Pythonic way of setting a default

Question

Is there a simple way of of setting a default in python - specifically setting a default in a dict?

For instance, let's say I have a dict called foo, which may or may not have something assigned on the key bar. The verbose way of doing this is:

if not foo.has_key('bar'):
 foo['bar'] = 123

One alternative would be:

foo['bar'] = foo.get('bar',123)

Is there some standard python way of doing this - something like the following, but that actually works?

foo['bar'] ||= 123

• jfs: don't use has_key() use 'bar' in foo to test for membership.
• Jamie Wong: @J.F. Sebastian Why? has_key seems more explicit to me - is there some case where 'bar' in foo is more appropriate? Besides when foo isn't necessarily a dict?
• jfs: has_key() is deprecated docs.python.org/library/stdtypes.html#dict.has_key (it is removed since python3.0). The reason might be "There should be one-- and preferably only one --obvious way to do it."
• Jamie Wong: @J. F. Sebastian - good to know - thanks.

Answer 1

Doesn't anyone read the documentation?

foo.setdefault('bar', 123)

Answer 2

You could check out defaultdict

• Jamie Wong: +1 Since that looks interesting for future purpose. Unfortunately, right now, I'm dealing with request.session in Django, so I can't redefine it to be a defaultdict.

Answer 3

(Wrong first part of the answer edited away)

Dicts have a setdefault() method that works just as get(), only it inserts the value if the key was missing.

foo.setdefault('bar', 123)

Cheers.

• D.Shawley: foo.get('bar') or 123 is quite different than checking foo.has_key('bar'). Consider foo['bar'] = 0.