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gh-110131: Document how a non-empty default works with append and extend #158155

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93406f1
gh-110131: Document argparse's extend action with a non-empty default
v0ropaev Sep 25, 2026
94bd220
gh-110131: Test argparse's extend action with a non-empty default
v0ropaev Sep 25, 2026
5f7766c
gh-110131: Restore the "for the option" scope in argparse's append docs
v0ropaev Sep 25, 2026
02246d2
gh-110131: Drop the test, show a non-empty default in the example
v0ropaev Sep 25, 2026
844bb63
gh-110131: Keep one extend example
v0ropaev Sep 26, 2026
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14 changes: 9 additions & 5 deletions Doc/library/argparse.rst
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Expand Up @@ -791,9 +791,9 @@ how the command-line arguments should be handled. The supplied actions are:

* ``'append'`` - This appends each argument value to a list.
It is useful for allowing an option to be specified multiple times.
If the default value is a non-empty list, the parsed value will start
with the default list's elements and any values from the command line
will be appended after those default values. Example usage::
If the default value is a non-empty list, the parsed value for the option
will start with the default list's elements and any values from the
command line will be appended after those default values. Example usage::

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='append', default=['0'])
Expand All @@ -818,12 +818,16 @@ how the command-line arguments should be handled. The supplied actions are:
value ``'+'`` or ``'*'``.
Note that when nargs_ is ``None`` (the default) or ``'?'``, each
character of the argument string will be appended to the list.
If the default value is a non-empty list, the parsed value for the option
will start with the default list's elements and any values from the
command line will be appended after those default values.
Example usage::

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("--foo", action="extend", nargs="+", type=str)
>>> parser.add_argument("--foo", action="extend", nargs="+", type=str,
... default=["d1"])
>>> parser.parse_args(["--foo", "f1", "--foo", "f2", "f3", "f4"])
Namespace(foo=['f1', 'f2', 'f3', 'f4'])
Namespace(foo=['d1', 'f1', 'f2', 'f3', 'f4'])

.. versionadded:: 3.8

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