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Your patch fails:
 python3 -m unittest test_argparse.TestEmptyAndSpaceContainingArguments
specifically these 2 subcases:
 (['-a badger'], NS(x='-a badger', y=None)),
 (['-y', '-a badger'], NS(x=None, y='-a badger')),
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At issue is the ambiguity when the user gives you a string that looks like an optionals flag, but doesn't match one of the defined arguments. When should it be treated as a positional, and when should it be treated as an unknown?
The code, and test says - if it has the space, treat it like a positional. You are trying to reverse that choice - if it has the prefix, treat it like an unknown optional. At the point where you apply the patch, we already know that the string has a prefixchar. So you are, effectively, eliminating the 'space' test.
I've added a simpler example of where the presence of the space flips that choice.