Function printing correct Output and None

Question

I have defined this function that takes a word and a string of required letters and that returns True if the word uses all the required letters at least once. When I run this, it gives me the correct output but I do not understand why it also outputs 'None'.

Here is my code:

 def uses_all(word, allused):
    boolean = False
    for e in allused:
        if e in word:
            boolean = True
        else:
            boolean = False
            break 
    print boolean 

print uses_all('facebook', 'd')

Output:

False
None

Comments on where I have gone wrong will be appreciated.

Answer 1

Your program prints boolean, which is False, so you know where that comes from.

If a function doesn't return anything explicitly, it automatically returns None, and when you use

print uses_all('facebook', 'd')

you're asking it to print what uses_all returns, which is None. Hence:

False
None

BTW, I think your function could be more concisely written as

def uses_all(word, allused):
    return all(e in word for e in allused)

Could make it more efficient, but that should be good enough for government work. The all function is really handy (see also any).

Answer 2

You print the result of the function, but the function doesn't return anything (--> None)

If you want the result to be False False, add return boolean in the function.

Alternatively, if a single False is enough, you could change print uses_all('facebook', 'd') to just uses_all('facebook', 'd')

Answer 3

Because uses_all() doesn't have a return statement. If you return a value from the function, it will be printed instead of None, unless of course you return None :)