paul j3 added the comment:
The suggestion in this issue is to add a 'mutually_inclusive_group' mechanism,
one that would let users specify that certain sets of arguments must occur
together. Furthermore there was mention of allowing some sort of nesting.
Modeling it on the mutually_exclusive_group would be straight forward. But
should it affect the usage and help display?mutually_exclusive_groups add a
messy layer to the usage formatting.
The only place such a group would act would be at the end of
'_parse_known_args', where the current code checks for things like required
actions (and mxgroups). A test at this point could use 'namespace',
'seen_actions' and 'seen_non_default_actions' to check whether the required
group actions were seen.
But the only thing that the argument_group contributes to this test is a list
of argument names ('dest'?). Why not provide this list directly? And what if
the user wants A to occur together with either B or C, but not both? Or make
the inclusivity conditional on the value of A?
Currently users can define argument interactions in a couple of ways. They can
define custom Actions. In test_argparse.py there's a custom Actions test that
does something like this (using '--spam' and 'badger'). But tests in Actions
depend on the order in which arguments are given.
An alternative is to test for interactions of arguments after `parse_args`.
However the only information that the user has at this point is the args
namespace. Reliably distinguishing between non-occurrence of arguments and
default values can be difficult.
I am proposing 'cross_test' mechanism that would give the user access to the
'seen_actions' and 'seen_non_default_actions' sets that
'mutually_exclusive_groups' use. Specifically an optional function can be
called at the end of '_parse_known_args' that has access to these sets as well
as the parser and the namespace.
The core of the change would be adding
cross_test = getattr(self, 'cross_test', None)
if cross_test:
cross_test(self, namespace, extras, seen_actions,
seen_non_default_actions)
at the end of 'parser._parse_known_args'. In addition 'cross_test' (or some
other name) could be added to the 'ArgumentParser.__init__' arguments.
The feature could be used by defining such a 'cross_test' function and adding
it to the parser (either instance or subclass)
def foobar(self, namespace, extras, seen_actions, seen_non_default_actions):
...
(raise self.error(...))
parser.cross_test = foobar
The patch proposed http://bugs.python.org/issue18943 should be included
with any changes here since it refines the setting of
'seen_non_default_actions'.
I am working on tests and examples of such functionality.
----------
nosy: +paul.j3
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<http://bugs.python.org/issue11588>
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