The mutable default argument in Python

>>> def a():
...     print "a executed"
...     return []
... 
>>>            
>>> def b(x=a()):
...     x.append(5)
...     print x
... 
a executed
>>> b()
[5]
>>> b()
[5, 5]

Actually, this is not a design flaw, and it is not because of internals, or performance. It comes simply from the fact that functions in Python are first-class objects, and not only a piece of code.

As soon as you get to think into this way, then it completely makes sense: a function is an object being evaluated on its definition; default parameters are kind of "member data" and therefore their state may change from one call to the other - exactly as in any other object.

In any case, Effbot has a very nice explanation of the reasons for this behavior in Default Parameter Values in Python. I found it very clear, and I really suggest reading it for a better knowledge of how function objects work.

Related Link: http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument