
Compound statements
*******************

Compound statements contain (groups of) other statements; they affect
or control the execution of those other statements in some way.  In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.

The ``if``, ``while`` and ``for`` statements implement traditional
control flow constructs.  ``try`` specifies exception handlers and/or
cleanup code for a group of statements, while the ``with`` statement
allows the execution of initialization and finalization code around a
block of code.  Function and class definitions are also syntactically
compound statements.

Compound statements consist of one or more 'clauses.'  A clause
consists of a header and a 'suite.'  The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon.  A suite is a group of statements controlled by a
clause.  A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines.  Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which ``if`` clause a following ``else`` clause would belong:

   if test1: if test2: print(x)

Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
``print()`` calls are executed:

   if x < y < z: print(x); print(y); print(z)

Summarizing:

   compound_stmt ::= if_stmt
                     | while_stmt
                     | for_stmt
                     | try_stmt
                     | with_stmt
                     | funcdef
                     | classdef
   suite         ::= stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
   statement     ::= stmt_list NEWLINE | compound_stmt
   stmt_list     ::= simple_stmt (";" simple_stmt)* [";"]

Note that statements always end in a ``NEWLINE`` possibly followed by
a ``DEDENT``.  Also note that optional continuation clauses always
begin with a keyword that cannot start a statement, thus there are no
ambiguities (the 'dangling ``else``' problem is solved in Python by
requiring nested ``if`` statements to be indented).

The formatting of the grammar rules in the following sections places
each clause on a separate line for clarity.


The ``if`` statement
====================

The ``if`` statement is used for conditional execution:

   if_stmt ::= "if" expression ":" suite
               ( "elif" expression ":" suite )*
               ["else" ":" suite]

It selects exactly one of the suites by evaluating the expressions one
by one until one is found to be true (see section *Boolean operations*
for the definition of true and false); then that suite is executed
(and no other part of the ``if`` statement is executed or evaluated).
If all expressions are false, the suite of the ``else`` clause, if
present, is executed.


The ``while`` statement
=======================

The ``while`` statement is used for repeated execution as long as an
expression is true:

   while_stmt ::= "while" expression ":" suite
                  ["else" ":" suite]

This repeatedly tests the expression and, if it is true, executes the
first suite; if the expression is false (which may be the first time
it is tested) the suite of the ``else`` clause, if present, is
executed and the loop terminates.

A ``break`` statement executed in the first suite terminates the loop
without executing the ``else`` clause's suite.  A ``continue``
statement executed in the first suite skips the rest of the suite and
goes back to testing the expression.


The ``for`` statement
=====================

The ``for`` statement is used to iterate over the elements of a
sequence (such as a string, tuple or list) or other iterable object:

   for_stmt ::= "for" target_list "in" expression_list ":" suite
                ["else" ":" suite]

The expression list is evaluated once; it should yield an iterable
object.  An iterator is created for the result of the
``expression_list``.  The suite is then executed once for each item
provided by the iterator, in the order of ascending indices.  Each
item in turn is assigned to the target list using the standard rules
for assignments (see *Assignment statements*), and then the suite is
executed.  When the items are exhausted (which is immediately when the
sequence is empty or an iterator raises a ``StopIteration``
exception), the suite in the ``else`` clause, if present, is executed,
and the loop terminates.

A ``break`` statement executed in the first suite terminates the loop
without executing the ``else`` clause's suite.  A ``continue``
statement executed in the first suite skips the rest of the suite and
continues with the next item, or with the ``else`` clause if there was
no next item.

The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.

Names in the target list are not deleted when the loop is finished,
but if the sequence is empty, it will not have been assigned to at all
by the loop.  Hint: the built-in function ``range()`` returns an
iterator of integers suitable to emulate the effect of Pascal's ``for
i := a to b do``; e.g., ``range(3)`` returns the list ``[0, 1, 2]``.

