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Python 1.5 Reference Manual

# Chapter 6: 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:

    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

## 6.1 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:

    expression_stmt: expression_list

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 `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.)

## 6.2 Assert statements

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

    assert_statement: "assert" expression ["," expression]

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 `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.

## 6.3 Assignment statements

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

    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.

- 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 has been relaxed since 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.)

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 global statement in the current code block: the name is bound to the object in the current local name space.

- Otherwise: the name is bound to the object in the current global name space.

  The name is rebound if it was already bound. This can cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its

- If the target is a target list enclosed in parentheses or square brackets: the object must be a sequence with the same number of items 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, 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 AttributeError).

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

  If the primary is a mutable sequence object (e.g. 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, IndexError is raised (assignment to a subscripted sequence cannot add new items to a list).

  If the primary is a mapping object (e.g. 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 (e.g. 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' (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

## 6.4 The pass statement

    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)

## 6.5 The del statement

    del_stmt:       "del" target_list

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 (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).

## 6.6 The print statement

    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.

## 6.7 The return statement

    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.

## 6.8 The raise statement

    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.

## 6.9 The break statement

    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.

## 6.10 The continue statement

    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.

## 6.11 The import statement

    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.]

## 6.12 The global statement

    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.

## 6.13 The exec statement

    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.

[5] Except that it may currently occur within an except clause.

6.1 - Expression statements
6.2 - Assert statements
6.3 - Assignment statements
6.4 - The pass statement
6.5 - The del statement
6.6 - The print statement
6.7 - The return statement
6.8 - The raise statement
6.9 - The break statement
6.10 - The continue statement
6.11 - The import statement
6.12 - The global statement
6.13 - The exec statement

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