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+
+.. _simple:
+
+*****************
+Simple statements
+*****************
+
+.. index:: pair: simple; statement
+
+Simple statements are comprised within a single logical line. Several simple
+statements may occur on a single line separated by semicolons. The syntax for
+simple statements is:
+
+.. productionlist::
+ simple_stmt: `expression_stmt`
+ : | `assert_stmt`
+ : | `assignment_stmt`
+ : | `augmented_assignment_stmt`
+ : | `pass_stmt`
+ : | `del_stmt`
+ : | `print_stmt`
+ : | `return_stmt`
+ : | `yield_stmt`
+ : | `raise_stmt`
+ : | `break_stmt`
+ : | `continue_stmt`
+ : | `import_stmt`
+ : | `global_stmt`
+ : | `exec_stmt`
+
+
+.. _exprstmts:
+
+Expression statements
+=====================
+
+.. index:: pair: expression; statement
+
+Expression statements are used (mostly interactively) to compute and write a
+value, or (usually) to call a procedure (a function that returns no meaningful
+result; in Python, procedures return the value ``None``). Other uses of
+expression statements are allowed and occasionally useful. The syntax for an
+expression statement is:
+
+.. productionlist::
+ expression_stmt: `expression_list`
+
+.. index:: pair: expression; list
+
+An expression statement evaluates the expression list (which may be a single
+expression).
+
+.. index::
+ builtin: repr
+ object: None
+ pair: string; conversion
+ single: output
+ pair: standard; output
+ pair: writing; values
+ pair: procedure; call
+
+In interactive mode, if the value is not ``None``, it is converted to a string
+using the built-in :func:`repr` function and the resulting string is written to
+standard output (see section :ref:`print`) on a line by itself. (Expression
+statements yielding ``None`` are not written, so that procedure calls do not
+cause any output.)
+
+
+.. _assert:
+
+Assert statements
+=================
+
+.. index::
+ statement: assert
+ pair: debugging; assertions
+
+Assert statements are a convenient way to insert debugging assertions into a
+program:
+
+.. productionlist::
+ assert_stmt: "assert" `expression` ["," `expression`]
+
+The simple form, ``assert expression``, is equivalent to ::
+
+ if __debug__:
+ if not expression: raise AssertionError
+
+The extended form, ``assert expression1, expression2``, is equivalent to ::
+
+ if __debug__:
+ if not expression1: raise AssertionError, expression2
+
+.. index::
+ single: __debug__
+ exception: AssertionError
+
+These equivalences assume that ``__debug__`` and :exc:`AssertionError` refer to
+the built-in variables with those names. In the current implementation, the
+built-in variable ``__debug__`` is ``True`` under normal circumstances,
+``False`` when optimization is requested (command line option -O). The current
+code generator emits no code for an assert statement when optimization is
+requested at compile time. Note that it is unnecessary to include the source
+code for the expression that failed in the error message; it will be displayed
+as part of the stack trace.
+
+Assignments to ``__debug__`` are illegal. The value for the built-in variable
+is determined when the interpreter starts.
+
+
+.. _assignment:
+
+Assignment statements
+=====================
+
+.. index::
+ pair: assignment; statement
+ pair: binding; name
+ pair: rebinding; name
+ object: mutable
+ pair: attribute; assignment
+
+Assignment statements are used to (re)bind names to values and to modify
+attributes or items of mutable objects:
+
+.. productionlist::
+ assignment_stmt: (`target_list` "=")+ (`expression_list` | `yield_expression`)
+ target_list: `target` ("," `target`)* [","]
+ target: `identifier`
+ : | "(" `target_list` ")"
+ : | "[" `target_list` "]"
+ : | `attributeref`
+ : | `subscription`
+ : | `slicing`
+
+(See section :ref:`primaries` for the syntax definitions for the last three
+symbols.)
+
+.. index:: pair: expression; list
+
+An assignment statement evaluates the expression list (remember that this can be
+a single expression or a comma-separated list, the latter yielding a tuple) and
+assigns the single resulting object to each of the target lists, from left to
+right.
+
+.. index::
+ single: target
+ pair: target; list
+
+Assignment is defined recursively depending on the form of the target (list).
+When a target is part of a mutable object (an attribute reference, subscription
+or slicing), the mutable object must ultimately perform the assignment and
+decide about its validity, and may raise an exception if the assignment is
+unacceptable. The rules observed by various types and the exceptions raised are
+given with the definition of the object types (see section :ref:`types`).
+
+.. index:: triple: target; list; assignment
+
+Assignment of an object to a target list is recursively defined as follows.
+
+* If the target list is a single target: The object is assigned to that target.
