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 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 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 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., list(range(3)) returns the list [0, 1, 2].
Note
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 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:
del N
This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback attached to them, they 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 the exception class, the exception instance and 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, the saved exception is set as the context of the new exception. If the finally clause 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).
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 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:
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.
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 6 below.
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 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
Changed in version 3.1: Support for multiple context expressions.
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 ["(" [parameter_list [","]] ")"] NEWLINE dotted_name ::= identifier ("." identifier)* parameter_list ::= (defparameter ",")* ( "*" [parameter] ("," defparameter)* ["," "**" parameter] | "**" parameter | defparameter [","] ) parameter ::= identifier [":" expression] defparameter ::= parameter ["="