Sunday, May 4, 2014

Storing state in a Python function

This one blew me away. You can store state in a function, just like you would any object. These are called function attributes. As an aside, I also learned that using a try: except clause is ever slightly so faster than an if.

def foo(a):
  try:
    foo.b += a
  except AttributeError:
    foo.b = a
  return foo.b

def foo2(a):
  if hasattr(foo2, 'b'):
    foo2.b += a
  else:
    foo2.b = a
  return foo2.b

if __name__ == '__main__':
  print [foo(x) for x in range(10)]
  print [foo2(x) for x in range(10)]

"""
python -mtimeit -s'import test' '[test.foo(x) for x in range(100)]'
python -mtimeit -s'import test' '[test.foo2(x) for x in range(100)]'
"""

Giving us:

python test.py
[0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
[0, 1, 3, 6, 10, 15, 21, 28, 36, 45]

python -mtimeit -s'import test' '[test.foo(x) for x in range(100)]'  ->  10000 loops, best of 3: 29.4 usec per loop
python -mtimeit -s'import test' '[test.foo2(x) for x in range(100)]' -> 10000 loops, best of 3: 39.1 usec per loop

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