In python, if a function doesn't have a return statement, what does it return?

Question

Given this example function:

def writeFile(listLine,fileName):
    '''put a list of str-line into a file named fileName'''
    with open(fileName,'a',encoding = 'utf-8') as f:
        for line in listLine:
            f.writelines(line+'\r\n')
    return True

Does this return True statement do anything useful?

What's the difference between with it and without it? What would happen if there were no return function?

Answer 1

If a function doesn't specify a return value, it returns None.

In an if/then conditional statement, None evaluates to False. So in theory you could check the return value of this function for success/failure. I say "in theory" because for the code in this question, the function does not catch or handle exceptions and may require additional hardening.

Answer 2

The function always returns None if explicit return is not written.

Answer 3

If you have the return True at the end of the function, you can say stuff like: a=writeFile(blah, blah)

However, because it will always be True, it is completely pointless. It would be better to return True if the file was written correctly, etc.

If you don't explicitly return anything, the value will be None