Python function prints None

问题:

I have the following exercise:

The parameter weekday is True if it is a weekday, and the parameter vacation is True if we are on vacation. We sleep in if it is not a weekday or we're on vacation. Return True if we sleep in.

Here's what I've done, but the second print function only prints 'None'.

def sleep_in(weekday, vacation):
    if(not weekday or vacation):
        return True

print(sleep_in(False, False))
print(sleep_in(True, False))
print(sleep_in(False, True))

Output:

True
None
True

回答1:

Functions in python return None unless explicitly instructed to do otherwise.

In your function above, you don't take into account the case in which weekday is True. The interpreter reaches the end of the function without reading a return statement (since the condition predecing yours evaluates to False), and returns None.

Edit:

def sleep_in(weekday, vacation):
    return (not weekday or vacation)

There you go =)