appear near the top of the module. The only lines that
|
|
can appear before a future statement are:
|
|
|
|
* the module docstring (if any),
|
|
* comments,
|
|
* blank lines, and
|
|
* other future statements.
|
|
|
|
.. XXX change this if future is cleaned out
|
|
|
|
The features recognized by Python 3.0 are ``absolute_import``, ``division``,
|
|
``generators``, ``unicode_literals``, ``print_function``, ``nested_scopes`` and
|
|
``with_statement``. They are all redundant because they are always enabled, and
|
|
only kept for backwards compatibility.
|
|
|
|
A future statement is recognized and treated specially at compile time: Changes
|
|
to the semantics of core constructs are often implemented by generating
|
|
different code. It may even be the case that a new feature introduces new
|
|
incompatible syntax (such as a new reserved word), in which case the compiler
|
|
may need to parse the module differently. Such decisions cannot be pushed off
|
|
until runtime.
|
|
|
|
For any given release, the compiler knows which feature names have been defined,
|
|
and raises a compile-time error if a future statement contains a feature not
|
|
known to it.
|
|
|
|
The direct runtime semantics are the same as for any import statement: there is
|
|
a standard module :mod:`__future__`, described later, and it will be imported in
|
|
the usual way at the time the future statement is executed.
|
|
|
|
The interesting runtime semantics depend on the specific feature enabled by the
|
|
future statement.
|
|
|
|
Note that there is nothing special about the statement::
|
|
|
|
import __future__ [as name]
|
|
|
|
That is not a future statement; it's an ordinary import statement with no
|
|
special semantics or syntax restrictions.
|
|
|
|
Code compiled by calls to the builtin functions :func:`exec` and :func:`compile`
|
|
that occur in a module :mod:`M` containing a future statement will, by default,
|
|
use the new syntax or semantics associated with the future statement. This can
|
|
be controlled by optional arguments to :func:`compile` --- see the documentation
|
|
of that function for details.
|
|
|
|
A future statement typed at an interactive interpreter prompt will take effect
|
|
for the rest of the interpreter session. If an interpreter is started with the
|
|
:option:`-i` option, is passed a script name to execute, and the script includes
|
|
a future statement, it will be in effect in the interactive session started
|
|
after the script is executed.
|
|
|
|
|
|
.. _global:
|
|
|
|
The :keyword:`global` statement
|
|
===============================
|
|
|
|
.. index::
|
|
statement: global
|
|
triple: global; name; binding
|
|
|
|
.. productionlist::
|
|
global_stmt: "global" `identifier` ("," `identifier`)*
|
|
|
|
The :keyword:`global` statement is a declaration which holds for the entire
|
|
current code block. It means that the listed identifiers are to be interpreted
|
|
as globals. It would be impossible to assign to a global variable without
|
|
:keyword:`global`, although free variables may refer to globals without being
|
|
declared global.
|
|
|
|
Names listed in a :keyword:`global` statement must not be used in the same code
|
|
block textually preceding that :keyword:`global` statement.
|
|
|
|
Names listed in a :keyword:`global` statement must not be defined as formal
|
|
parameters or in a :keyword:`for` loop control target, :keyword:`class`
|
|
definition, function definition, or :keyword:`import` statement.
|
|
|
|
(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.)
|
|
|
|
.. index::
|
|
builtin: exec
|
|
builtin: eval
|
|
builtin: compile
|
|
|
|
**Programmer's note:** the :keyword:`global` is a directive to the parser. It
|
|
applies only to code parsed at the same time as the :keyword:`global` statement.
|
|
In particular, a :keyword:`global` statement contained in a string or code
|
|
object supplied to the builtin :func:`exec` function does not affect the code
|
|
block *containing* the function call, and code contained in such a string is
|
|
unaffected by :keyword:`global` statements in the code containing the function
|
|
call. The same applies to the :func:`eval` and :func:`compile` functions.
|
|
|
|
|
|
.. _nonlocal:
|
|
|
|
The :keyword:`nonlocal` statement
|
|
=================================
|
|
|
|
.. index:: statement: nonlocal
|
|
|
|
.. productionlist::
|
|
nonlocal_stmt: "nonlocal" `identifier` ("," `identifier`)*
|
|
|
|
.. XXX add when implemented
|
|
: ["=" (`target_list` "=")+ `expression_list`]
|
|
: | "nonlocal" `identifier` `augop` `expression_list`
|
|
|
|
The :keyword:`nonlocal` statement causes the listed identifiers to refer to
|
|
previously bound variables in the nearest enclosing scope. This is important
|
|
because the default behavior for binding is to search the local namespace
|
|
first. The statement allows encapsulated code to rebind variables outside of
|
|
the local scope besides the global (module) scope.
|
|
|
|
.. XXX not implemented
|
|
The :keyword:`nonlocal` statement may prepend an assignment or augmented
|
|
assignment, but not an expression.
|
|
|
|
Names listed in a :keyword:`nonlocal` statement, unlike to those listed in a
|
|
:keyword:`global` statement, must refer to pre-existing bindings in an
|
|
enclosing scope (the scope in which a new binding should be created cannot
|
|
be determined unambiguously).
|
|
|
|
Names listed in a :keyword:`nonlocal` statement must not collide with
|
|
pre-existing bindings in the local scope.
|
|
|
|
.. seealso::
|
|
|
|
:pep:`3104` - Access to Names in Outer Scopes
|
|
The specification for the :keyword:`nonlocal` statement.
|
|
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] It may occur within an :keyword:`except` or :keyword:`else` clause. The
|
|
restriction on occurring in the :keyword:`try` clause is implementor's
|
|
laziness and will eventually be lifted.
|