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Python argparse has a limitation on argument groups that makes me sad

June 10, 2025

Argparse is the straightforward standard library module for handling
command line arguments, with a number of nice features. One of those
nice features is groups of mutually exclusive arguments. If people
can only give one of '--quiet' and '--verbose' and both together make
no sense, you can put them in a mutually exclusive group and argparse
will check for you and generate an appropriate error. However,
mutually exclusive groups have a little limitation that makes me sad.

Suppose, not hypothetically, that you have a Python program that has
some timeouts. You'd like people using the program to be able to
adjust the various sorts of timeouts away from their default values
and also to be able to switch it to a mode where it never times out
at all. Generally it makes no sense to adjust the timeouts and also
to say not to have any timeouts, so you'd like to put these in a
mutually exclusive group. If you have only a single timeout, this
works fine; you can have a group with '--no-timeout' and '--timeout
<TIME>' and it works. However, if you have multiple sorts of timeouts
that people may want adjust all of, this doesn't work. If you put all
of the options in a single mutually exclusive group, people can only
adjust one timeout, not several of them. What you want is for the
'--no-timeouts' switch to be mutually exclusive with a group of all
of the timeout switches.

Unfortunately, if you read the current argparse documentation, you
will find this note:

    Changed in version 3.11: Calling add_argument_group() or
    add_mutually_exclusive_group() on a mutually exclusive group is
    deprecated. These features were never supported and do not always
    work correctly. The functions exist on the API by accident
    through inheritance and will be removed in the future.

You can nest a mutually exclusive group inside a regular group, and
there are some uses for this. But you can't nest any sort of group
inside a mutually exclusive group (or a regular group inside of a
regular group). At least not officially, and there are apparently
known issues with doing so that won't ever be fixed, so you probably
shouldn't do it at all.

Oh well, it would have been nice.

(I suspect one reason that this isn't officially supported is that
working out just what was conflicting with what in a pile of nested
groups (and what error message to emit) might be a bit complex and
require explicit code to handle this case.)

As an extended side note, checking this by hand isn't necessarily all
that easy. If you have something, such as timeouts, that have a
default value but can be changed by the user, the natural way to set
them up in argparse is to make the argparse default value your real
default value and then use the value argparse sets in your program.
If the person running the program used the switch, you'll get their
value, and if not you'll get your default value, and everything works
out. Unfortunately this usage makes it difficult or impossible to see
if the person running your program explicitly gave a particular
switch. As far as I know, argparse doesn't expose this information,
so at a minimum you have to know what your default value is and then
check to see if the current value is different (and this doesn't
catch the admittedly unlikely case of the person using the switch
with the default value).

Written on 10 June 2025.

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