Demo of Python one-time eval of function declaration default params, used for caching.

private_cache_in_func.py

def foo(a, b=None, cache={}):
  if b is not None:
    cache[a] = b
    return b
  if a in cache:
    return cache[a]
  raise KeyError('Bleh, no %s' % a)

Grabbed this snippet to test gist.

Note that normally using a mutable object as a default value for a function param is a bad idea, but this is the rare case where it's the correct approach and turns what's normally a programming bug into a neat hack. [The default value is evaluated when the function definition is executed, not when the function is invoked, and the result is used for each call to the function]