gistfile1.py

# -*- coding: utf-8 -*-

# foo()'s a_value is unmodified.
def foo():
    a_value = "Hello"

    def bar():
        a_value = "Goodbye"

    bar()
    print a_value # -> "Hello"

foo() # -> "Hello"

# But can be read.
def foo():
    a_value = "Hello"

    def bar():
        print a_value

    bar()
    print a_value # -> "Hello"

foo() # -> "Hello"

# If it's set in bar(), it becomes a different var, even before the
# assignment.
def foo():
    a_value = "Hello"

    def bar():
        print a_value
        a_value = "Goodbye"

    bar()
    print a_value

try:
    foo()
except Exception, e:
    print e # -> UnboundLocalError: local variable 'a_value' referenced
            #    before assignment

# Attributes of vars can be changed.
def foo():
    some_values = {'a_value': "Hello"}

    def bar():
        print some_values
        some_values['a_value'] = "Goodbye"

    bar()
    print some_values  # -> {'a_value': 'Hello'}


foo() # -> {'a_value': 'Goodbye'}