python explicitly not passing optional parameter

Question

imagine the following scenario:

class A:
    def __init__(self, arg1=3, arg2=5):
        pass

def createA(arg1=None, arg2=None):
    if arg1 is None:
        if arg2 is None:
            a = A()
        else:
            a = A(arg2=arg2)
    else:
        if arg2 is None:
            a = A(arg1=arg1)
        else:
            a = A(arg1=arg1, arg2=arg2)
    return a

Whats the best way to implement this behavior, with the followings in mind:

I can't/don't want to change class A
I don't want to explicitly add the default values of A's constructor's parameters to the createA function?

For example is there any value that indicates a not passed optional argument? Something like:

if arg1 is None:
    newArg1 = NotPassed
else:
    newArg1 = arg1

if arg2 is None:
    newArg2 = NotPassed
else:
    newArg2 = arg2
A(arg1=newArg1, arg2=newArg2)

runDOSrun · Accepted Answer · 2015-12-19 12:28:52Z

8

def create_A(arg1=None, arg2=None):
    kwargs = {}
    if arg1 is not None: kwargs['arg1'] = arg1
    if arg1 is not None: kwargs['arg2'] = arg2
    return A(**kwargs)

or maybe even

def create_A(**kwargs):
    return A(**kwargs)

edited Dec 19, 2015 at 12:28

runDOSrun

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answered Oct 18, 2012 at 17:16

glglgl

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1 Comment

quazgar Over a year ago

Very nice, I'm just wondering why Python as a modern language does not provide a special value for this behaviour. Something like A(arg1=1, arg2=Empty) maybe...

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