The os.environ documentation does state it’s a mapping:
A mapping object representing the string environment.
As such it behaves according to the python mapping documentation of which dict is the standard implementation.
os.environ therefor behaves just like the standard dict, it has all the same methods:
>>> import os
>>> len(os.environ)
36
>>> 'USER' in os.environ
True
>>> os.environ.fromkeys
<bound method classobj.fromkeys of <class os._Environ at 0x107096ce8>>
The .setdefault method is documented on the same page as the rest of the mapping methods, and you can use it just fine as is.