Python if more than one of N variables is true

Question

I need to check if more than one of a, b, c, and d are being defined:

def myfunction(input, a=False, b=False, c=False, d=False):
    if <more than one True> in a, b, c, d:
    print("Please specify only one of 'a', 'b', 'c', 'd'.)

I am currently nesting if statements, but that looks horrid. Could you suggest anything better?

Answer 1

Try adding the values:

if sum([a,b,c,d]) > 1:
    print("Please specify at most one of 'a', 'b', 'c', 'd'.")

This works because boolean values inherit from int, but it could be subject to abuse if someone passed integers. If that is a risk cast them all to booleans:

if sum(map(bool, [a,b,c,d])) > 1:
    print("Please specify at most one of 'a', 'b', 'c', 'd'.")

Or if you want exactly one flag to be True:

if sum(map(bool, [a,b,c,d])) != 1:
    print("Please specify exactly one of 'a', 'b', 'c', 'd'.")

Answer 2

If you want exactly 1 True value:

 def myfunction(input, a=False, b=False, c=False, d=False):
    if filter(None,[a, b, c, d]) != [True]:
        print("Please specify only one of 'a', 'b', 'c', 'd'.)")

Answer 3

First thing that comes to mind:

if [a, b, c, d].count(True) > 1: