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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())
-----
$ 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)

Strange, this will only trigger when that argument type is int.

Maybe documentation should note that:
"""
# error if this argument is not allowed with other previously
# seen arguments, assuming that actions that use the default
# value don't really count as "present"
"""

And only for integers smaller than 257!
The problem is with this test in `take_action` function
 # error if this argument is not allowed with other previously
 # seen arguments, assuming that actions that use the default
 # value don't really count as "present"
 if argument_values is not action.default:
 seen_non_default_actions.add(action)
 for conflict_action in action_conflicts.get(action, []):
 if conflict_action in seen_non_default_actions:
 msg = _('not allowed with argument %s')
 action_name = _get_action_name(conflict_action)
 raise ArgumentError(action, msg % action_name)
Specifically with the
 argument_values is not action.default
test. In CPython integers below 257 are unique, so that
 In [33]: int('10') is 10
 Out[33]: True
 In [34]: int('257') is 257
 Out[34]: False
(and strings from 'sys.argv' do not match in the 'is' sense with strings set via 'default'. So the problem is unique to small integers.)
The test is really meant to catch the default that is added in '_get_values()' for positionals with optional nargs ('?').
This issue was raised by some PyPy developers several years ago. 
 PyPy does not treat the small integers as unique. I'll have to dig around to see what the resolution was, if any.
For now the solution is to use a string '10' as the default.

This issue was raised in 2013. The proposed patch is still pending
http://bugs.python.org/issue18943
I'm going to close this new one, though it is a good reminder of a rare, but persistent bug.

Thanks, paul. Your msg help a lot.
Will you work on #18943? I can help for this or review the pending patch.