[issue43160] argparse: add extend_const action

[Tony Lykke]

Tony Lykke  added the comment:

Sorry, there's a typo in my last comment.

--store --foo a
Namespace(foo=['a', 'b', 'c'])

from the first set of examples should have been

--store --foo c
Namespace(foo=['a', 'b', 'c'])

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[issue43160] argparse: add extend_const action

[Tony Lykke]

Tony Lykke  added the comment:

Perhaps the example I added to the docs isn't clear enough and should be 
changed because you're right, that specific one can be served by store_const. 
Turns out coming up with examples that are minimal but not too contrived is 
hard! Let me try again with a longer example that hopefully shows more clearly 
how the existing action's behaviours differ from my patch.

parser = argparse.ArgumentParser()
parser.add_argument("--foo", action="append", default=[])
    parser.add_argument("--append", action="append_const", dest="foo", 
const=["a", "b"])
    parser.add_argument("--store", action="store_const", dest="foo", 
const=["a", "b"])

When run on master the following behaviour is observed:

--foo a --foo b --foo c
Namespace(foo=['a', 'b', 'c'])
--foo c --append
Namespace(foo=['c', ['a', 'b']])
--foo c --store
Namespace(foo=['a', 'b'])
--store --foo a
Namespace(foo=['a', 'b', 'c'])

If we then add the following:

parser.add_argument("--extend", action="extend_const", dest="foo", 
const=["a", "b"])

and then run it with my patch the following can be observed:

--foo c --extend
Namespace(foo=['c', 'a', 'b'])
--extend --foo c
Namespace(foo=['a', 'b', 'c'])

store_const is actually a pretty close fit, but the way it makes order 
significant (specifically in that it will silently drop prev values) seems like 
it'd be rather surprising to users and makes it a big enough footgun for this 
use case that I don't think it's a satisfactory alternative.

> I suspect users of your addition will get a surprise if they aren't careful 
> to provide a list or tuple 'const'

I did consider that, but I don't think they'd get any more of a surprise than 
for doing the same with list.extend vs list.append.

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[issue43160] argparse: add extend_const action

[paul j3]

paul j3  added the comment:

It's not clear what your patch does that the existing 'store_const' doesn't.   
Even 'append_const' does the same except for a extra level of list/tuple 
nesting.

But I'll admit that I didn't much need for  'extend' addition, but others, 
including the original developer liked it.

I suspect users of your addition will get a surprise if they aren't careful to 
provide a list or tuple 'const':

const='foobar'
const=str

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[issue43160] argparse: add extend_const action

[hai shi]

Change by hai shi :


--
nosy: +paul.j3

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[issue43160] argparse: add extend_const action

[Tony Lykke]

Change by Tony Lykke :


--
keywords: +patch
pull_requests: +23269
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/24478

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[issue43160] argparse: add extend_const action

[Tony Lykke]

New submission from Tony Lykke :

I submitted this to the python-ideas mailing list early last year: 
https://mail.python.org/archives/list/python-id...@python.org/thread/7ZHY7HFFQHIX3YWWCIJTNB4DRG2NQDOV/.
 Recently I had some time to implement it (it actually turned out to be pretty 
trivial), so thought I'd put forward a PR.

Here's the summary from the mailing list submission:

I have found myself a few times in a position where I have a repeated argument 
that uses the append action, along with some convenience arguments that append 
a specific const to that same dest (eg:  --filter-x being made equivalent to 
--filter x via append_const). This is particularly useful in cli apps that 
expose some kind of powerful-but-verbose filtering capability, while also 
providing shorter aliases for common invocations. I'm sure there are other use 
cases, but this is the one I'm most familiar with.

The natural extension to this filtering idea are convenience args that set two 
const values (eg: --filter x --filter y being equivalent to --filter-x-y), but 
there is no extend_const action to enable this.

While this is possible (and rather straight forward) to add via a custom 
action, I feel like this should be a built-in action instead. append has 
append_const, it seems intuitive and reasonable to expect extend to have 
extend_const too (my anecdotal experience the first time I came across this 
need was that I simply tried using extend_const without checking the docs, 
assuming it already existed).

Here's an excerpt from the docs I drafted for this addition that hopefully 
convey the intent and use case clearly.

+* ``'extend_const'`` - This stores a list, and extends each argument value to 
the list.
+  The ``'extend_const'`` action is typically useful when you want to provide 
an alias
+  that is the combination of multiple other arguments. For example::
+
+>>> parser = argparse.ArgumentParser()
+>>> parser.add_argument('--str', dest='types', action='append_const', 
const=str)
+>>> parser.add_argument('--int', dest='types', action='append_const', 
const=int)
+>>> parser.add_argument('--both', dest='types', action='extend_const', 
const=(str, int))
+>>> parser.parse_args('--str --int'.split())
+Namespace(types=[, ])
+>>> parser.parse_args('--both'.split())
+Namespace(types=[, ])

--
components: Library (Lib)
messages: 386614
nosy: rhettinger, roganartu
priority: normal
severity: normal
status: open
title: argparse: add extend_const action
type: enhancement
versions: Python 3.10

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