Warning: There is a subtlety when the sequence is being modified by the loop
  (this can only occur for mutable sequences, i.e. lists).  An
  internal counter is used to keep track of which item is used next,
  and this is incremented on each iteration.  When this counter has
  reached the length of the sequence the loop terminates.  This means
  that if the suite deletes the current (or a previous) item from the
  sequence, the next item will be skipped (since it gets the index of
  the current item which has already been treated).  Likewise, if the
  suite inserts an item in the sequence before the current item, the
  current item will be treated again the next time through the loop.
  This can lead to nasty bugs that can be avoided by making a
  temporary copy using a slice of the whole sequence, e.g.,

     for x in a[:]:
         if x < 0: a.remove(x)


The ``try`` statement
=====================

The ``try`` statement specifies exception handlers and/or cleanup code
for a group of statements:

   try_stmt  ::= try1_stmt | try2_stmt
   try1_stmt ::= "try" ":" suite
                 ("except" [expression ["as" target]] ":" suite)+
                 ["else" ":" suite]
                 ["finally" ":" suite]
   try2_stmt ::= "try" ":" suite
                 "finally" ":" suite

The ``except`` clause(s) specify one or more exception handlers. When
no exception occurs in the ``try`` clause, no exception handler is
executed. When an exception occurs in the ``try`` suite, a search for
an exception handler is started.  This search inspects the except
clauses in turn until one is found that matches the exception.  An
expression-less except clause, if present, must be last; it matches
any exception.  For an except clause with an expression, that
expression is evaluated, and the clause matches the exception if the
resulting object is "compatible" with the exception.  An object is
compatible with an exception if it is the class or a base class of the
exception object or a tuple containing an item compatible with the
exception.

If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.
[1]

If the evaluation of an expression in the header of an except clause
raises an exception, the original search for a handler is canceled and
a search starts for the new exception in the surrounding code and on
the call stack (it is treated as if the entire ``try`` statement
raised the exception).

When a matching except clause is found, the exception is assigned to
the target specified after the ``as`` keyword in that except clause,
if present, and the except clause's suite is executed.  All except
clauses must have an executable block.  When the end of this block is
reached, execution continues normally after the entire try statement.
(This means that if two nested handlers exist for the same exception,
and the exception occurs in the try clause of the inner handler, the
outer handler will not handle the exception.)

When an exception has been assigned using ``as target``, it is cleared
at the end of the except clause.  This is as if

   except E as N:
       foo

was translated to

   except E as N:
       try:
           foo
       finally:
           N = None
           del N

That means that you have to assign the exception to a different name
if you want to be able to refer to it after the except clause.  The
reason for this is that with the traceback attached to them,
exceptions will form a reference cycle with the stack frame, keeping
all locals in that frame alive until the next garbage collection
occurs.

Before an except clause's suite is executed, details about the
exception are stored in the ``sys`` module and can be access via
``sys.exc_info()``. ``sys.exc_info()`` returns a 3-tuple consisting
of: ``exc_type``, the exception class; ``exc_value``, the exception
instance; ``exc_traceback``, a traceback object (see section *The
standard type hierarchy*) identifying the point in the program where
the exception occurred. ``sys.exc_info()`` values are restored to
their previous values (before the call) when returning from a function
that handled an exception.

The optional ``else`` clause is executed if and when control flows off
the end of the ``try`` clause. [2] Exceptions in the ``else`` clause
are not handled by the preceding ``except`` clauses.

If ``finally`` is present, it specifies a 'cleanup' handler.  The
``try`` clause is executed, including any ``except`` and ``else``
clauses.  If an exception occurs in any of the clauses and is not
handled, the exception is temporarily saved. The ``finally`` clause is
executed.  If there is a saved exception, it is re-raised at the end
of the ``finally`` clause. If the ``finally`` clause raises another
exception or executes a ``return`` or ``break`` statement, the saved
exception is lost.  The exception information is not available to the
program during execution of the ``finally`` clause.

When a ``return``, ``break`` or ``continue`` statement is executed in
the ``try`` suite of a ``try``...``finally`` statement, the
``finally`` clause is also executed 'on the way out.' A ``continue``
statement is illegal in the ``finally`` clause. (The reason is a
problem with the current implementation --- this restriction may be
lifted in the future).

