The corrosion of Aaron Stone


Python's Lame Lexical Closures

Here’s a simple generator function in Python that you can use in a test harness to find out how many times the function was called. Call the function once with an argument, and then each time you call the function again it will return that argument and increment an internal counter. Call the function with the named argument ‘got_called’ and it will reset and return its current call count.

def GotCalled(ret):
  """Stub that counts how many times it was called."""
  count = [0]

  def _GotCalled(*unused_args, **kwargs):
    if kwargs.get('got_called'):
      lame = count[0]
      count[0] = 0
      return lame
      count[0] += 1
      return ret

  return _GotCalled

But note how count is a list. And I’m incrementing and returning the first member of the list. Guess what – you can’t use a variable directly in this situation! It’ll error out on the ‘count += 1’ because the act of assigning to the variable in the inner function creates a new instance of that variable, one that hasn’t been used before and so fails the + part of the +=. LAME.