Setting default value for optional argument that depends on another optional argument in python argparse

Question

I am using python argparse() for command line parsing. I want something like this,

-a -> optional argument

-b -> optional argument that depends on a with default value x

Condition : -b should set(either to default or custom value) if and only if a is set, otherwise b should be None .

Could someone help me to achieve this?

Answer 1

@hpaulj 's comment still holds. This is best done in code after parsing

But you could do it with the @property and your own MyNamespace object instead of default argparse namespace

The @property lets you write arbitrary code for getters/setters of a particular class object (in this case b)

class MyNamespace(object):
    def __init__(self):
        self.a = None
        self._b = None   # this is a variable to store the actual value of -b

    @property
    def b(self):
        if self.a:
            return "A_IS_SET"
        else:
            return self._b or None
    @b.setter
    def b(self, input):
        self._b = input

def parse():
    parse.avalue = []
    parse.bvalue = []

    parser = argparse.ArgumentParser()
    parser.add_argument("-a")
    parser.add_argument("-b")

    out = parser.parse_args(namespace=MyNamespace())
    print out.a, out.b

if __name__ == "__main__":
    parse()

When both a, b are passed (in any order)

$>python testparse.py -b BTEST 
None BTEST

$>python testparse.py -a atest
atest A_IS_SET

When only -b is passed

$>python testparse.py -b BTEST None BTEST

When only -a is passed

$>python testparse.py -a atest atest A_IS_SET

When none of them is passed

$>python testparse.py None None

Hope this helps!