paul j3 added the comment:
A related issue which Sworddragon found just before starting this issue is
http://bugs.python.org/issue9625
The handling of defaults in argparse is a bit complicated. In general it errs
on the side of giving the user/developer freedom to set them how ever they
want. That's especially true in the case of non-string defaults. A default
does not have to be something that the user could give. In particular the
default 'default' is None, which can't be parsed from the command line. The
same for boolean values (the default 'type' does not translate string 'False'
to boolean 'False'.)
Errors in the defaults are usually caught during program development.
If the calling code generates the defaults dynamically, it seems reasonable
that it should also check that they are valid. The validity test for choices
is a simple 'in' (contains) one. 'choices' can be a list or dictionary (keys),
and can even be dynamically generated.
parser = ....
choices = <get a list>
default = <get a value>
if default not in choices:
<scream>
parser.add_argument(..., choices=choices, default=default)
etc.
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nosy: +paul.j3
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<http://bugs.python.org/issue24956>
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