+
+* If the target list is a comma-separated list of targets: The object must be a
+ sequence with the same number of items as there are targets in the target list,
+ and the items are assigned, from left to right, to the corresponding targets.
+ (This rule is relaxed as of Python 1.5; in earlier versions, the object had to
+ be a tuple. Since strings are sequences, an assignment like ``a, b = "xy"`` is
+ now legal as long as the string has the right length.)
+
+Assignment of an object to a single target is recursively defined as follows.
+
+* If the target is an identifier (name):
+
+ .. index:: statement: global
+
+* If the name does not occur in a :keyword:`global` statement in the current
+ code block: the name is bound to the object in the current local namespace.
+
+* Otherwise: the name is bound to the object in the current global namespace.
+
+ .. index:: single: destructor
+
+ The name is rebound if it was already bound. This may cause the reference count
+ for the object previously bound to the name to reach zero, causing the object to
+ be deallocated and its destructor (if it has one) to be called.
+
+ .. % nested
+
+* If the target is a target list enclosed in parentheses or in square brackets:
+ The object must be a sequence with the same number of items as there are targets
+ in the target list, and its items are assigned, from left to right, to the
+ corresponding targets.
+
+ .. index:: pair: attribute; assignment
+
+* If the target is an attribute reference: The primary expression in the
+ reference is evaluated. It should yield an object with assignable attributes;
+ if this is not the case, :exc:`TypeError` is raised. That object is then asked
+ to assign the assigned object to the given attribute; if it cannot perform the
+ assignment, it raises an exception (usually but not necessarily
+ :exc:`AttributeError`).
+
+ .. index::
+ pair: subscription; assignment
+ object: mutable
+
+* If the target is a subscription: The primary expression in the reference is
+ evaluated. It should yield either a mutable sequence object (such as a list) or
+ a mapping object (such as a dictionary). Next, the subscript expression is
+ evaluated.
+
+ .. index::
+ object: sequence
+ object: list
+
+ If the primary is a mutable sequence object (such as a list), the subscript must
+ yield a plain integer. If it is negative, the sequence's length is added to it.
+ The resulting value must be a nonnegative integer less than the sequence's
+ length, and the sequence is asked to assign the assigned object to its item with
+ that index. If the index is out of range, :exc:`IndexError` is raised
+ (assignment to a subscripted sequence cannot add new items to a list).
+
+ .. index::
+ object: mapping
+ object: dictionary
+
+ If the primary is a mapping object (such as a dictionary), the subscript must
+ have a type compatible with the mapping's key type, and the mapping is then
+ asked to create a key/datum pair which maps the subscript to the assigned
+ object. This can either replace an existing key/value pair with the same key
+ value, or insert a new key/value pair (if no key with the same value existed).
+
+ .. index:: pair: slicing; assignment
+
+* If the target is a slicing: The primary expression in the reference is
+ evaluated. It should yield a mutable sequence object (such as a list). The
+ assigned object should be a sequence object of the same type. Next, the lower
+ and upper bound expressions are evaluated, insofar they are present; defaults
+ are zero and the sequence's length. The bounds should evaluate to (small)
+ integers. If either bound is negative, the sequence's length is added to it.
+ The resulting bounds are clipped to lie between zero and the sequence's length,
+ inclusive. Finally, the sequence object is asked to replace the slice with the
+ items of the assigned sequence. The length of the slice may be different from
+ the length of the assigned sequence, thus changing the length of the target
+ sequence, if the object allows it.
+
+(In the current implementation, the syntax for targets is taken to be the same
+as for expressions, and invalid syntax is rejected during the code generation
+phase, causing less detailed error messages.)
+
+WARNING: Although the definition of assignment implies that overlaps between the
+left-hand side and the right-hand side are 'safe' (for example ``a, b = b, a``
+swaps two variables), overlaps *within* the collection of assigned-to variables
+are not safe! For instance, the following program prints ``[0, 2]``::
+
+ x = [0, 1]
+ i = 0
+ i, x[i] = 1, 2
+ print x
+
+
+.. _augassign:
+
+Augmented assignment statements
+-------------------------------
+
+.. index::
+ pair: augmented; assignment
+ single: statement; assignment, augmented
+
+Augmented assignment is the combination, in a single statement, of a binary
+operation and an assignment statement:
+
+.. productionlist::
+ augmented_assignment_stmt: `target` `augop` (`expression_list` | `yield_expression`)
+ augop: "+=" | "-=" | "*=" | "/=" | "%=" | "**="
+ : | ">>=" | "<<=" | "&=" | "^=" | "|="
+
+(See section :ref:`primaries` for the syntax definitions for the last three
+symbols.)
+
+An augmented assignment evaluates the target (which, unlike normal assignment
+statements, cannot be an unpacking) and the expression list, performs the binary
+operation specific to the type of assignment on the two operands, and assigns
+the result to the original target. The target is only evaluated once.
+
+An augmented assignment expression like ``x += 1`` can be rewritten as ``x = x +
+1`` to achieve a similar, but not exactly equal effect. In the augmented
+version, ``x`` is only evaluated once. Also, when possible, the actual operation
+is performed *in-place*, meaning that rather than creating a new object and
+assigning that to the target, the old object is modified instead.
+
+With the exception of assigning to tuples and multiple targets in a single
+statement, the assignment done by augmented assignment statements is handled the
+same way as normal assignments. Similarly, with the exception of the possible
+*in-place* behavior, the binary operation performed by augmented assignment is
+the same as the normal binary operations.
+
+For targets which are attribute references, the initial value is retrieved with
+a :meth:`getattr` and the result is assigned with a :meth:`setattr`. Notice
+that the two methods do not necessarily refer to the same variable. When
+:meth:`getattr` refers to a class variable, :meth:`setattr` still writes to an
+instance variable. For example::
+
+ class A:
+ x = 3 # class variable
+ a = A()
+ a.x += 1 # writes a.x as 4 leaving A.x as 3
+
+
+.. _pass:
+
+The :keyword:`pass` statement
+=============================
+
+.. index:: statement: pass
+
+.. productionlist::
+ pass_stmt: "pass"
+
+.. index:: pair: null; operation
+
+:keyword:`pass` is a null operation --- when it is executed, nothing happens.
+It is useful as a placeholder when a statement is required syntactically, but no
+code needs to be executed, for example::
+
+ def f(arg): pass # a function that does nothing (yet)
+
+ class C: pass # a class with no methods (yet)
+
+
+.. _del:
+
+The :keyword:`del` statement
+============================
+
+.. index:: statement: del
+
+.. productionlist::
+ del_stmt: "del" `target_list`
+
+.. index::
+ pair: deletion; target
+ triple: deletion; target; list
+
+Deletion is recursively defined very similar to the way assignment is defined.
+Rather that spelling it out in full details, here are some hints.
+
+Deletion of a target list recursively deletes each target, from left to right.
+
+.. index::
+ statement: global
+ pair: unbinding; name
+
+Deletion of a name removes the binding of that name from the local or global
+namespace, depending on whether the name occurs in a :keyword:`global` statement
+in the same code block. If the name is unbound, a :exc:`NameError` exception
+will be raised.
+
+.. index:: pair: free; variable
+
+It is illegal to delete a name from the local namespace if it occurs as a free
+variable in a nested block.
+
+.. index:: pair: attribute; deletion
+
+Deletion of attribute references, subscriptions and slicings is passed to the
+primary object involved; deletion of a slicing is in general equivalent to
+assignment of an empty slice of the right type (but even this is determined by
+the sliced object).
+
+
+.. _print:
+
+The :keyword:`print` statement
+==============================
+
+.. index:: statement: print
+
+.. productionlist::
+ print_stmt: "print" ([`expression` ("," `expression`)* [","]
+ : | ">>" `expression` [("," `expression`)+ [","])
+
+:keyword:`print` evaluates each expression in turn and writes the resulting
+object to standard output (see below). If an object is not a string, it is
+first converted to a string using the rules for string conversions. The
+(resulting or original) string is then written. A space is written before each
+object is (converted and) written, unless the output system believes it is
+positioned at the beginning of a line. This is the case (1) when no characters
+have yet been written to standard output, (2) when the last character written to
+standard output is ``'\n'``, or (3) when the last write operation on standard
+output was not a :keyword:`print` statement. (In some cases it may be
+functional to write an empty string to standard output for this reason.)
+
+.. note::
+
+ Objects which act like file objects but which are not the built-in file objects
+ often do not properly emulate this aspect of the file object's behavior, so it
+ is best not to rely on this.
+
+.. index::
+ single: output
+ pair: writing; values
+
+.. index::
+ pair: trailing; comma
+ pair: newline; suppression
+
+A ``'\n'`` character is written at the end, unless the :keyword:`print`
+statement ends with a comma. This is the only action if the statement contains
+just the keyword :keyword:`print`.
+
+.. index::
+ pair: standard; output
+ module: sys
+ single: stdout (in module sys)
+ exception: RuntimeError
+
+Standard output is defined as the file object named ``stdout`` in the built-in
+module :mod:`sys`. If no such object exists, or if it does not have a
+:meth:`write` method, a :exc:`RuntimeError` exception is raised.
