Below "Examples and Recipes", the Documentation of collections.ChainMap has an "Example of letting user specified command-line arguments take precedence over environment variables which in turn take precedence over default values:" In there, a ChainMap is created which represents the default values, updated by the command-line arguments, if they have been set. The relevant code snippet is the following:
   
    parser = argparse.ArgumentParser()
    parser.add_argument('-u', '--user')
    parser.add_argument('-c', '--color')
    namespace = parser.parse_args()
    command_line_args = {k:v for k, v in vars(namespace).items() if v}

If user passes an empty string as value for any of the command-line arguments, that argument would not appear in `command_line_args` (because the boolean value of empty string is `False`). However, passing the empty string as a value for a command-line argument, would reflect the intent of overwriting the default value (setting it to the empty string). With the current example code, this would erroneously not be reflected in the `command_line_args`. This is caused by checking for the boolean of `v` instead of checking for `v` not being `None`. So, this should be handled correctly by writing

    command_line_args = {k: v for k, v in vars(namespace).items() if v is not None}

The example was just meant to be a quick snippet to suggest a possible use case.  It isn't a stand-alone application.  However, stylistically, this is an improvement and I'll accept some variation of the PR.

New changeset 98480cef9dba04794bd61c7e7cca643d384c8c35 by Raymond Hettinger (Jonathan Scholbach) in branch 'master':
bpo-38771:  Explict test for None in code example (GH-17108)
https://github.com/python/cpython/commit/98480cef9dba04794bd61c7e7cca643d384c8c35

New changeset cb1c06e3bfbce7f4e8dcb0db84bcee54c8d4fa21 by Raymond Hettinger (Miss Islington (bot)) in branch '3.7':
bpo-38771:  Explict test for None in code example (GH-17108) (GH-17110)
https://github.com/python/cpython/commit/cb1c06e3bfbce7f4e8dcb0db84bcee54c8d4fa21

New changeset d8e08456025d9349abbf76035c821d3f7b2d722a by Raymond Hettinger (Miss Islington (bot)) in branch '3.8':
bpo-38771:  Explict test for None in code example (GH-17108) (GH-17109)
https://github.com/python/cpython/commit/d8e08456025d9349abbf76035c821d3f7b2d722a