Additional information on exceptions can be found in section
*Exceptions*, and information on using the ``raise`` statement to
generate exceptions may be found in section *The raise statement*.


The ``with`` statement
======================

The ``with`` statement is used to wrap the execution of a block with
methods defined by a context manager (see section *With Statement
Context Managers*). This allows common
``try``...``except``...``finally`` usage patterns to be encapsulated
for convenient reuse.

   with_stmt ::= "with" expression ["as" target] ":" suite

The execution of the ``with`` statement proceeds as follows:

1. The context expression is evaluated to obtain a context manager.

2. The context manager's ``__enter__()`` method is invoked.

3. If a target was included in the ``with`` statement, the return
   value from ``__enter__()`` is assigned to it.

   Note: The ``with`` statement guarantees that if the ``__enter__()``
     method returns without an error, then ``__exit__()`` will always
     be called.  Thus, if an error occurs during the assignment to the
     target list, it will be treated the same as an error occurring
     within the suite would be.  See step 5 below.

4. The suite is executed.

5. The context manager's ``__exit__()`` method is invoked.  If an
   exception caused the suite to be exited, its type, value, and
   traceback are passed as arguments to ``__exit__()``. Otherwise,
   three ``None`` arguments are supplied.

   If the suite was exited due to an exception, and the return value
   from the ``__exit__()`` method was false, the exception is
   reraised.  If the return value was true, the exception is
   suppressed, and execution continues with the statement following
   the ``with`` statement.

   If the suite was exited for any reason other than an exception, the
   return value from ``__exit__()`` is ignored, and execution proceeds
   at the normal location for the kind of exit that was taken.

See also:

   **PEP 0343** - The "with" statement
      The specification, background, and examples for the Python
      ``with`` statement.


Function definitions
====================

A function definition defines a user-defined function object (see
section *The standard type hierarchy*):

   funcdef        ::= [decorators] "def" funcname "(" [parameter_list] ")" ["->" expression] ":" suite
   decorators     ::= decorator+
   decorator      ::= "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
   dotted_name    ::= identifier ("." identifier)*
   parameter_list ::= (defparameter ",")*
                      (  "*" [parameter] ("," defparameter)*
                      [, "**" parameter]
                      | "**" parameter
                      | defparameter [","] )
   parameter      ::= identifier [":" expression]
   defparameter   ::= parameter ["=" expression]
   funcname       ::= identifier

A function definition is an executable statement.  Its execution binds
the function name in the current local namespace to a function object
(a wrapper around the executable code for the function).  This
function object contains a reference to the current global namespace
as the global namespace to be used when the function is called.

The function definition does not execute the function body; this gets
executed only when the function is called. [3]

A function definition may be wrapped by one or more *decorator*
expressions. Decorator expressions are evaluated when the function is
defined, in the scope that contains the function definition.  The
result must be a callable, which is invoked with the function object
as the only argument. The returned value is bound to the function name
instead of the function object.  Multiple decorators are applied in
nested fashion. For example, the following code

   @f1(arg)
   @f2
   def func(): pass

is equivalent to

   def func(): pass
   func = f1(arg)(f2(func))

When one or more parameters have the form *parameter* ``=``
*expression*, the function is said to have "default parameter values."
For a parameter with a default value, the corresponding argument may
be omitted from a call, in which case the parameter's default value is
substituted.  If a parameter has a default value, all following
parameters up until the "``*``" must also have a default value ---
this is a syntactic restriction that is not expressed by the grammar.

