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.
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
if x < y < z: print x; print y; print z
stmt_listNEWLINE | NEWLINE INDENT
statement+ DEDENT statement ::=
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
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.
if statement is used for conditional execution:
if_stmt ::= "if"
suite)* ["else" ":"
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.
while statement is used for repeated execution as long as an
expression is true:
while_stmt ::= "while"
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
break statement executed in the first suite terminates the loop
without executing the
else clause’s suite. A
statement executed in the first suite skips the rest of the suite and goes back
to testing the expression.
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"
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
else clause, if present, is executed, and the loop terminates.
break statement executed in the first suite terminates the loop
without executing the
else clause’s suite. A
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
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
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].
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)
try statement specifies exception handlers and/or cleanup code
for a group of statements:
try_stmt ::= try1_stmt | try2_stmt try1_stmt ::= "try" ":"
expression[("as" | ",")
suite)+ ["else" ":"
suite] ["finally" ":"
suite] try2_stmt ::= "try" ":"
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. 
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
the object identifying the exception;
sys.exc_value receives the exception’s
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
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.
finally is present, it specifies a ‘cleanup’ handler. The
try clause is executed, including any
else clauses. If an exception occurs in any of the clauses and is
not handled, the exception is temporarily saved. The
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
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
continue statement is
executed in the
try suite of a
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
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'
New in version 2.5.
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
patterns to be encapsulated for convenient reuse.
with_stmt ::= "with" with_item ("," with_item)* ":"
The execution of the
with statement with one “item” proceeds as follows:
The context expression (the expression given in the
with_item) is evaluated to obtain a context manager.
The context manager’s
__exit__()is loaded for later use.
The context manager’s
__enter__()method is invoked.
The suite is executed.
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
Nonearguments 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
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
In Python 2.5, the
with statement is only allowed when the
with_statement feature has been enabled. It is always enabled in
Changed in version 2.7: Support for multiple context expressions.
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 ::= "@"
argument_list[","]] ")"] NEWLINE funcdef ::= "def"
parameter_list] ")" ":"
identifier)* parameter_list ::= (
defparameter",")* ( "*"
identifier] | "**"
defparameter[","] ) defparameter ::=
expression] sublist ::=
parameter)* [","] parameter ::=
sublist")" funcname ::=
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. 
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
= 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
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 “
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 “
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"
suiteinheritance ::= "(" [
expression_list] ")" classname ::=
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.  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
classes, descriptors can be used to create instance variables with different
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.
|||The exception is propagated to the invocation stack unless
there is a |
|||Currently, control “flows off the end” except in the case of an exception or the
execution of a |
|||A string literal appearing as the first statement in the function body is
transformed into the function’s |
|||A string literal appearing as the first statement in the class body is
transformed into the namespace’s |