1006 lines
39 KiB
ReStructuredText
1006 lines
39 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: The object must be an
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iterable with the same number of items as there are targets in the target list,
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and the items are assigned, from left to right, to the corresponding 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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.. index:: single: destructor
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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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* If the target is a target list enclosed in parentheses or in square brackets:
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The object must be an iterable with the same number of items as there are
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targets in the target list, and its items are assigned, from left to right,
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to the 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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.. _attr-target-note:
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Note: If the object is a class instance and the attribute reference occurs on
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both sides of the assignment operator, the RHS expression, ``a.x`` can access
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either an instance attribute or (if no instance attribute exists) a class
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attribute. The LHS target ``a.x`` is always set as an instance attribute,
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creating it if necessary. Thus, the two occurrences of ``a.x`` do not
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necessarily refer to the same attribute: if the RHS expression refers to a
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class attribute, the LHS creates a new instance attribute as the target of the
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assignment::
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class Cls:
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x = 3 # class variable
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inst = Cls()
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inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3
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This description does not necessarily apply to descriptor attributes, such as
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properties created with :func:`property`.
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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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.. impl-detail::
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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: `augtarget` `augop` (`expression_list` | `yield_expression`)
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augtarget: `identifier` | `attributeref` | `subscription` | `slicing`
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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 same :ref:`caveat about class
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and instance attributes <attr-target-note>` applies as for regular assignments.
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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 than 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: 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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.. versionchanged:: 3.2
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Previously it was illegal to delete a name from the local namespace if it
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occurs as a free variable in a nested block.
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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 indicates that the
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generator is done and will cause :exc:`StopIteration` to be raised. The returned
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value (if any) is used as an argument to construct :exc:`StopIteration` and
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becomes the :attr:`StopIteration.value` attribute.
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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 :func:`next` function on the generator repeatedly until
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it 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 :func:`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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When ``yield from <expr>`` is used, it treats the supplied expression as
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a subiterator, producing values from it until the underlying iterator is
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exhausted.
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.. versionchanged:: 3.3
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Added ``yield from <expr>`` to delegate control flow to a subiterator
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For full details of :keyword:`yield` semantics, refer to the :ref:`yieldexpr`
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section.
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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 to enhance the API and syntax of generators, making them
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usable as simple coroutines.
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:pep:`0380` - Syntax for Delegating to a Subgenerator
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The proposal to introduce the :token:`yield_from` syntax, making delegation
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to sub-generators easy.
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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:`RuntimeError` exception is raised indicating that this is an
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error.
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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 Exception("foo occurred").with_traceback(tracebackobj)
|
|
|
|
.. index:: pair: exception; chaining
|
|
__cause__ (exception attribute)
|
|
__context__ (exception attribute)
|
|
|
|
The ``from`` clause is used for exception chaining: if given, the second
|
|
*expression* must be another exception class or instance, which will then be
|
|
attached to the raised exception as the :attr:`__cause__` attribute (which is
|
|
writable). If the raised exception is not handled, both exceptions will be
|
|
printed::
|
|
|
|
>>> try:
|
|
... print(1 / 0)
|
|
... except Exception as exc:
|
|
... raise RuntimeError("Something bad happened") from exc
|
|
...
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 2, in <module>
|
|
ZeroDivisionError: int division or modulo by zero
|
|
|
|
The above exception was the direct cause of the following exception:
|
|
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 4, in <module>
|
|
RuntimeError: Something bad happened
|
|
|
|
A similar mechanism works implicitly if an exception is raised inside an
|
|
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 statement comes in two
|
|
forms differing on whether it uses the :keyword:`from` keyword. 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. For a reference implementation of step (1), see the
|
|
:mod:`importlib` module.
|
|
|
|
.. index::
|
|
single: package
|
|
|
|
To understand how step (1) occurs, one must first understand how Python handles
|
|
hierarchical naming of modules. To help organize modules and provide a
|
|
hierarchy in naming, Python has a concept of packages. A package can contain
|
|
other packages and modules while modules cannot contain other modules or
|
|
packages. From a file system perspective, packages are directories and modules
|
|
are files. The original `specification for packages
|
|
<http://www.python.org/doc/essays/packages.html>`_ is still available to read,
|
|
although minor details have changed since the writing of that document.
|
|
|
|
.. index::
|
|
single: sys.modules
|
|
|
|
Once the name of the module is known (unless otherwise specified, the term
|
|
"module" will refer to both packages and modules), searching
|
|
for the module or package can begin. The first place checked is
|
|
:data:`sys.modules`, the cache of all modules that have been imported
|
|
previously. If the module is found there then it is used in step (2) of import
|
|
unless ``None`` is found in :data:`sys.modules`, in which case
|
|
:exc:`ImportError` is raised.
|
|
|
|
.. index::
|
|
single: sys.meta_path
|
|
single: finder
|
|
pair: finder; find_module
|
|
single: __path__
|
|
|
|
If the module is not found in the cache, then :data:`sys.meta_path` is searched
|
|
(the specification for :data:`sys.meta_path` can be found in :pep:`302`).
|
|
The object is a list of :term:`finder` objects which are queried in order as to
|
|
whether they know how to load the module by calling their :meth:`find_module`
|
|
method with the name of the module. If the module happens to be contained
|
|
within a package (as denoted by the existence of a dot in the name), then a
|
|
second argument to :meth:`find_module` is given as the value of the
|
|
:attr:`__path__` attribute from the parent package (everything up to the last
|
|
dot in the name of the module being imported). If a finder can find the module
|
|
it returns a :term:`loader` (discussed later) or returns ``None``.
