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Argparse: "Default" interacts incorrectly/non-intuitively with action='append' and action='extend' #110131
Description
Bug report
Bug description:
The documentation for default claims:
The default keyword argument of add_argument(), whose value defaults to None, specifies what value should be used if the command-line argument is not present
which (implicitly) sets the expectation that the default is not used if the command-line argument is present
in it's defense, the strange behaviour is documented in append but not extend
If the default value is non-empty, the default elements will be present in the parsed value for the option, with any values from the command line appended after those default values
However, this behaviour is non-intuitive. The fact that the values will be prepended to the end-user's input is not obvious, furthermore, the end-user is unable to specify a set of parameters that don't contain undesired default values.
DEFAULT_SET=['a','b','c','d']
parser = argparse.ArgumentParser()
parser.add_argument('--items',nargs='+',action='extend',dest='items',choices=DEFAULT_SET,default=DEFAULT_SET)
args = parser.parse_args()
print(args.items)$ python3 example.py --items a --items b c # expected: items will contain 'a', 'b', and 'c'
['a', 'b', 'c', 'd', 'a', 'b', 'c']
Now it's fair to say that there's a workaround:
parser.add_argument('--items',nargs='+',action='extend',dest='items',choices=DEFAULT_SET,default=None)
args = parser.parse_args()
if args.items == None:
args.items = DEFAULT_SET however, isn't this the pattern that default= aims to eliminate/alleviate in the first place?
If the purpose of DEFAULT is to supply a value in the absense of user input, the fact that it supplies a value even when the user supplies input is an undesired side effect and not really default behaviour.
If the code needs to always be processing a set of values regardless of user input, then that really should be clearly stated in the code/logic, rather than a side effect of the input parsing
do_something(DEFAULT_SET + args.items) # no ambiguityhope this makes sense. thank you!
CPython versions tested on:
3.8
Operating systems tested on:
macOS
Linked PRs
- gh-110131: Document how a non-empty default works with append and extend #158155
- [3.15] gh-110131: Document how a non-empty default works with append and extend (GH-158155) #158352
- [3.14] gh-110131: Document how a non-empty default works with append and extend (GH-158155) #158353
- [3.13] gh-110131: Document how a non-empty default works with append and extend (GH-158155) #158354
Activity
hpaulj commentedon Feb 14, 2024
With all action classes, the
defaultis inserted into the args Namespace at the start of parsing.At the end of parsing, if the
argsvalue is still the same as the default, and it is a string, it (may) be passed through thetypefunction. I'd have to check if it's passed by thechoices. Quick tests withextendsuggest it is not.Anyways, with
extend, and thedefaultis a list, each user input will be added to that list in the normalextendmethod.In your example, both
--items aand--items b cextendwhat was already in the Namespace. It does not know, in any way, that 'a' is the first item, and 'b c' the second.So
defaultisn't 'prepended to the user inputs. User inputs are added to the default, and any previous user inputs.For default 'store' action, user inputs overwrite the default in a transparent manner. 'append' and 'extend' handle the defaults in the same way, just differing in what list method is used. If you have more questions, I suggest looking at the
__call__method of the respectiveActionsubclasses.serhiy-storchaka commentedon Sep 25, 2024
The documentation for
extendrequires improvements (see also #84545). But the behavior is correct and pretty straightforward. Reclassified this issue as the documentation issue.pythongh-110131: Drop the test, show a non-empty default in the example
pythongh-110131: Keep one extend example
Merge branch 'main' into pythongh-110131-argparse-append-default
gh-110131: Document how a non-empty default works with append and ext…