Issue 39058: argparse should preserve argument ordering in Namespace

Currently, Namespace() objects sort the attributes in the __repr__.  This is annoying because argument order matters and because everywhere else in the module we preserve order (i.e. users see help in the order that arguments are added).

Note, the docs do not promise that Namespace is displayed with a sort.  This is likely just an artifact of older dictionaries having arbitrary or randomised ordering.


>>> from argparse import ArgumentParser
>>> parser = ArgumentParser()
>>> _ = parser.add_argument('source')
>>> _ = parser.add_argument('destination')

# Order matters to the user inputing the arguments 
# (source must go first and destination must go last
>>> args = parser.parse_args(['input.txt', 'output.txt'])

# Order is preserved internally
>>> vars(args)
{'source': 'input.txt', 'destination': 'output.txt'}

# Despite this, the Namespace() repr alphabetizes the output
>>> args
Namespace(destination='output.txt', source='input.txt')

# Order is preserved in help()
>>> parser.parse_args(['-h'])       
usage: [-h] source destination

positional arguments:
  source
  destination

optional arguments:
  -h, --help   show this help message and exit

> Currently, Namespace() objects sort the attributes in the __repr__.  This is annoying because argument
> order matters and because everywhere else in the module we preserve order (i.e. users see help in the
> order that arguments are added).

Hmm, I was going to suggest switching to types.SimpleNamespace, but
realized we sort that repr too (likely for the same reason).  I've
opened issue #39075 to address that.

In writing up that issue, I considered that a sorted repr can be
useful in some cases.  However, I don't think any of those cases
really apply here.

Anyway, I agree with your conclusion. :)

FWIW, I've also opened issue #39076 about subclassing types.SimpleNamespace.

This patch changes the super class, _AttributeHolder.  ArgumentParser and Actions also inherit from this, though they have their own _get_kwargs methods, and so aren't affected by the sort and its removal.

I just had occasion on stackoverflow to discuss the order in which attributes are added.  A poster wanted to preserve the sys.argv order.

https://stackoverflow.com/questions/58904423/find-the-order-of-arguments-in-argparse-python3/58905067#58905067

Most attributes are added as defaults at the start of parsing - via a loop through parser._actions.  Predefining the namespace, SUPPRESS defaults, parser.set_defaults may alter this default order.

Anyways removing the sort makes sense, and the proposed change phrase "in the order attributes were added" is sufficiently general.

New changeset 9681953c99b686cf23d1c476a2b26d2ddbec7694 by Raymond Hettinger in branch 'master':
bpo-39058:  Preserve attribute order in argparse Namespace reprs. (GH-17621)
https://github.com/python/cpython/commit/9681953c99b686cf23d1c476a2b26d2ddbec7694