Is Python Dictionary Really Mutable?

Yes, Python Dictionary is mutable. Changing references to keys doesn't lead to the creation of new dictionaries. Rather it updates the current dictionary in place. 

example

a = {'foo': 1, 'bar': 12}
b = a
b['foo'] = 20

print(a)
print(b)

Output

This will give the output −

{'foo': 20, 'bar': 12}
{'foo': 20, 'bar': 12}