python: pitfalls

http://zephyrfalcon.org/labs/python_pitfalls.html

5. Mutable default arguments

This one bites beginners over and over again. It's really a variant of #2, combined with unexpected behavior of default arguments. Consider this function:

>>> def popo(x=[]):
...     x.append(666)
...     print x
...     
>>> popo([1, 2, 3])
[1, 2, 3, 666]
>>> x = [1, 2]
>>> popo(x)
[1, 2, 666]
>>> x
[1, 2, 666]

This was expected. But now:

>>> popo()
[666]
>>> popo()
[666, 666]
>>> popo()
[666, 666, 666]

Maybe you expected that the output would be [666] in all cases... after all, when popo() is called without arguments, it takes [] as the default argument for x, right? Wrong. The default argument is bound *once*, when the function is *created*, not when it's called. (In other words, for a function f(x=[]), x is *not* bound whenever the function is called. x got bound to [] when we defined f, and that's it.) So if it's a mutable object, and it has changed, then the next function call will take this same list (which has different contents now) as its default argument.
Solution: This behavior can occasionally be useful. In general, just watch out for unwanted side effects.