Best way to deal with default params in Python?

Question

I've got some code (that someone else wrote):

def render(self, name, value, attrs=None):
    if not attrs:
        attrs = {}
    attrs.update({'class': 'ui-autocomplete-input'})

which I think could be re-written as:

def render(self, name, value, attrs={}):
    attrs.update({'class': 'ui-autocomplete-input'})

This would of course fail if someone passed in attrs=None, but is that something to be expected? Is there a penalty for the attrs={} (extra wasted dict creation?)

I'm still too new to python to answer these questions, but I am curious.

When I test this in the interpreter, I get:

>>> def x(attrs={}):
...   attrs.update({'a': 'b'})
...   print attrs
... 
>>> x({'b': 'c'})
{'a': 'b', 'b': 'c'}
>>> x({'d': 'e'})
{'a': 'b', 'd': 'e'}
>>> x()
{'a': 'b'}

How does this ever cause a problem? Note that I'm ALWAYS adding that dict to the attrs, even if the user specified one (which may actually point to a different problem (I should probably merge the class attribute with an existing one if present).

------------------- And to point out the flaw in the above ------------------

>>> def yikes(attrs):
...   attrs.update({'something': 'extra'})
...   print attrs
>>> def x(attrs={}):
...   print "Before:", attrs
...   attrs.update({'a': 'b'})
...   print "After:", attrs
...   yikes(attrs)

>>> x({"b": "c"})
Before: {'b': 'c'}
After: {'a': 'b', 'b': 'c'}
{'a': 'b', 'b': 'c', 'something': 'extra'}
>>> x()
Before: {}
After: {'a': 'b'}
{'a': 'b', 'something': 'extra'}

Still seems ok, what's the problem?

>>> x()
Before: {'a': 'b', 'something': 'extra'}
After: {'a': 'b', 'something': 'extra'}
{'a': 'b', 'something': 'extra'}

Ahhh, now I get it, if {'something': 'extra'} is added by some other bit of code, that never gets cleaned up. Sure, the attrs I force to be there are there, but so is the {'something': 'extra'} that shouldn't be. This is subtle enough to be good fodder for an obfuscation contest, lookout PERL!

Answer 1

Using attrs = {} in the function signature will bite you, since it will keep its value on successive calls to the function. The original code is best.

eg.

>>> def a(attrs= {}):
...   print attrs
...   attrs.update({1:1})
...
>>> a()
{}
>>> a()
{1: 1}

Note how it kept the value assigned the first time, on the second call.