Source

sphinx / Doc-26 / reference / simple_stmts.rst

Simple statements

Simple statements are comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is:

Expression statements

Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value None). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:

An expression statement evaluates the expression list (which may be a single expression).

In interactive mode, if the value is not None, it is converted to a string using the built-in :func:`repr` function and the resulting string is written to standard output (see section :ref:`print`) on a line by itself. (Expression statements yielding None are not written, so that procedure calls do not cause any output.)

Assert statements

Assert statements are a convenient way to insert debugging assertions into a program:

The simple form, assert expression, is equivalent to

if __debug__:
   if not expression: raise AssertionError

The extended form, assert expression1, expression2, is equivalent to

if __debug__:
   if not expression1: raise AssertionError, expression2

These equivalences assume that __debug__ and :exc:`AssertionError` refer to the built-in variables with those names. In the current implementation, the built-in variable __debug__ is True under normal circumstances, False when optimization is requested (command line option -O). The current code generator emits no code for an assert statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.

Assignments to __debug__ are illegal. The value for the built-in variable is determined when the interpreter starts.

Assignment statements

Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects:

(See section :ref:`primaries` for the syntax definitions for the last three symbols.)

An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.

Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section :ref:`types`).

Assignment of an object to a target list is recursively defined as follows.

  • If the target list is a single target: The object is assigned to that target.
  • If the target list is a comma-separated list of targets: The object must be a sequence with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets. (This rule is relaxed as of Python 1.5; in earlier versions, the object had to be a tuple. Since strings are sequences, an assignment like a, b = "xy" is now legal as long as the string has the right length.)

Assignment of an object to a single target is recursively defined as follows.

  • If the target is an identifier (name):

  • If the name does not occur in a :keyword:`global` statement in the current

    code block: the name is bound to the object in the current local namespace.

  • Otherwise: the name is bound to the object in the current global namespace.

    The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called.

  • If the target is a target list enclosed in parentheses or in square brackets: The object must be a sequence with the same number of items as there are targets in the target list, and its items are assigned, from left to right, to the corresponding targets.

  • If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, :exc:`TypeError` is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily :exc:`AttributeError`).

  • If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object (such as a list) or a mapping object (such as a dictionary). Next, the subscript expression is evaluated.

    If the primary is a mutable sequence object (such as a list), the subscript must yield a plain integer. If it is negative, the sequence's length is added to it. The resulting value must be a nonnegative integer less than the sequence's length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range, :exc:`IndexError` is raised (assignment to a subscripted sequence cannot add new items to a list).

    If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping's key type, and the mapping is then asked to create a key/datum pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed).

  • If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type. Next, the lower and upper bound expressions are evaluated, insofar they are present; defaults are zero and the sequence's length. The bounds should evaluate to (small) integers. If either bound is negative, the sequence's length is added to it. The resulting bounds are clipped to lie between zero and the sequence's length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the object allows it.

(In the current implementation, the syntax for targets is taken to be the same as for expressions, and invalid syntax is rejected during the code generation phase, causing less detailed error messages.)

WARNING: Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are 'safe' (for example a, b = b, a swaps two variables), overlaps within the collection of assigned-to variables are not safe! For instance, the following program prints [0, 2]:

x = [0, 1]
i = 0
i, x[i] = 1, 2
print x

Augmented assignment statements

Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement:

(See section :ref:`primaries` for the syntax definitions for the last three symbols.)

An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once.

An augmented assignment expression like x += 1 can be rewritten as x = x + 1 to achieve a similar, but not exactly equal effect. In the augmented version, x is only evaluated once. Also, when possible, the actual operation is performed in-place, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.

With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible in-place behavior, the binary operation performed by augmented assignment is the same as the normal binary operations.

For targets which are attribute references, the initial value is retrieved with a :meth:`getattr` and the result is assigned with a :meth:`setattr`. Notice that the two methods do not necessarily refer to the same variable. When :meth:`getattr` refers to a class variable, :meth:`setattr` still writes to an instance variable. For example:

class A:
    x = 3    # class variable
a = A()
a.x += 1     # writes a.x as 4 leaving A.x as 3

The :keyword:`pass` statement

:keyword:`pass` is a null operation --- when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:

def f(arg): pass    # a function that does nothing (yet)

class C: pass       # a class with no methods (yet)

The :keyword:`del` statement

Deletion is recursively defined very similar to the way assignment is defined. Rather that spelling it out in full details, here are some hints.

Deletion of a target list recursively deletes each target, from left to right.

Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a :keyword:`global` statement in the same code block. If the name is unbound, a :exc:`NameError` exception will be raised.

It is illegal to delete a name from the local namespace if it occurs as a free variable in a nested block.

Deletion of attribute references, subscriptions and slicings is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object).

The :keyword:`print` statement

:keyword:`print` evaluates each expression in turn and writes the resulting object to standard output (see below). If an object is not a string, it is first converted to a string using the rules for string conversions. The (resulting or original) string is then written. A space is written before each object is (converted and) written, unless the output system believes it is positioned at the beginning of a line. This is the case (1) when no characters have yet been written to standard output, (2) when the last character written to standard output is '\n', or (3) when the last write operation on standard output was not a :keyword:`print` statement. (In some cases it may be functional to write an empty string to standard output for this reason.)

Note

Objects which act like file objects but which are not the built-in file objects often do not properly emulate this aspect of the file object's behavior, so it is best not to rely on this.

A '\n' character is written at the end, unless the :keyword:`print` statement ends with a comma. This is the only action if the statement contains just the keyword :keyword:`print`.

Standard output is defined as the file object named stdout in the built-in module :mod:`sys`. If no such object exists, or if it does not have a :meth:`write` method, a :exc:`RuntimeError` exception is raised.

:keyword:`print` also has an extended form, defined by the second portion of the syntax described above. This form is sometimes referred to as ":keyword:`print` chevron." In this form, the first expression after the >> must evaluate to a "file-like" object, specifically an object that has a :meth:`write` method as described above. With this extended form, the subsequent expressions are printed to this file object. If the first expression evaluates to None, then sys.stdout is used as the file for output.

The :keyword:`return` statement

:keyword:`return` may only occur syntactically nested in a function definition, not within a nested class definition.

If an expression list is present, it is evaluated, else None is substituted.

:keyword:`return` leaves the current function call with the expression list (or None) as return value.

When :keyword:`return` passes control out of a :keyword:`try` statement with a :keyword:`finally` clause, that :keyword:`finally` clause is executed before really leaving the function.

In a generator function, the :keyword:`return` statement is not allowed to include an :token:`expression_list`. In that context, a bare :keyword:`return` indicates that the generator is done and will cause :exc:`StopIteration` to be raised.

The :keyword:`yield` statement

The :keyword:`yield` statement is only used when defining a generator function, and is only used in the body of the generator function. Using a :keyword:`yield` statement in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.

When a generator function is called, it returns an iterator known as a generator iterator, or more commonly, a generator. The body of the generator function is executed by calling the generator's :meth:`next` method repeatedly until it raises an exception.

When a :keyword:`yield` statement is executed, the state of the generator is frozen and the value of :token:`expression_list` is returned to :meth:`next`'s caller. By "frozen" we mean that all local state is retained, including the current bindings of local variables, the instruction pointer, and the internal evaluation stack: enough information is saved so that the next time :meth:`next` is invoked, the function can proceed exactly as if the :keyword:`yield` statement were just another external call.

As of Python version 2.5, the :keyword:`yield` statement is now allowed in the :keyword:`try` clause of a :keyword:`try` ... :keyword:`finally` construct. If the generator is not resumed before it is finalized (by reaching a zero reference count or by being garbage collected), the generator-iterator's :meth:`close` method will be called, allowing any pending :keyword:`finally` clauses to execute.

Note

In Python 2.2, the :keyword:`yield` statement is only allowed when the generators feature has been enabled. It will always be enabled in Python 2.3. This __future__ import statement can be used to enable the feature:

from __future__ import generators

The :keyword:`raise` statement

If no expressions are present, :keyword:`raise` re-raises the last exception that was active in the current scope. If no exception is active in the current scope, a :exc:`TypeError` exception is raised indicating that this is an error (if running under IDLE, a :exc:`Queue.Empty` exception is raised instead).

Otherwise, :keyword:`raise` evaluates the expressions to get three objects, using None as the value of omitted expressions. The first two objects are used to determine the type and value of the exception.

If the first object is an instance, the type of the exception is the class of the instance, the instance itself is the value, and the second object must be None.

If the first object is a class, it becomes the type of the exception. The second object is used to determine the exception value: If it is an instance of the class, the instance becomes the exception value. If the second object is a tuple, it is used as the argument list for the class constructor; if it is None, an empty argument list is used, and any other object is treated as a single argument to the constructor. The instance so created by calling the constructor is used as the exception value.

