bug or feature?

[Steven D'Aprano]

On Wed, 05 Oct 2005 03:39:30 -0700, beza1e1 wrote:

> Coming back from a bug hunt, i am not sure what to think of this python
> behaviour.

[snip code]

> The point seems to be, that lst=[] creates a class attribute (correct
> name?), which is shared by all instances of A. So a.lst ist the same
> object as b.lst, despite the fact, that object a is different to object
> b.

Not a bug, but not really a feature as such, it is a side-effect of
the way Python works. I guess that makes it a gotcha.

Argument defaults are set at compile time. You set the argument default to
a mutable object, an empty list. Every time you call the function,
you are appending to the same list.

This is not a problem if your argument default is a string, or a number,
or None, since these are all immutable objects that can't be changed.

I suppose someone might be able to come up with code that deliberately
uses this feature for good use, but in general it is something you want to
avoid. Instead of:

def spam(obj, L=[]):
    L.append(obj)

do this:

def spam(obj, L=None):
    if L is None: L = []
    L.append(obj)


-- 
Steven.