+
+.. index:: single: extended print statement
+
+:keyword:`print` also has an extended form, defined by the second portion of the
+syntax described above. This form is sometimes referred to as ":keyword:`print`
+chevron." In this form, the first expression after the ``>>`` must evaluate to a
+"file-like" object, specifically an object that has a :meth:`write` method as
+described above. With this extended form, the subsequent expressions are
+printed to this file object. If the first expression evaluates to ``None``,
+then ``sys.stdout`` is used as the file for output.
+
+
+.. _return:
+
+The :keyword:`return` statement
+===============================
+
+.. index:: statement: return
+
+.. productionlist::
+ return_stmt: "return" [`expression_list`]
+
+.. index::
+ pair: function; definition
+ pair: class; definition
+
+:keyword:`return` may only occur syntactically nested in a function definition,
+not within a nested class definition.
+
+If an expression list is present, it is evaluated, else ``None`` is substituted.
+
+:keyword:`return` leaves the current function call with the expression list (or
+``None``) as return value.
+
+.. index:: keyword: finally
+
+When :keyword:`return` passes control out of a :keyword:`try` statement with a
+:keyword:`finally` clause, that :keyword:`finally` clause is executed before
+really leaving the function.
+
+In a generator function, the :keyword:`return` statement is not allowed to
+include an :token:`expression_list`. In that context, a bare :keyword:`return`
+indicates that the generator is done and will cause :exc:`StopIteration` to be
+raised.
+
+
+.. _yield:
+
+The :keyword:`yield` statement
+==============================
+
+.. index:: statement: yield
+
+.. productionlist::
+ yield_stmt: `yield_expression`
+
+.. index::
+ single: generator; function
+ single: generator; iterator
+ single: function; generator
+ exception: StopIteration
+
+The :keyword:`yield` statement is only used when defining a generator function,
+and is only used in the body of the generator function. Using a :keyword:`yield`
+statement in a function definition is sufficient to cause that definition to
+create a generator function instead of a normal function.
+
+When a generator function is called, it returns an iterator known as a generator
+iterator, or more commonly, a generator. The body of the generator function is
+executed by calling the generator's :meth:`next` method repeatedly until it
+raises an exception.
+
+When a :keyword:`yield` statement is executed, the state of the generator is
+frozen and the value of :token:`expression_list` is returned to :meth:`next`'s
+caller. By "frozen" we mean that all local state is retained, including the
+current bindings of local variables, the instruction pointer, and the internal
+evaluation stack: enough information is saved so that the next time :meth:`next`
+is invoked, the function can proceed exactly as if the :keyword:`yield`
+statement were just another external call.
+
+As of Python version 2.5, the :keyword:`yield` statement is now allowed in the
+:keyword:`try` clause of a :keyword:`try` ... :keyword:`finally` construct. If
+the generator is not resumed before it is finalized (by reaching a zero
+reference count or by being garbage collected), the generator-iterator's
+:meth:`close` method will be called, allowing any pending :keyword:`finally`
+clauses to execute.
+
+.. note::
+
+ In Python 2.2, the :keyword:`yield` statement is only allowed when the
+ ``generators`` feature has been enabled. It will always be enabled in Python
+ 2.3. This ``__future__`` import statement can be used to enable the feature::
+
+ from __future__ import generators
+
+
+.. seealso::
+
+ :pep:`0255` - Simple Generators
+ The proposal for adding generators and the :keyword:`yield` statement to Python.
+
+ :pep:`0342` - Coroutines via Enhanced Generators
+ The proposal that, among other generator enhancements, proposed allowing
+ :keyword:`yield` to appear inside a :keyword:`try` ... :keyword:`finally` block.
+
+
+.. _raise:
+
+The :keyword:`raise` statement
+==============================
+
+.. index:: statement: raise
+
+.. productionlist::
+ raise_stmt: "raise" [`expression` ["," `expression` ["," `expression`]]]
+
+.. index::
+ single: exception
+ pair: raising; exception
+
+If no expressions are present, :keyword:`raise` re-raises the last exception
+that was active in the current scope. If no exception is active in the current
+scope, a :exc:`TypeError` exception is raised indicating that this is an error
+(if running under IDLE, a :exc:`Queue.Empty` exception is raised instead).
+
+Otherwise, :keyword:`raise` evaluates the expressions to get three objects,
+using ``None`` as the value of omitted expressions. The first two objects are
+used to determine the *type* and *value* of the exception.
+
+If the first object is an instance, the type of the exception is the class of
+the instance, the instance itself is the value, and the second object must be
+``None``.
