Python-查找函数内传递的参数数量

Question

Imagine I want to define a function with all arguments having default values:

def fun(a=2, b=1):
    ...

The logic inside the function is that: if nothing is passed, use the default value; if only a is passed, then b is always equal to a/2; if both a and b are passed, then use the passed values.

The problem in completing this function is that it seems difficult to tell the difference between (for example) fun(4) and fun(4,1) since I don't know whether the b=1 is provided outside or is just the default value.

One obvious way to get around is to use *args, however, in my case I want to keep the default values and names. Therefore it seems the only way out is to know inside fun() how many arguments are passed when it's called. I guess there might be some system function to use, but I didn't succeed in finding them. Any idea how could this work would be very helpful.

Thanks! :)

Answer 1

You can have (for instance) the default value of b be None , and then work out inside your function what value b should take.

def fun(a=2, b=None):
    if b is None:
        b = a/2
    ...

This will give you:

f()    : a=2, b=1
f(4)   : a=4, b=2
f(4,1) : a=4, b=1