Author paul.j3
Recipients n8falke, paul.j3
Date 2019-02-22.17:57:05
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Defaults are handled into two stages.

At the start of parsing defaults are added to the Namespace.

At the end of parsing intact defaults are evaluated with 'type'.

But a nargs='?' positional gets special handling.  It matches an empty string, so it is always 'seen'.  If its default is not None, that default is put in the Namespace instead of the matching empty list.

It's this special default handling that lets us use a ?-positional in a mutually exclusive group.

I suspect the error arises from this special default handling, but I'll have to look at the code to verify the details.
Date User Action Args
2019-02-22 17:57:05paul.j3setrecipients: + paul.j3, n8falke
2019-02-22 17:57:05paul.j3setmessageid: <>
2019-02-22 17:57:05paul.j3linkissue36078 messages
2019-02-22 17:57:05paul.j3create