Can't directly update a dict deepcopy

I can update a deepcopy of a dictionary in two steps. But when I try to make it a single step it returns None.

from copy import deepcopy

d0 = {'a': 1}
d1 = deepcopy(d0)
d1.update([('b', 2)])
print(d0, d1)

d0 = {'a': 1}
d1 = deepcopy(d0).update([('b', 2)])
print(d0, d1)

Output:

{'a': 1} {'a': 1, 'b': 2}
{'a': 1} None

Why? How to?

Thanks to @tkaus now I know update returns None. This is what I would like to do:

d0 = {'a': 1}
d = {
    'x': deepcopy(d0).update([('b', 2)]),
    'y': deepcopy(d0).update([('b', 3)]),
}

Any ideas?

Why? Because dict.update always operates in place and returns None.

How? You can invoke an in-place union:

>>> d0 = {"a": [1]}
>>> d1 = deepcopy(d0).__ior__([("b", 2)])
>>> d0["a"].append(3)
>>> print(d0, d1)
{'a': [1, 3]} {'a': [1], 'b': 2}

Note that in example cases shown in the original question, a deepcopy isn't required. The values are immutable, so you can safely shallow copy and update in a single operation, which will be simpler and more efficient:

>>> d0 = {"a": 1}
>>> d1 = d0 | {"b": 2}  # or: d1 = {**d0, "b": 2}
>>> print(d0, d1)
{'a': 1} {'a': 1, 'b': 2}