+
+If the first object is a class, it becomes the type of the exception. The second
+object is used to determine the exception value: If it is an instance of the
+class, the instance becomes the exception value. If the second object is a
+tuple, it is used as the argument list for the class constructor; if it is
+``None``, an empty argument list is used, and any other object is treated as a
+single argument to the constructor. The instance so created by calling the
+constructor is used as the exception value.
+
+.. index:: object: traceback
+
+If a third object is present and not ``None``, it must be a traceback object
+(see section :ref:`types`), and it is substituted instead of the current
+location as the place where the exception occurred. If the third object is
+present and not a traceback object or ``None``, a :exc:`TypeError` exception is
+raised. The three-expression form of :keyword:`raise` is useful to re-raise an
+exception transparently in an except clause, but :keyword:`raise` with no
+expressions should be preferred if the exception to be re-raised was the most
+recently active exception in the current scope.
+
+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
+
+.. productionlist::
+ break_stmt: "break"
+
+.. index::
+ statement: for
+ statement: while
+ pair: loop; statement
+
+: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
+
+It terminates the nearest enclosing loop, skipping the optional :keyword:`else`
+clause if the loop has one.
+
+.. index:: pair: loop control; target
+
+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
+
+.. productionlist::
+ continue_stmt: "continue"
+
+.. index::
+ statement: for
+ statement: while
+ pair: loop; statement
+ keyword: finally
+
+: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` statement within that loop. [#]_ It continues with the next
+cycle of the nearest enclosing loop.
+
+
+.. _import:
+
+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
+ module: sys
+ pair: filename; extension
+ triple: module; search; path
+
+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
+
+.. index::
+ 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.
+
+The features recognized by Python 2.5 are ``absolute_import``, ``division``,
+``generators``, ``nested_scopes`` and ``with_statement``. ``generators`` and
+``nested_scopes`` are redundant in Python version 2.3 and above because they
+are always enabled.
+
+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 an :keyword:`exec` statement or calls to the builtin functions
+:func:`compile` and :func:`execfile` 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, starting with Python 2.2 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
+
+.. productionlist::
+ global_stmt: "global" `identifier` ("," `identifier`)*
+
+.. index:: triple: global; name; binding
+
+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::
+ statement: exec
+ builtin: eval
+ builtin: execfile
+ 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 an :keyword:`exec`
+statement does not affect the code block *containing* the :keyword:`exec`
+statement, and code contained in an :keyword:`exec` statement is unaffected by
+:keyword:`global` statements in the code containing the :keyword:`exec`
+statement. The same applies to the :func:`eval`, :func:`execfile` and
+:func:`compile` functions.
+
+
+.. _exec:
+
+The :keyword:`exec` statement
+=============================
+
+.. index:: statement: exec
+
+.. productionlist::
+ exec_stmt: "exec" `or_expr` ["in" `expression` ["," `expression`]]
+
+This statement supports dynamic execution of Python code. The first expression
+should evaluate to either a string, an open file object, or a code object. If
+it is a string, the string is parsed as a suite of Python statements which is
+then executed (unless a syntax error occurs). If it is an open file, the file
+is parsed until EOF and executed. If it is a code object, it is simply
+executed. In all cases, the code that's executed is expected to be valid as
+file input (see section :ref:`file-input`). Be aware that the
+:keyword:`return` and :keyword:`yield` statements may not be used outside of
+function definitions even within the context of code passed to the
+:keyword:`exec` statement.
+
+In all cases, if the optional parts are omitted, the code is executed in the
+current scope. If only the first expression after :keyword:`in` is specified,
+it should be a dictionary, which will be used for both the global and the local
+variables. If two expressions are given, they are used for the global and local
+variables, respectively. If provided, *locals* can be any mapping object.
+
+.. versionchanged:: 2.4
+ formerly *locals* was required to be a dictionary.
+
+.. index::
+ single: __builtins__
+ module: __builtin__
+
+As a side effect, an implementation may insert additional keys into the
+dictionaries given besides those corresponding to variable names set by the
+executed code. For example, the current implementation may add a reference to
+the dictionary of the built-in module :mod:`__builtin__` under the key
+``__builtins__`` (!).
+
+.. index::
+ builtin: eval
+ builtin: globals
+ builtin: locals
+
+**Programmer's hints:** dynamic evaluation of expressions is supported by the
+built-in function :func:`eval`. The built-in functions :func:`globals` and
+:func:`locals` return the current global and local dictionary, respectively,
+which may be useful to pass around for use by :keyword:`exec`.
+
+.. 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.
+