Let's play 'Feature or Defect!'

Lolwhut?!

Sometimes you run into programming language “features” that make you wonder if that was the intended effect. This particular one involves python and a mutable default function argument. Here’s a quick example:

>>> def foo(a=[]):
...   a.append(1)
...   return a
... 
>>> foo()
[1]
>>> foo()
[1, 1]
>>> foo()
[1, 1, 1]
>>> foo()
[1, 1, 1, 1]

On the one hand, this can be used as a weird little way of caching items. Since a is mutable, it acts as a global variable. To me, this is counterintuitive. If I say, def foo(a=[]): I expect a to be an empty list every time I run that function unless I tell it otherwise.