If a third object is present and not None, it must be a traceback object (see section :ref:`traceback`), and it is substituted instead of the current location as the place where the exception occurred. If the third object is present and not a traceback object or None, a :exc:`TypeError` exception is raised. The three-expression form of :keyword:`raise` is useful to re-raise an exception transparently in an except clause, but :keyword:`raise` with no expressions should be preferred if the exception to be re-raised was the most recently active exception in the current scope.

Additional information on exceptions can be found in section :ref:`exceptions`, and information about handling exceptions is in section :ref:`try`.

The :keyword:`break` statement

:keyword:`break` may only occur syntactically nested in a :keyword:`for` or :keyword:`while` loop, but not nested in a function or class definition within that loop.

It terminates the nearest enclosing loop, skipping the optional :keyword:`else` clause if the loop has one.

If a :keyword:`for` loop is terminated by :keyword:`break`, the loop control target keeps its current value.

When :keyword:`break` passes control out of a :keyword:`try` statement with a :keyword:`finally` clause, that :keyword:`finally` clause is executed before really leaving the loop.

The :keyword:`continue` statement

:keyword:`continue` may only occur syntactically nested in a :keyword:`for` or :keyword:`while` loop, but not nested in a function or class definition or :keyword:`finally` statement within that loop. [1] It continues with the next cycle of the nearest enclosing loop.

The :keyword:`import` statement

Import statements are executed in two steps: (1) find a module, and initialize it if necessary; (2) define a name or names in the local namespace (of the scope where the :keyword:`import` statement occurs). The first form (without :keyword:`from`) repeats these steps for each identifier in the list. The form with :keyword:`from` performs step (1) once, and then performs step (2) repeatedly.

In this context, to "initialize" a built-in or extension module means to call an initialization function that the module must provide for the purpose (in the reference implementation, the function's name is obtained by prepending string "init" to the module's name); to "initialize" a Python-coded module means to execute the module's body.

The system maintains a table of modules that have been or are being initialized, indexed by module name. This table is accessible as sys.modules. When a module name is found in this table, step (1) is finished. If not, a search for a module definition is started. When a module is found, it is loaded. Details of the module searching and loading process are implementation and platform specific. It generally involves searching for a "built-in" module with the given name and then searching a list of locations given as sys.path.

If a built-in module is found, its built-in initialization code is executed and step (1) is finished. If no matching file is found, :exc:`ImportError` is raised. If a file is found, it is parsed, yielding an executable code block. If a syntax error occurs, :exc:`SyntaxError` is raised. Otherwise, an empty module of the given name is created and inserted in the module table, and then the code block is executed in the context of this module. Exceptions during this execution terminate step (1).

When step (1) finishes without raising an exception, step (2) can begin.

The first form of :keyword:`import` statement binds the module name in the local namespace to the module object, and then goes on to import the next identifier, if any. If the module name is followed by :keyword:`as`, the name following :keyword:`as` is used as the local name for the module.

The :keyword:`from` form does not bind the module name: it goes through the list of identifiers, looks each one of them up in the module found in step (1), and binds the name in the local namespace to the object thus found. As with the first form of :keyword:`import`, an alternate local name can be supplied by specifying ":keyword:`as` localname". If a name is not found, :exc:`ImportError` is raised. If the list of identifiers is replaced by a star ('*'), all public names defined in the module are bound in the local namespace of the :keyword:`import` statement..

The public names defined by a module are determined by checking the module's namespace for a variable named __all__; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in __all__ are all considered public and are required to exist. If __all__ is not defined, the set of public names includes all names found in the module's namespace which do not begin with an underscore character ('_'). __all__ should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).

The :keyword:`from` form with * may only occur in a module scope. If the wild card form of import --- import * --- is used in a function and the function contains or is a nested block with free variables, the compiler will raise a :exc:`SyntaxError`.

Hierarchical module names: when the module names contains one or more dots, the module search path is carried out differently. The sequence of identifiers up to the last dot is used to find a "package"; the final identifier is then searched inside the package. A package is generally a subdirectory of a directory on sys.path that has a file :file:`__init__.py`. [XXX Can't be bothered to spell this out right now; see the URL http://www.python.org/doc/essays/packages.html for more details, also about how the module search works from inside a package.]

The built-in function :func:`__import__` is provided to support applications that determine which modules need to be loaded dynamically; refer to Built-in Functions (XXX reference: ../lib/built-in-funcs.html) in the Python Library Reference (XXX reference: ../lib/lib.html) for additional information.

