Python 1.5 Reference Manual
simple_stmt: expression_stmt
| assert_stmt
| assignment_stmt
| pass_stmt
| del_stmt
| print_stmt
| return_stmt
| raise_stmt
| break_stmt
| continue_stmt
| import_stmt
| global_stmt
| exec_stmt
expression_stmt: expression_listAn 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
repr() function and the resulting string is written to standard output (see "The print statement" on page 42) on a line by itself. (Expression statements yielding None are not written, so that procedure calls do not cause any output.)
assert_statement: "assert" expression ["," expression]The simple form, "
assert expression", is equivalent to
if __debug__: if not expression: raise AssertionErrorThe extended form, "
assert expression1, expression2", is equivalent to
if __debug__: if not expression1: raise AssertionError, expression2These equivalences assume that
__debug__ and AssertionError refer to the built-in variables with those names. In the current implementation, the built-in variable __debug__ is 1 under normal circumstances, 0 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.
assignment_stmt: (target_list "=")+ expression_list
target_list: target ("," target)* [","]
target: identifier | "(" target_list ")" | "[" target_list "]"
| attributeref | subscription | slicing
(See "Primaries" on page 29 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 "The standard type hierarchy" on page 12.)
Assignment of an object to a target list is recursively defined as follows.
Warning: Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are 'safe' (e.g. ''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
pass_stmt: "pass"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)
del_stmt: "del" target_listDeletion 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 (which must exist) from the local or global name space, depending on whether the name occurs in a global statement in the same code 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).
print_stmt: "print" [ expression ("," expression)* [","] ]
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; or (2) when the last character written to standard output is \n; or (3) when the last write operation on standard output was not a print statement. (In some cases it may be functional to write an empty string to standard output for this reason.)
A "\n" character is written at the end, unless the print statement ends with a comma. This is the only action if the statement contains just the keyword print. Standard output is defined as the object named stdout in the built-in module sys. If no such object exists, or if it does not have a write() method, an exception is raised.
return_stmt: "return" [expression_list]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.
return leaves the current function call with the expression list (or None) as return value.
When return passes control out of a try statement with a finally clause, that finally clause is executed before really leaving the function.
raise_stmt: "raise" expression ["," expression ["," expression]]raise evaluates its first expression, which must yield a string, class, or instance object. If there is a second expression, this is evaluated, else None is substituted. If the first expression is a class object, then the second expression must be an instance of that class or one of its derivatives. If the first expression is an instance object, the second expression must be None.
If the first object is a class or string, it then raises the exception identified by the first object, with the second one (or None) as its parameter. If the first object is an instance, it raises the exception identified by the class of the object, with the instance as its parameter (and there should be no second object, or the second object should be None).
If a third object is present, and it is not None, it should be a traceback object (see page 17 traceback objects), and it is substituted instead of the current location as the place where the exception occurred. This is useful to re-raise an exception transparently in an except clause.
break_stmt: "break"break may only occur syntactically nested in a for or while loop, but not nested in a function or class definition within that loop.
It terminates the nearest enclosing loop, skipping the optional else clause if the loop has one.
If a for loop is terminated by break, the loop control target keeps its current value.
When break passes control out of a try statement with a finally clause, that finally clause is executed before really leaving the loop.
continue_stmt: "continue"continue may only occur syntactically nested in a for or while loop, but not nested in a function or class definition or try statement within that loop.[5] It continues with the next cycle of the nearest enclosing loop.
import_stmt: "import" module ("," module)*
| "from" module "import" identifier ("," identifier)*
| "from" module "import" "*"
module: (identifier ".")* identifier
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 name space (of the scope where the import statement occurs). The first form (without from) repeats these steps for each identifier in the list. The form with from performs step (1) once, and then performs step (2) repeatedly.
The system maintains a table of modules that have been initialized, indexed by module name. (The current implementation makes this table 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.
When step (1) finishes without raising an exception, step (2) can begin.
The first form of import statement binds the module name in the local name space to the module object, and then goes on to import the next identifier, if any. The 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 name space to the object thus found. If a name is not found, ImportError is raised. If the list of identifiers is replaced by a star (*), all names defined in the module are bound, except those beginning with an underscore(_).
Names bound by import statements should not occur in global statements in the same scope.
The from form with * should only occur in a module scope.
(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.)
Hierarchical module names: when the module names contains aone 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. [XXX Can't be bothered to spell this out right now; see the URL http://grail.cnri.reston.va.us/python/essays/packages.hmtl for more details, also about how the module search works from inside a package.]
global_stmt: "global" identifier ("," identifier)*
The 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. While using global names is automatic if they are not defined in the local scope, assigning to global names would be impossible without global. Names listed in a global statement must not be used in the same code block before that global statement is executed.
Names listed in a global statement must not be defined as formal parameters or in a for loop control target, class definition, function definition, or 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 global is a directive to the parser. It applies only to code parsed at the same time as the global statement. In particular, a global statement contained in an exec statement does not affect the code block containing the exec statement, and code contained in an exec statement is unaffected by global statements in the code containing the exec statement. The same applies to the eval(), execfile() and compile() functions.
exec_stmt: "exec" expression ["in" expression ["," expression]]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, if the optional parts are omitted, the code is executed in the current scope. If only the first expression after 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, both must be dictionaries and they are used for the global and local variables, respectively.
Programmer's hints: dynamic evaluation of expressions is supported by the built-in function eval(). The built-in functions globals() and locals() return the current global and local dictionary, respectively, which may be useful to pass around for use by exec. When assigning to a global variable, a global statement for that variable should be present in the source code string passed to the exec statement.
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