7. 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.  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" statements 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
                     | decorated
   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.


7.1. 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.


7.2. 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.


7.3. 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, and then the suite is executed.  When the items are
exhausted (which is immediately when the sequence is empty), 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.

The target list is 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 a sequence 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]".

備註: There is a subtlety when the sequence is being modified by the
  loop (this can only occur for mutable sequences, e.g. 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)


7.4. 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" | ",") identifier]] ":" suite)+
                 ["else" ":" suite]
                 ["finally" ":" suite]
   try2_stmt ::= "try" ":" suite
                 "finally" ":" suite

2.5 版更變: In previous versions of Python, "try"…"except"…"finally"
did not work. "try"…"except" had to be nested in "try"…"finally".

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 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.)

Before an except clause’s suite is executed, details about the
exception are assigned to three variables in the "sys" module:
"sys.exc_type" receives the object identifying the exception;
"sys.exc_value" receives the exception’s parameter;
"sys.exc_traceback" receives a traceback object (see section The
standard type hierarchy) identifying the point in the program where
the exception occurred. These details are also available through the
"sys.exc_info()" function, which returns a tuple "(exc_type,
exc_value, exc_traceback)".  Use of the corresponding variables is
deprecated in favor of this function, since their use is unsafe in a
threaded program.  As of Python 1.5, the variables 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 the control flow leaves the
"try" suite, no exception was raised, and no "return", "continue", or
"break" statement was executed.  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
discarded:

   >>> def f():
   ...     try:
   ...         1/0
   ...     finally:
   ...         return 42
   ...
   >>> f()
   42

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).

The return value of a function is determined by the last "return"
statement executed.  Since the "finally" clause always executes, a
"return" statement executed in the "finally" clause will always be the
last one executed:

   >>> def foo():
   ...     try:
   ...         return 'try'
   ...     finally:
   ...         return 'finally'
   ...
   >>> foo()
   'finally'

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.


7.5. The "with" statement
=========================

2.5 版新加入.

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" with_item ("," with_item)* ":" suite
   with_item ::= expression ["as" target]

The execution of the "with" statement with one 「item」 proceeds as
follows:

1. The context expression (the expression given in the "with_item")
   is evaluated to obtain a context manager.

2. The context manager’s "__exit__()" is loaded for later use.

3. The context manager’s "__enter__()" method is invoked.

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

   備註: 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 6 below.

5. The suite is executed.

6. 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.

With more than one item, the context managers are processed as if
multiple "with" statements were nested:

   with A() as a, B() as b:
       suite

is equivalent to

   with A() as a:
       with B() as b:
           suite

備註: In Python 2.5, the "with" statement is only allowed when the
  "with_statement" feature has been enabled.  It is always enabled in
  Python 2.6.

2.7 版更變: Support for multiple context expressions.

也參考:

  **PEP 343** - The 「with」 statement
     The specification, background, and examples for the Python "with"
     statement.


7.6. Function definitions
=========================

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

   decorated      ::= decorators (classdef | funcdef)
   decorators     ::= decorator+
   decorator      ::= "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
   funcdef        ::= "def" funcname "(" [parameter_list] ")" ":" suite
   dotted_name    ::= identifier ("." identifier)*
   parameter_list ::= (defparameter ",")*
                      (  "*" identifier ["," "**" identifier]
                      | "**" identifier
                      | defparameter [","] )
   defparameter   ::= parameter ["=" expression]
   sublist        ::= parameter ("," parameter)* [","]
   parameter      ::= identifier | "(" sublist ")"
   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. [2]

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 top-level *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 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 the 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.

It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions.  This uses lambda
expressions, described in section Lambdas.  Note that the lambda
expression 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
expression.  The 「"def"」 form is actually more powerful since it
allows the execution of multiple statements.

**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.


7.7. Class definitions
======================

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

   classdef    ::= "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. [3] 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.

**Programmer’s note:** Variables defined in the class definition are
class variables; they are shared by all instances.  To create instance
variables, they 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.  For *new-style class*es, descriptors can
be used to create instance variables with different implementation
details.

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.

-[ 註解 ]-

[1] The exception is propagated to the invocation stack unless
    there is a "finally" clause which happens to raise another
    exception. That new exception causes the old one to be lost.

[2] 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*.

[3] 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*.
