Support Nested/Dependent Mutually Exclusive Arguments in argparse

[dhature]

Feature or enhancement

Proposal:

Currently, Python’s argparse module supports mutually exclusive groups at a single level, meaning you can enforce constraints like:

a or b or c

However, there is no built-in way to express nested or dependent exclusivity, such as:

a or b or (c and d)

In this case, if argument c is provided, d must also be provided, but a and b remain mutually exclusive with the combination (c and d).

Result:

This would allow developers to write complex argument logic withou writing code manually for such validation: I actually needed for a cli app. This would be something like:

# Valid
mytool.py --a
mytool.py --b
mytool.py --c --d

# Invalid
mytool.py --c          # missing --d
mytool.py --c --d --a  # mutually exclusive with a

[serhiy-storchaka]

argparse do not even have support for mutually required groups ("if -a, then -b is required, if -b, then -a is required"), neither for one-side dependencies ("if -a, then -b is required" or "if -a, then -b is forbidden"), neither intersecting mutually exclusive groups. It is not easy to design simple but powerful API to specify complex dependencies, and it even more difficult, and not always possible to produce good usage string for them. All these conditions you can check manually, you have Python -- a programming language. Mutually exclusive groups is just the most common type of dependencies which was worth to support naturally. I do not think that other types are even close to it.

[picnixz]

I had times when I really needed required arguments conditioned to others but if we want to have a simple way to provide this I feel we should instead offer it as a pluggable extension rather than having to provide each condition separately. Since we do not have any actionable item now I will close the issue as not planned

[picnixz]

FTR I would not be really against but I think we should not expand argparse with too many niche features. I think we should instead make it more extensible by third-party extensions, but this requires a more general discussion on discuss.python.org

[dhature]

Yeah,cool, I was convinced by @serhiy-storchaka's comment. making it extensible with third party extnesions could be a solution, but that would just result in fragmented code.