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Diffstat (limited to 'Doc/reference/compound_stmts.rst')
| -rw-r--r-- | Doc/reference/compound_stmts.rst | 303 |
1 files changed, 182 insertions, 121 deletions
diff --git a/Doc/reference/compound_stmts.rst b/Doc/reference/compound_stmts.rst index 4a616eb..d0d0646 100644 --- a/Doc/reference/compound_stmts.rst +++ b/Doc/reference/compound_stmts.rst @@ -13,8 +13,10 @@ statement may be contained in one line. The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement traditional control flow constructs. :keyword:`try` specifies exception -handlers and/or cleanup code for a group of statements. Function and class -definitions are also syntactically compound statements. +handlers and/or cleanup code for a group of statements, while the +:keyword:`with` statement allows the execution of initialization and +finalization code around a block of code. Function and class definitions are +also syntactically compound statements. .. index:: single: clause @@ -29,15 +31,15 @@ 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 :keyword:`if` clause a following -:keyword:`else` clause would belong: :: +:keyword:`else` clause would belong:: - if test1: if test2: print x + 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 :keyword:`print` -statements are executed:: +that in the following example, either all or none of the :func:`print` calls are +executed:: - if x < y < z: print x; print y; print z + if x < y < z: print(x); print(y); print(z) Summarizing: @@ -49,7 +51,6 @@ Summarizing: : | `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`)* [";"] @@ -60,7 +61,7 @@ Summarizing: pair: dangling; else Note that statements always end in a ``NEWLINE`` possibly followed by a -``DEDENT``. Also note that optional continuation clauses always begin with 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 :keyword:`else`' problem is solved in Python by requiring nested :keyword:`if` statements to be indented). @@ -80,6 +81,8 @@ The :keyword:`if` statement statement: if keyword: elif keyword: else + keyword: elif + keyword: else The :keyword:`if` statement is used for conditional execution: @@ -102,6 +105,7 @@ The :keyword:`while` statement .. index:: statement: while + keyword: else pair: loop; statement keyword: else @@ -134,6 +138,9 @@ The :keyword:`for` statement .. index:: statement: for + keyword: in + keyword: else + pair: target; list pair: loop; statement keyword: in keyword: else @@ -151,8 +158,9 @@ 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 +standard rules for assignments (see :ref:`assignment`), and then the suite is +executed. When the items are exhausted (which is immediately when the sequence +is empty or an iterator raises a :exc:`StopIteration` exception), the suite in the :keyword:`else` clause, if present, is executed, and the loop terminates. .. index:: @@ -170,13 +178,12 @@ the next item assigned to it. .. index:: builtin: range - pair: Pascal; language -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 :func:`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]``. +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 :func:`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:: @@ -185,16 +192,16 @@ effect of Pascal's ``for i := a to b do``; e.g., ``range(3)`` returns the list single: mutable sequence; loop over 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 + 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., :: + 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) @@ -211,6 +218,7 @@ The :keyword:`try` statement statement: try keyword: except keyword: finally +.. index:: keyword: except The :keyword:`try` statement specifies exception handlers and/or cleanup code for a group of statements: @@ -218,16 +226,12 @@ for a group of statements: .. productionlist:: try_stmt: try1_stmt | try2_stmt try1_stmt: "try" ":" `suite` - : ("except" [`expression` [("as" | ",") `target`]] ":" `suite`)+ + : ("except" [`expression` ["as" `target`]] ":" `suite`)+ : ["else" ":" `suite`] : ["finally" ":" `suite`] try2_stmt: "try" ":" `suite` : "finally" ":" `suite` -.. versionchanged:: 2.5 - In previous versions of Python, :keyword:`try`...\ :keyword:`except`...\ - :keyword:`finally` did not work. :keyword:`try`...\ :keyword:`except` had to be - nested in :keyword:`try`...