New submission from Jonathan Scholbach :
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@python
components: Documentation
messages: 356398
nosy: docs@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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<https://bugs.python.org/issue38771>
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