Future statements

A :dfn:`future statement` is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python. The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.

A future statement must appear near the top of the module. The only lines that can appear before a future statement are:

  • the module docstring (if any),
  • comments,
  • blank lines, and
  • other future statements.

The features recognized by Python 2.5 are absolute_import, division, generators, nested_scopes and with_statement. generators and nested_scopes are redundant in Python version 2.3 and above because they are always enabled.

A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.

For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.

The direct runtime semantics are the same as for any import statement: there is a standard module :mod:`__future__`, described later, and it will be imported in the usual way at the time the future statement is executed.

The interesting runtime semantics depend on the specific feature enabled by the future statement.

Note that there is nothing special about the statement:

import __future__ [as name]

That is not a future statement; it's an ordinary import statement with no special semantics or syntax restrictions.

Code compiled by an :keyword:`exec` statement or calls to the builtin functions :func:`compile` and :func:`execfile` that occur in a module :mod:`M` containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can, starting with Python 2.2 be controlled by optional arguments to :func:`compile` --- see the documentation of that function in the Python Library Reference (XXX reference: ../lib/built-in-funcs.html) for details.

A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the :option:`-i` option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed.

The :keyword:`global` statement

The :keyword:`global` statement is a declaration which holds for the entire current code block. It means that the listed identifiers are to be interpreted as globals. It would be impossible to assign to a global variable without :keyword:`global`, although free variables may refer to globals without being declared global.

Names listed in a :keyword:`global` statement must not be used in the same code block textually preceding that :keyword:`global` statement.

Names listed in a :keyword:`global` statement must not be defined as formal parameters or in a :keyword:`for` loop control target, :keyword:`class` definition, function definition, or :keyword:`import` statement.

(The current implementation does not enforce the latter two restrictions, but programs should not abuse this freedom, as future implementations may enforce them or silently change the meaning of the program.)

Programmer's note: the :keyword:`global` is a directive to the parser. It applies only to code parsed at the same time as the :keyword:`global` statement. In particular, a :keyword:`global` statement contained in an :keyword:`exec` statement does not affect the code block containing the :keyword:`exec` statement, and code contained in an :keyword:`exec` statement is unaffected by :keyword:`global` statements in the code containing the :keyword:`exec` statement. The same applies to the :func:`eval`, :func:`execfile` and :func:`compile` functions.

The :keyword:`exec` statement

This statement supports dynamic execution of Python code. The first expression should evaluate to either a string, an open file object, or a code object. If it is a string, the string is parsed as a suite of Python statements which is then executed (unless a syntax error occurs). If it is an open file, the file is parsed until EOF and executed. If it is a code object, it is simply executed. In all cases, the code that's executed is expected to be valid as file input (see section :ref:`file-input`, "File input"). Be aware that the :keyword:`return` and :keyword:`yield` statements may not be used outside of function definitions even within the context of code passed to the :keyword:`exec` statement.

In all cases, if the optional parts are omitted, the code is executed in the current scope. If only the first expression after :keyword:`in` is specified, it should be a dictionary, which will be used for both the global and the local variables. If two expressions are given, they are used for the global and local variables, respectively. If provided, locals can be any mapping object.

As a side effect, an implementation may insert additional keys into the dictionaries given besides those corresponding to variable names set by the executed code. For example, the current implementation may add a reference to the dictionary of the built-in module :mod:`__builtin__` under the key __builtins__ (!).

Programmer's hints: dynamic evaluation of expressions is supported by the built-in function :func:`eval`. The built-in functions :func:`globals` and :func:`locals` return the current global and local dictionary, respectively, which may be useful to pass around for use by :keyword:`exec`.

Footnotes

[1]It may occur within an :keyword:`except` or :keyword:`else` clause. The restriction on occurring in the :keyword:`try` clause is implementor's laziness and will eventually be lifted.
Tip: Filter by directory path e.g. /media app.js to search for public/media/app.js.
Tip: Use camelCasing e.g. ProjME to search for ProjectModifiedEvent.java.
Tip: Filter by extension type e.g. /repo .js to search for all .js files in the /repo directory.
Tip: Separate your search with spaces e.g. /ssh pom.xml to search for src/ssh/pom.xml.
Tip: Use ↑ and ↓ arrow keys to navigate and return to view the file.
Tip: You can also navigate files with Ctrl+j (next) and Ctrl+k (previous) and view the file with Ctrl+o.
Tip: You can also navigate files with Alt+j (next) and Alt+k (previous) and view the file with Alt+o.