\ :keyword:`finally`. The :keyword:`except` clause(s) specify one or more exception handlers. When no exception occurs in the :keyword:`try` clause, no exception handler is executed. @@ -238,7 +242,7 @@ 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. +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. [#]_ @@ -249,31 +253,43 @@ the new exception in the surrounding code and on the call stack (it is treated as if the entire :keyword:`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.) +specified after the :keyword:`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. .. index:: module: sys object: traceback - single: exc_type (in module sys) - single: exc_value (in module sys) - single: exc_traceback (in module sys) Before an except clause's suite is executed, details about the exception are -assigned to three variables in the :mod:`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 -:ref:`types`) identifying the point in the program where the exception -occurred. These details are also available through the :func:`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. +stored in the :mod:`sys` module and can be access via :func:`sys.exc_info`. +:func:`sys.exc_info` returns a 3-tuple consisting of the exception class, the +exception instance and a traceback object (see section :ref:`types`) identifying +the point in the program where the exception occurred. :func:`sys.exc_info` +values are restored to their previous values (before the call) when returning +from a function that handled an exception. .. index:: keyword: else @@ -291,10 +307,11 @@ If :keyword:`finally` is present, it specifies a 'cleanup' handler. The :keyword:`try` clause is executed, including any :keyword:`except` and :keyword:`else` clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The :keyword:`finally` clause -is executed. If there is a saved exception, it is re-raised at the end of the -:keyword:`finally` clause. If the :keyword:`finally` clause raises another -exception or executes a :keyword:`return` or :keyword:`break` statement, the -saved exception is dicarded:: +is executed. If there is a saved exception it is re-raised at the end of the +:keyword:`finally` clause. If the :keyword:`finally` clause raises another +exception, the saved exception is set as the context of the new exception. +If the :keyword:`finally` clause executes a :keyword:`return` or :keyword:`break` +statement, the saved exception is discarded:: def f(): try: @@ -333,12 +350,10 @@ The :keyword:`with` statement .. index:: statement: with -.. versionadded:: 2.5 - The :keyword:`with` statement is used to wrap the execution of a block with -methods defined by a context manager (see section :ref:`context-managers`). This -allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` usage -patterns to be encapsulated for convenient reuse. +methods defined by a context manager (see section :ref:`context-managers`). +This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally` +usage patterns to be encapsulated for convenient reuse. .. productionlist:: with_stmt: "with" with_item ("," with_item)* ":" `suite` @@ -358,26 +373,27 @@ The execution of the :keyword:`with` statement with one "item" proceeds as follo .. note:: - The :keyword:`with` statement guarantees that if the :meth:`__enter__` method - returns without an error, then :meth:`__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 :keyword:`with` statement guarantees that if the :meth:`__enter__` + method returns without an error, then :meth:`__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 :meth:`__exit__` method is invoked. If an exception +#. The context manager's :meth:`__exit__` method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are supplied. If the suite was exited due to an exception, and the return value from the - :meth:`__exit__` method was false, the exception is reraised. If the return + :meth:`__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 :keyword:`with` statement. - If the suite was exited for any reason other than an exception, the return value - from :meth:`__exit__` is ignored, and execution proceeds at the normal location - for the kind of exit that was taken. + If the suite was exited for any reason other than an exception, the return + value from :meth:`__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 :keyword:`with` statements were nested:: @@ -391,13 +407,7 @@ is equivalent to :: with B() as b: suite -.. note:: - - In Python 2.5, the :keyword:`with` statement is only allowed when the - ``with_statement`` feature has been enabled. It is always enabled in - Python 2.6. - -.. versionchanged:: 2.7 +.. versionchanged:: 3.1 Support for multiple context expressions. .. seealso:: @@ -423,25 +433,26 @@ Function definitions pair: name; binding object: user-defined function object: function + pair: function; name + pair: name; binding A function definition defines a user-defined function object (see section :ref:`types`): .. productionlist:: - decorated: decorators (classdef | funcdef) + funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`] ":" `suite` decorators: `decorator`+ - decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE - funcdef: "def" `funcname` "(" [`parameter_list`] ")" ":" `suite` + decorator: "@" `dotted_name` ["(" [`parameter_list` [","]] ")"] NEWLINE dotted_name: `identifier` ("." `identifier`)* parameter_list: (`defparameter` ",")* - : ( "*" `identifier` ["," "**" `identifier`] - : | "**" `identifier` + : ( "*" [`parameter`] ("," `defparameter`)* ["," "**" `parameter`] + : | "**" `parameter` : | `defparameter` [","] ) + parameter: `identifier` [":" `expression`] 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 @@ -459,13 +470,13 @@ 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:: +are applied in