[New-bugs-announce] [issue38771] Bug in example of collections.ChainMap

[Jonathan Scholbach]

New submission from Jonathan Scholbach <j.scholbach at posteo.de>:

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}

----------
assignee: docs at python
components: Documentation
messages: 356398
nosy: docs at python, jonathan.scholbach
priority: normal
severity: normal
status: open
title: Bug in example of collections.ChainMap
versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38771>
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