Experiments with scope in Python.

ex1.py

"""
Python scope is not dynamic.
"""

def foo():
  print x

x = 0


def bar(func):
  x = 1
  func()


foo()  # prints 0
bar(foo)  # prints 0

ex2.py

"""
Non-global variables that are in scope are strictly read-only.

In Python 3, this restriction is removed via the `nonlocal` keyword.
"""

x = 10

def foo():
    global x
    x += 5
    print x
    
foo()  # prints 15


def bar():
    x = 10
    def foobar():
        # global x # uncommenting this line doesn't help
        x += 5
        print x
    foobar()
    
bar()  # causes "UnboundLocalError: local variable 'x' referenced before assignment"


def car():
    print car
    
car()  # prints <function car at 0x...>


def dar():
    dar = 1
    print dar

dar()  # prints 1


ear = 1

def ear():
    print ear
    
ear()  # prints <function ear at 0x...>


def far():
    far = far
    print far
    
far()  # UnboundLocalError: local variable 'far' referenced before assignment

ex3.py

def enclosing_function():
    def factorial(n):
        if n < 2:
            return 1
        return n * factorial(n - 1)  # does not fail with NameError
    print factorial(5)

enclosing_function()  # prints 120

retry.py

import functools
import time


class TimeoutException(Exception):
    pass


def retry(exceptions, truncate=5, delay=0.25):
    """Retry the decorated function using an exponential backoff strategy.
    If the function does not complete successfully a TimeoutException is
    raised."""
    def wrapper(func):
        @functools.wraps(func)
        def wrapped(*args, **kwargs):
            tries = 0
            while tries < truncate:
                try:
                    return func(*args, **kwargs)
                except exceptions, e:
                    print "%s, Retrying in %d seconds..." % (str(e), delay)
                    time.sleep(delay)
                    delay += delay  # W804 local variable 'delay' (defined in enclosing scope on line 9) referenced before assignment
                    tries += 1
            else:
                raise TimeoutException()
        return wrapped
    return wrapper

Lots of reading to do. http://www.python.org/dev/peps/pep-3104/#id42