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Parameter Groups

Parameter groups allow you to (as the name implies!) group parameters into re-usable chunks. A parameter group can then be used for multiple commands, and each of those commands will have all of those params defined.

Creating A group

Parameter groups are fairly simple to create, you just need to decorate a class with the decorator.
class MyGroup:

Adding Parameters to a group

Now, to make a group useful, we need to actually add parameters to it. To do so, we add class-variables with our desired parameter definitions.

For example, let's take this example from the previous page:

import arc

def hello(firstname: str, reverse: bool):
    if reverse:
        firstname = firstname[::-1]

    arc.print(f"Hello, {firstname}! Hope you have a wonderful day!")


and convert it to use a parameter group instead.

For this example, the transfer is very straight forward, we can just take the argument list definition, and add it to the class body:
class MyGroup:
    firstname: str
    reverse: bool

Using a group

To use a Parameter Group, all we need to do is add an argument to the argument list of a command, with the param group as the type hint.

def hello(group: MyGroup): # Don't need firstname or reverse here anymore

Putting it together

import arc
class MyGroup:
    firstname: str
    reverse: bool

def hello(group: MyGroup):
    if group.reverse:
        group.firstname = group.firstname[::-1]

    arc.print(f"Hello, {group.firstname}! Hope you have a wonderful day!")

$ python --help
USAGE [-h] [--reverse] [--] firstname


    --help (-h)  Displays this help message

$ python Joseph --reverse
Hello, hpesoJ! Hope you have a wonderful day!

And just like that, we have a set of re-usable parameters that we can add to any command at-will!


  • Anything that works for regular parameters also works for Parameter groups. This means that arc.Argument() and it's cohorts can be used to expand the use of a parameter in a group.
  • Because there is no bare * or equivelant, there isn't a good way to distinguish between arguments and options. So, any non-flag will be presumed to be an argument unless given an explicit arc.Option() as a default value.
  • Groups are not permitted to have a default value assigned to them
    def hello(group: MyGroup = ...): # This isn't allowed
  • Groups are allowed to be nested arbitrarily