OR and AND do WHAT now?

Computers will do exactly what you tell them to do, not what you want them to do.

Hey pips!

We looked at truthiness last week, which was a bit of a wacky concept. Today, I’m going to shatter how you understand or and and.

It’s pretty self explanatory that if x and y only executes if x and y are true – or truthy, as we now know – and if x or y will execute if either (or both) are truthy.

Let’s try storing one of these expressions in a variable, and see what happens.

>>> win = 0 or 6
>>> if win:
>>>     print("yay")
yay

Remember that 6 is considered truthy, which means the print() nested inside if does run. What’s actually the value of win though?

>>> win
6

Huh? Not True?

Contrary to what you may have been led to believe, or and and are not like == and <, which output True or False. Actually, they return 1 of the 2 expressions they’re given!

So here, we really have:

>>> if 6:
        print("yay")
yay

But how do we know which expression is returned?

Let’s think about how an OR statement works. In x or y, if we know 1 of x or y is truthy, then we immediately know the whole expression must be truthy.

Python will look at x first. If it’s truthy, then it doesn’t matter what y is – no matter what, the whole expression is truthy.

So Python returns x from the expression:

>>> 3 or 4
3

>>> "life" or None
'life'

>>> [0, 1, 2] or [3, 4, 5]
[0, 1, 2]

But if x is falsy, then we do need to look at the truthiness of y. In fact, the truthiness of the whole expression is now determined by y – if it’s falsy, the expression is falsy; if it’s truthy, the expression is truthy.

So in this case, Python returns y from the expression:

>>> None or 2.0
2.0

>>> False or ""
''

>>> [] or [9, 9, 6]
[9, 9, 6]

And guess what, this is actually just a ternary conditional!

# this conditional...
x if x else y

# is identical to:
x or y

Now let’s look at and. Can guess how it works?

x and y is only truthy if both x and y are truthy. So (contrapositively 🔥) if either x or y is falsy, then the whole expression must be false.

Once again, we’ll look at the expressions from left to right. If Python comes across x and sees that it’s falsy, then it knows the whole expression is falsy. So it returns x:

>>> 0 and 5
0

>>> None and "link"
None

>>> {} and {"state": "pondering"}
{}

But if x is truthy, then like before, the truthiness of the whole expression now rests on the metaphorical shoulders of y – if it’s truthy, the expression is truthy, if it’s falsy, the expression is falsy.

So in this case, Python returns y:

>>> "project" and "sekai"
'sekai'

>>> True and 3
3

>>> [-1, 1] and []
[]

It takes a little more time to get comfortable with using or and and in this way, but you’ll find some neat situations where they come in quite handy!

Further Reading

• Boolean algebra – Wikipedia^↗

Challenge

You’re given 2 objects, py and co. 1 of them is truthy, 1 is falsy. Can you write an expression to print the truthy one first, followed by the falsy one?

>>> py = "aleph"
>>> co = 0

>>> (your_expression)
aleph
0

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