**Default parameter values are evaluated when the function definition
is executed.** This means that the expression is evaluated once, when
the function is defined, and that that same "pre-computed" value is
used for each call.  This is especially important to understand when a
default parameter is a mutable object, such as a list or a dictionary:
if the function modifies the object (e.g. by appending an item to a
list), the default value is in effect modified. This is generally not
what was intended.  A way around this is to use ``None`` as the
default, and explicitly test for it in the body of the function, e.g.:

   def whats_on_the_telly(penguin=None):
       if penguin is None:
           penguin = []
       penguin.append("property of the zoo")
       return penguin

Function call semantics are described in more detail in section
*Calls*. A function call always assigns values to all parameters
mentioned in the parameter list, either from position arguments, from
keyword arguments, or from default values.  If the form
"``*identifier``" is present, it is initialized to a tuple receiving
any excess positional parameters, defaulting to the empty tuple.  If
the form "``**identifier``" is present, it is initialized to a new
dictionary receiving any excess keyword arguments, defaulting to a new
empty dictionary. Parameters after "``*``" or "``*identifier``" are
keyword-only parameters and may only be passed used keyword arguments.

Parameters may have annotations of the form "``: expression``"
following the parameter name.  Any parameter may have an annotation
even those of the form ``*identifier`` or ``**identifier``.  Functions
may have "return" annotation of the form "``-> expression``" after the
parameter list.  These annotations can be any valid Python expression
and are evaluated when the function definition is executed.
Annotations may be evaluated in a different order than they appear in
the source code.  The presence of annotations does not change the
semantics of a function.  The annotation values are available as
values of a dictionary keyed by the parameters' names in the
``__annotations__`` attribute of the function object.

It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions.  This uses lambda forms,
described in section *Expression lists*.  Note that the lambda form is
merely a shorthand for a simplified function definition; a function
defined in a "``def``" statement can be passed around or assigned to
another name just like a function defined by a lambda form.  The
"``def``" form is actually more powerful since it allows the execution
of multiple statements and annotations.

**Programmer's note:** Functions are first-class objects.  A "``def``"
form executed inside a function definition defines a local function
that can be returned or passed around.  Free variables used in the
nested function can access the local variables of the function
containing the def.  See section *Naming and binding* for details.


Class definitions
=================

A class definition defines a class object (see section *The standard
type hierarchy*):

   classdef    ::= [decorators] "class" classname [inheritance] ":" suite
   inheritance ::= "(" [expression_list] ")"
   classname   ::= identifier

A class definition is an executable statement.  It first evaluates the
inheritance list, if present.  Each item in the inheritance list
should evaluate to a class object or class type which allows
subclassing.  The class's suite is then executed in a new execution
frame (see section *Naming and binding*), using a newly created local
namespace and the original global namespace. (Usually, the suite
contains only function definitions.)  When the class's suite finishes
execution, its execution frame is discarded but its local namespace is
saved. [4] A class object is then created using the inheritance list
for the base classes and the saved local namespace for the attribute
dictionary.  The class name is bound to this class object in the
original local namespace.

Classes can also be decorated; as with functions,

   @f1(arg)
   @f2
   class Foo: pass

is equivalent to

   class Foo: pass
   Foo = f1(arg)(f2(Foo))

**Programmer's note:** Variables defined in the class definition are
class variables; they are shared by instances. Instance variables can
be set in a method with ``self.name = value``.  Both class and
instance variables are accessible through the notation
"``self.name``", and an instance variable hides a class variable with
the same name when accessed in this way.  Class variables can be used
as defaults for instance variables, but using mutable values there can
lead to unexpected results.  Descriptors can be used to create
instance variables with different implementation details.

See also:

   **PEP 3129** - Class Decorators

Class definitions, like function definitions, may be wrapped by one or
more *decorator* expressions.  The evaluation rules for the decorator
expressions are the same as for functions.  The result must be a class
object, which is then bound to the class name.

-[ Footnotes ]-

[1] The exception is propagated to the invocation stack only if there
    is no ``finally`` clause that negates the exception.

[2] Currently, control "flows off the end" except in the case of an
    exception or the execution of a ``return``, ``continue``, or
    ``break`` statement.

[3] A string literal appearing as the first statement in the function
    body is transformed into the function's ``__doc__`` attribute and
    therefore the function's *docstring*.

[4] A string literal appearing as the first statement in the class
    body is transformed into the namespace's ``__doc__`` item and
    therefore the class's *docstring*.
