one line if else condition in python

This is what you want.

def sum10(a, b):
    return sum([a, b]) % 10 == 0

Also the ternary If in Python works like this

<True Statment> if <Conditional Expression> else <False Statement>

True if sum([a,b]) % 10 == 0 else False

Might i also recommend using the plus operator?

True if (a+b) % 10 == 0 else False

Your code

def sum10(a, b):
    if sum([a, b]) % 10 == 0: return True; return False

is equivalent to

def sum10(a, b):
    if sum([a, b]) % 10 == 0: 
        return True; return False

so return False is never evaluated.

Some (of the probably endless) alternatives:

    if sum([a, b]) % 10 == 0: 
        return True
    return False

    return sum([a, b]) % 10 == 0

    return True if sum([a, b]) % 10 == 0 else False

    return False if (a+b) % 10 else True

or (the most readable IMHO)

    return not (a + b) % 10

Related