where is this documented?

[DeaD_EyE]

foo = {}.update(os.environ)

foo is None because the method update returns None.
You can do this technique with data types, which returns a new copy of them selfs if mutated (str does this).

It's the same pitfall like this:

sorted_list = [1,4,5,6,7,3,2].sort()

list.sort() makes an inline sort (mutating the object itself) and returns None.