[issue43220] Explicit default required arguments with add_mutually_exclusive_group are rejected

[Keith Smiley]

Keith Smiley <keithbsmiley at gmail.com> added the comment:

Here's an example outside of argparse showing this is caused by the `is` comparison with interned string:

```
import sys

short_string = sys.argv[1]
short_default = '1'
long_string = sys.argv[2]
long_default = 'not-interned'

print(f"short comparisons: id1: {id(short_default)} id2: {id(short_string)}, eq: {short_default == short_string}, is: {short_default is short_string}")
print(f"long comparisons: id1: {id(long_default)} id2: {id(long_string)}, eq: {long_default == long_string}, is: {long_default is long_string}")
```

```
% ./python.exe /tmp/foo.py 1 not-interned
short comparisons: id1: 4523386416 id2: 4523386416, eq: True, is: True
long comparisons: id1: 4524440064 id2: 4523395296, eq: True, is: False
```

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43220>
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