For the following lines of code, "dictionary changed size during iteration" would be thrown when the environment variables change while in iteration.

Python-3.5.0/Lib/urllib/request.py
2393 for name, value in os.environ.items():
2394 name = name.lower()
2395 if value and name[-6:] == '_proxy':
2396 proxies[name[:-6]] = value

How are you managing to get os.environ to change inside that function? Another thread or garbage collector callback perhaps?

My understanding is that os.environ is coupled to the C library’s environment, and that is not thread-safe.

For a simple fix, 2393 should probably read:

    for name, value in list(os.environ.items()):

os.environ is never changed by getproxies_environment()

Like Martin Panter, I will curious to know as how the user experienced this issue in the place.

@Raymond Hettinger: If the dict is only read and never modified in the fuction, does it still make sense to have a defensive list() as you suggested? If yes, then there will be couple of places in the stdlib code that will need this change.

$ ag os.environ.items Lib
Lib/distutils/_msvccompiler.py
80:            for key, value in os.environ.items()

Lib/test/test_os.py
679:        for key, val in os.environ.items():

Lib/test/test_wsgiref.py
427:    os_environ = dict(os.environ.items())

Lib/urllib/request.py
2453:    for name, value in os.environ.items():

Lib/wsgiref/handlers.py
47:    for k, v in os.environ.items():

My understanding is, if the function is not manipulation and only reading the dictionary, a defensive list or copy is not required. And, this bug report should be closed.