How to define default argument value based on previous arguments?

问题

I want to define a resize(h, w) method, and I want to be able to call it in one of two ways:

1. resize(x,y)
2. resize(x)

Where, in the second call, I want y to be equal to x. Can I do this in the method definition or should I do something like resize(x,y=None) and check inside:

if y is None:
    y = x

回答1:

Can I do this in the method definition

No. During the method definition there's no way to know what value x might have at run-time. Default arguments are evaluated once at definition time, there's no way to save a dynamic value for y.

or should I do something like resize(x,y=None) and check inside

exactly. This is a common idiom in Python.

回答2:

To complete Jim's answer, in the case that None is valid as a parameter value, you could use variable length arguments feature (positional, and/or keyword). Example of use for positional:

def resize(x,*args):
    if args:
        if len(args)>1:
           raise Exception("Too many arguments")
        y = args[0]
    else:
        y = x

in that example, you have to pass x, but you can pass 0 or more extra positional arguments. Of course you have to do the checking manually, and you lose the y keyword.

来源：https://stackoverflow.com/questions/44025228/how-to-define-default-argument-value-based-on-previous-arguments