Question

Using this code, many keys are output, but I expected no output:

import os

for i in os.environ:
    print  i

This is the code from os.py:

try:
    environ
except NameError:
    environ = {}

Where does os.environ get its values from? Where is it initialized?

Answer 1

The os module starts by importing all names from a platform-specific submodule (such as _nt or _posix) then does a little normalization. Clearly the environ name (standing for the system environment) was defined by the platform-specific submodule (as it’s normally expected to be!!!), so the except clause in os.py didn’t trigger and os.environ is just the rich dictionary it’s normally supposed to be.