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And, or, not in Python: combine conditions into rules

Real rules are rarely one test. "Free shipping over $75, or for members." "A credit if you returned an item and spent $30." Three keywords cover all of it.

The big idea

and is true when both sides are true, or when at least one is, and not inverts a boolean.

See it in code

1The basics

or needs only one side to be true. A member with a small order still ships free:

python
order_total = 50
free_shipping_min = 75
is_member = True

ships_free = order_total >= free_shipping_min or is_member
print("Ships free:", ships_free)
Run it. A $50 order from a member:
Ships free: True

The first test was false (50 is under 75), but is_member was true, and one true side is enough for or.

2A step further

and is stricter: both sides must hold. The recycling credit applies only to customers who returned an item and met the minimum:

python
credit_min = 30

returned_item = True
order_total = 90
print("Gets credit:", returned_item and order_total >= credit_min)

returned_item = True
order_total = 20
print("Gets credit:", returned_item and order_total >= credit_min)
Run it. Two customers, one rule:
Gets credit: True
Gets credit: False

Both customers returned an item. Only the first also spent at least $30, so only the first satisfies and.

3At Evergreen

not inverts a boolean, and the three operators combine. Without parentheses, not binds tightest, then and, then or:

python
order_total = 50
is_member = False
returned_item = False
free_shipping_min = 75
credit_min = 30

if not returned_item:
    print("Tip: send back a worn-out item for $5 off your next order")

vip = is_member or order_total >= free_shipping_min and returned_item
print("VIP perks:", vip)
Run it. A nudge, and a combined rule:
Tip: send back a worn-out item for $5 off your next order
VIP perks: False

The last rule reads as is_member or (order_total >= 75 and returned_item), because and is evaluated before or. Writing those parentheses yourself costs nothing and removes all doubt.

The same idea, everywhere

Python also short-circuits: in a and b, if a is false it never evaluates b. That lets you guard a risky check, like testing that a list isn't empty before reading its first item.

Try it yourself

Set is_member = True and re-run; vip flips to true whatever the other values are. Then rewrite the rule so a customer needs to be a member and have either a $75 order or a returned item.

The common mistake

Writing order_total == 40 or 75 and expecting it to test both. Python reads that as (order_total == 40) or 75, and 75 counts as true, so the whole thing is always true. Repeat the comparison: order_total == 40 or order_total == 75.

What it unlocks

Each side of a rule is a comparison, and the finished rule goes inside an if or keeps a while loop running.

Want the simpler version? Read the Kids version →

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