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When adding mutually exclusive group and required is True, and the group argument has default value. If we type its default value, argparse will ignore the input and return `argument is required`
------- PoC --------
import argparse
parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('-v', type=int, default=10)
print(parser.parse_args())
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$ python tests.py -v 10
usage: tests.py [-h] -v V
tests.py: error: one of the arguments -v is required
$ python tests.py -v 11
Namespace(v=11)