Support Nested/Dependent Mutually Exclusive Arguments in argparse

[Tailung42]

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.