A name that was never filled is an error. A name that was filled with nothing is a value, and that difference is how a program records that something has not happened yet.
About 2 min of reading
Before this new lesson, one small question about the lesson you just finished. Recall it, do not look it up.
Record the absence of a value and fill it later.
Store nothing under a name.
Before you run it, say out loud what it will print. Then open the answer.
Ask whether it is still nothing.
Before you run it, say out loud what it will print. Then open the answer.
Replace it with a real value and ask again.
Before you run it, say out loud what it will print. Then open the answer.
Store the result of a function that only prints. You get nothing.
Before you run it, say out loud what it will print. Then open the answer.
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
Every function you write answers with something, and if you did not say what, it answers with nothing. Reading a function means asking whether it returns. That one question catches a whole class of bug where a value is printed and never passed on.
Zero is a real number you can add to. Empty text is a real empty message. None is the statement that no value was produced. Mixing them up makes a program compute with a fact that was never there, which is why the check is worth writing.
best = None
print(best)None means "nothing here yet". It is a value, so it prints, and it can be stored. It is not zero and not empty text, it is its own kind with one value.
best = None
print(best is None)The word is asks whether a name holds exactly that nothing. Testing with is None is the readable check, because other empty things can be false for their own reasons.
def show(x):
print(x)
print(show(3))show prints but never says "give back". A function like that answers with None, so storing its result stores nothing useful. That is the difference between showing and returning.
def twice(x):
return x * 2
print(twice(3))return hands a value back to where the call was written. Only then does print have something to show. A function either shows for you or gives a value, and giving is the useful one.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.