Question

I know you can use setdefault(key, value) to set default value for a given key, but is there a way to set default values of all keys to some value after creating a dict ?

Put it another way, I want the dict to return the specified default value for every key I didn’t yet set.

Answer 1

You can replace your old dictionary with a defaultdict:

>>> from collections import defaultdict
>>> d = {'foo': 123, 'bar': 456}
>>> d['baz']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'baz'
>>> d = defaultdict(lambda: -1, d)
>>> d['baz']
-1

The “trick” here is that a defaultdict can be initialized with another dict. This means
that you preserve the existing values in your normal dict:

>>> d['foo']
123