nested fashion. For example, the following code :: @f1(arg) @f2 def func(): pass -is equivalent to:: +is equivalent to :: def func(): pass func = f1(arg)(f2(func)) @@ -474,21 +485,21 @@ is equivalent to:: triple: default; parameter; value single: argument; function definition -When one or more top-level :term:`parameters <parameter>` have the form -*parameter* ``=`` *expression*, the function is said to have "default parameter -values." For a parameter with a default value, the corresponding -:term:`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 +When one or more :term:`parameters <parameter>` have the form *parameter* ``=`` +*expression*, the function is said to have "default parameter values." For a +parameter with a default value, the corresponding :term:`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 +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`` +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): @@ -508,6 +519,21 @@ 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. + +.. index:: pair: function; annotations + +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 :attr:`__annotations__` attribute of the +function object. .. index:: pair: lambda; form @@ -517,7 +543,7 @@ section :ref:`lambda`. Note that the lambda form is merely a shorthand for a simplified function definition; a function defined in a ":keyword:`def`" statement can be passed around or assigned to another name just like a function defined by a lambda form. The ":keyword:`def`" form is actually more powerful -since it allows the execution of multiple statements. +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 @@ -525,6 +551,11 @@ returned or passed around. Free variables used in the nested function can access the local variables of the function containing the def. See section :ref:`naming` for details. +.. seealso:: + + :pep:`3107` - Function Annotations + The original specification for function annotations. + .. _class: @@ -544,35 +575,65 @@ Class definitions A class definition defines a class object (see section :ref:`types`): .. productionlist:: - classdef: "class" `classname` [`inheritance`] ":" `suite` - inheritance: "(" [`expression_list`] ")" + classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite` + inheritance: "(" [`parameter_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 :ref:`naming`), 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. +A class definition is an executable statement. The inheritance list usually +gives a list of base classes (see :ref:`metaclasses` for more advanced uses), so +each item in the list should evaluate to a class object which allows +subclassing. Classes without an inheritance list inherit, by default, from the +base class :class:`object`; hence, :: + + class Foo: + pass + +is equivalent to :: + + class Foo(object): + pass + +The class's suite is then executed in a new execution frame (see :ref:`naming`), +using a newly created local namespace and the original global namespace. +(Usually, the suite contains mostly 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. + +Class creation can be customized heavily using :ref:`metaclasses <metaclasses>`. + +Classes can also be decorated: just like when decorating functions, :: + + @f1(arg) + @f2 + class Foo: pass + +is equivalent to :: + + class Foo: pass + Foo = f1(arg)(f2(Foo)) + +The evaluation rules for the decorator expressions are the same as for function +decorators. The result must be a class object, which is then bound to the class +name. **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 :term:`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 -:term:`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. +attributes; they are shared by instances. Instance attributes can be set in a +method with ``self.name = value``. Both class and instance attributes are +accessible through the notation "``self.name``", and an instance attribute hides +a class attribute with the same name when accessed in this way. Class +attributes can be used as defaults for instance attributes, but using mutable +values there can lead to unexpected results. :ref:`Descriptors <descriptors>` +can be used to create instance variables with different implementation details. + + +.. seealso:: + + :pep:`3115` - Metaclasses in Python 3 + :pep:`3129` - Class Decorators + .. rubric:: Footnotes @@ -580,9 +641,9 @@ which is then bound to the class name. there is a :keyword:`finally` clause which happens to raise another exception. That new exception causes the old one to be lost. -.. [#] Currently, control "flows off the end" except in the case of an exception or the - execution of a :keyword:`return`, :keyword:`continue`, or :keyword:`break` - statement. +.. [#] Currently, control "flows off the end" except in the case of an exception + or the execution of a :keyword:`return`, :keyword:`continue`, or + :keyword:`break` statement. .. [#] A string literal appearing as the first statement in the function body is transformed into the function's ``__doc__`` attribute and therefore the |