pytutorial/python_basics/namespaces

Python Scopes and Namespaces

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The goal

Questions to David Rotermund

Shortened quotes from https://docs.python.org/3/tutorial/classes.html

Scopes and Namespaces Example

This is an example demonstrating how to reference the different scopes and namespaces, and how globalhttps://docs.python.org/3/reference/simple_stmts.html#global and nonlocalhttps://docs.python.org/3/reference/simple_stmts.html#nonlocal affect variable binding:

def scope_test():
    def do_local():
        spam = "local spam"

    def do_nonlocal():
        nonlocal spam
        spam = "nonlocal spam"

    def do_global():
        global spam
        spam = "global spam"

    spam = "test spam"
    do_local()
    print("After local assignment:", spam) # -> After local assignment: test spam
    do_nonlocal()
    print("After nonlocal assignment:", spam) # -> After nonlocal assignment: nonlocal spam
    do_global()
    print("After global assignment:", spam) # -> After global assignment: nonlocal spam

scope_test()
print("In global scope:", spam) # -> In global scope: global spam

Note how the local assignment (which is default) didn’t change scope_test's binding of spam. The nonlocal assignment changed scope_test's binding of spam, and the global assignment changed the module-level binding.

You can also see that there was no previous binding for spam before the global assignment.

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. It would be impossible to assign to a global variable without global, although free variables may refer to globals without being declared global.

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

Programmer’s note: 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 a string or code object supplied to the built-in exec() function does not affect the code block containing the function call, and code contained in such a string is unaffected by global statements in the code containing the function call. The same applies to the eval() and compile() functions.

The nonlocal statement

nonlocal_stmt ::=  "nonlocal" identifier ("," identifier)*

The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope excluding globals. This is important because the default behavior for binding is to search the local namespace first. The statement allows encapsulated code to rebind variables outside of the local scope besides the global (module) scope.

Names listed in a nonlocal statement, unlike those listed in a global statement, must refer to pre-existing bindings in an enclosing scope (the scope in which a new binding should be created cannot be determined unambiguously).

Names listed in a nonlocal statement must not collide with pre-existing bindings in the local scope.