I ran into this problem when working with defaultdict. Here’s a program that demonstrates it:

from collections import defaultdict

d1 = defaultdict(default_factory=dict)
d2 = defaultdict(dict)

print("d1's default_factory:", d1.default_factory)
print("d2's default_factory:", d2.default_factory)

try:
    d1['key'].update({'a': 'b'})
except KeyError:
    print("d1 caused an exception")

try:
    d2['key'].update({'a': 'b'})
except KeyError:
    print("d2 caused an exception")

The above outputs:

d1's default_factory: None
d2's default_factory: <class 'dict'>
d1 caused an exception

Should this happen?