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gh-59330: Improve error message for dest= for positionals #125215

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Oct 12, 2024
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gh-59330: Improve error message for dest= for positionals #125215

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gh-59330: Improve error message for dest= for positionals
serhiy-storchaka Oct 9, 2024
36c5c20
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serhiy-storchaka Oct 11, 2024
a9295f0
Merge branch 'main' into argparse-positional-dest
serhiy-storchaka Oct 11, 2024
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19 changes: 19 additions & 0 deletions Doc/library/argparse.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -636,6 +636,25 @@ be positional::
usage: PROG [-h] [-f FOO] bar
PROG: error: the following arguments are required: bar

By default, argparse automatically handles the internal naming and
display names of arguments, simplifying the process without requiring
additional configuration.
As such, you do not need to specify the dest_ and metavar_ parameters.
The dest_ parameter defaults to the argument name with underscores ``_``
replacing hyphens ``-`` . The metavar_ parameter defaults to the
upper-cased name. For example::
Comment on lines +639 to +645
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Suggested change
By default, argparse automatically handles the internal naming and
display names of arguments, simplifying the process without requiring
additional configuration.
As such, you do not need to specify the dest_ and metavar_ parameters.
The dest_ parameter defaults to the argument name with underscores ``_``
replacing hyphens ``-`` . The metavar_ parameter defaults to the
upper-cased name. For example::
By default, argparse automatically handles the internal naming and
display names of arguments, simplifying the process without requiring
additional configuration. As such, you do not need to specify the dest_
and metavar_ parameters. The dest_ parameter defaults to the argument
name with underscores ``_`` replacing hyphens ``-`` . The metavar_
parameter defaults to the upper-cased name. For example::

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Just removing the newline as I think that this can be one short paragraph.

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@serhiy-storchaka serhiy-storchaka Oct 12, 2024 •
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This does not affect the result. For Sphinx this is the same.

I prefer to start new sentence from a new line. This will help to keep the text more readable in future without reformatting the whole paragraph. Adding or removing will only affect one sentence.

I don't know if this is already in the style guide, but I try to follow this rule.


>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo-bar')
>>> parser.parse_args(['--foo-bar', 'FOO-BAR']
Namespace(foo_bar='FOO-BAR')
>>> parser.print_help()
usage: [-h] [--foo-bar FOO-BAR]

optional arguments:
-h, --help show this help message and exit
--foo-bar FOO-BAR


.. _action:

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3 changes: 2 additions & 1 deletion Lib/argparse.py
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Expand Up @@ -1417,7 +1417,8 @@ def add_argument(self, *args, **kwargs):
chars = self.prefix_chars
if not args or len(args) == 1 and args[0][0] not in chars:
if args and 'dest' in kwargs:
raise ValueError('dest supplied twice for positional argument')
raise ValueError('dest supplied twice for positional argument,'
' did you mean metavar?')
kwargs = self._get_positional_kwargs(*args, **kwargs)

# otherwise, we're adding an optional argument
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3 changes: 2 additions & 1 deletion Lib/test/test_argparse.py
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Expand Up @@ -5380,7 +5380,8 @@ def test_multiple_dest(self):
parser.add_argument(dest='foo')
with self.assertRaises(ValueError) as cm:
parser.add_argument('bar', dest='baz')
self.assertIn('dest supplied twice for positional argument',
self.assertIn('dest supplied twice for positional argument,'
' did you mean metavar?',
str(cm.exception))

def test_no_argument_actions(self):
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