|
|
|
|
.. index::
|
|
single: sys.path_hooks
|
|
single: sys.path_importer_cache
|
|
single: sys.path
|
|
|
|
If none of the finders on :data:`sys.meta_path` are able to find the module
|
|
then some implicitly defined finders are queried. Implementations of Python
|
|
vary in what implicit meta path finders are defined. The one they all do
|
|
define, though, is one that handles :data:`sys.path_hooks`,
|
|
:data:`sys.path_importer_cache`, and :data:`sys.path`.
|
|
|
|
The implicit finder searches for the requested module in the "paths" specified
|
|
in one of two places ("paths" do not have to be file system paths). If the
|
|
module being imported is supposed to be contained within a package then the
|
|
second argument passed to :meth:`find_module`, :attr:`__path__` on the parent
|
|
package, is used as the source of paths. If the module is not contained in a
|
|
package then :data:`sys.path` is used as the source of paths.
|
|
|
|
Once the source of paths is chosen it is iterated over to find a finder that
|
|
can handle that path. The dict at :data:`sys.path_importer_cache` caches
|
|
finders for paths and is checked for a finder. If the path does not have a
|
|
finder cached then :data:`sys.path_hooks` is searched by calling each object in
|
|
the list with a single argument of the path, returning a finder or raises
|
|
:exc:`ImportError`. If a finder is returned then it is cached in
|
|
:data:`sys.path_importer_cache` and then used for that path entry. If no finder
|
|
can be found but the path exists then a value of ``None`` is
|
|
stored in :data:`sys.path_importer_cache` to signify that an implicit,
|
|
file-based finder that handles modules stored as individual files should be
|
|
used for that path. If the path does not exist then a finder which always
|
|
returns ``None`` is placed in the cache for the path.
|
|
|
|
.. index::
|
|
single: loader
|
|
pair: loader; load_module
|
|
exception: ImportError
|
|
|
|
If no finder can find the module then :exc:`ImportError` is raised. Otherwise
|
|
some finder returned a loader whose :meth:`load_module` method is called with
|
|
the name of the module to load (see :pep:`302` for the original definition of
|
|
loaders). A loader has several responsibilities to perform on a module it
|
|
loads. First, if the module already exists in :data:`sys.modules` (a
|
|
possibility if the loader is called outside of the import machinery) then it
|
|
is to use that module for initialization and not a new module. But if the
|
|
module does not exist in :data:`sys.modules` then it is to be added to that
|
|
dict before initialization begins. If an error occurs during loading of the
|
|
module and it was added to :data:`sys.modules` it is to be removed from the
|
|
dict. If an error occurs but the module was already in :data:`sys.modules` it
|
|
is left in the dict.
|
|
|
|
.. index::
|
|
single: __name__
|
|
single: __file__
|
|
single: __path__
|
|
single: __package__
|
|
single: __loader__
|
|
|
|
The loader must set several attributes on the module. :data:`__name__` is to be
|
|
set to the name of the module. :data:`__file__` is to be the "path" to the file
|
|
unless the module is built-in (and thus listed in
|
|
:data:`sys.builtin_module_names`) in which case the attribute is not set.
|
|
If what is being imported is a package then :data:`__path__` is to be set to a
|
|
list of paths to be searched when looking for modules and packages contained
|
|
within the package being imported. :data:`__package__` is optional but should
|
|
be set to the name of package that contains the module or package (the empty
|
|
string is used for module not contained in a package). :data:`__loader__` is
|
|
also optional but should be set to the loader object that is loading the
|
|
module.
|
|
|
|
.. index::
|
|
exception: ImportError
|
|
|
|
If an error occurs during loading then the loader raises :exc:`ImportError` if
|
|
some other exception is not already being propagated. Otherwise the loader
|
|
returns the module that was loaded and initialized.
|
|
|
|
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. The wild
|
|
card form of import --- ``import *`` --- is only allowed at the module level.
|
|
Attempting to use it in class or function definitions will raise a
|
|
:exc:`SyntaxError`.
|
|
|
|
.. index::
|
|
single: relative; import
|
|
|
|
When specifying what module to import you do not have to specify the absolute
|
|
name of the module. When a module or package is contained within another
|
|
package it is possible to make a relative import within the same top package
|
|
without having to mention the package name. By using leading dots in the
|
|
specified module or package after :keyword:`from` you can specify how high to
|
|
traverse up the current package hierarchy without specifying exact names. One
|
|
leading dot means the current package where the module making the import
|
|
exists. Two dots means up one package level. Three dots is up two levels, etc.
|
|
So if you execute ``from . import mod`` from a module in the ``pkg`` package
|
|
then you will end up importing ``pkg.mod``. If you execute ``from ..subpkg2
|
|
import mod`` from within ``pkg.subpkg1`` you will import ``pkg.subpkg2.mod``.
|
|
The specification for relative imports is contained within :pep:`328`.
|
|
|
|
:func:`importlib.import_module` is provided to support applications that
|
|
determine which modules need to be loaded dynamically.
|
|
|
|
|
|
.. _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 built-in 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.
|
|
|
|
.. seealso::
|
|
|
|
:pep:`236` - Back to the __future__
|
|
The original proposal for the __future__ mechanism.
|
|
|
|
|
|
.. _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.
|
|
|
|
.. impl-detail::
|
|
|
|
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 built-in :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.
|