Operators and precedence

Most Python operators behave the way arithmetic class taught you. A handful do not, and those are the ones that cause bugs: -2 ** 2, a comparison that chains, and and returning something that is not a boolean.

The fundamentals lesson explains precedence and short-circuiting in general. This lesson covers how Python applies them.

Precedence in practice

From tightest to loosest, the groups you will meet most are: **, unary -, then * / // %, then + -, then comparisons, then not, and and or. When in doubt, add parentheses. They cost nothing and tell the next reader what you meant.

Start with arithmetic:

print(2 + 3 * 4)
print(-2 ** 2)
print((-2) ** 2)
print(2 ** 3 ** 2)
print(10 - 4 - 3)

14
-4
4
512
3

• -2 ** 2 is -(2 ** 2), because ** binds tighter than a unary minus on its left.
• ** groups right to left, so 2 ** 3 ** 2 is 2 ** (3 ** 2), which is 512.
• Subtraction groups left to right: 10 - 4 - 3 is (10 - 4) - 3.

Comparisons produce True or False, and not, and and or combine them:

print(not 1 == 2)
print(1 + 2 == 3 and 4 > 3)

True
True

not 1 == 2 is not (1 == 2), because comparisons bind tighter than not. The second line works out both comparisons first, then combines them with and.

Chained comparisons

Python lets you chain comparisons the way mathematics does. a < b < c means a < b and b < c, with b evaluated only once.

age = 25
print(18 <= age < 65)
print(1 < 3 < 2)

True
False

The first line asks whether age lies in the range from 18 up to, but not including, 65. The chain 1 < 3 < 2 fails on its second link.

and and or return operands

and and or do not return True or False. They return one of their two operands:

• a or b returns a if a is truthy, otherwise b.
• a and b returns a if a is falsy, otherwise b.

Here “truthy” and “falsy” mean what they sound like: 0 and the empty string "" count as false, and almost everything else counts as true. Lesson 13 covers the full rules.

print(0 or "default")
print("name" or "default")
print(5 and 7)
print("" and "never")

default
name
7

Both operators stop as soon as the answer is known, which is called short-circuiting. That is why "" and "never" gives the empty string: the right side is never looked at. The blank last line in the output is that empty string.

This makes value or default a popular way to supply a fallback:

nickname = ""
print(nickname or "anonymous")

anonymous

Be careful, though: it replaces every falsy value, including 0 and "", not just missing ones.

The walrus operator

The assignment expression :=, nicknamed the walrus, binds a name in the middle of an expression. It exists since Python 3.8.

print((squared := 7 * 7) + 1)
print(squared)

50
49

The part in parentheses worked out 7 * 7, bound the name squared to the result, and then handed that value to the + 1. Afterwards squared is an ordinary name. You will find the walrus most useful inside the conditions of if and while, once you have met them, where it avoids calling something twice. Wrap it in parentheses unless it stands alone, and use it sparingly. If a plain assignment on its own line reads better, use that.

Common pitfalls

• x == 1 or 2. This parses as (x == 1) or 2, which is always truthy. Write x == 1 or x == 2.
• Misusing a chain. 1 < x > 0 is legal but confusing. Keep chains in one direction.
• Reading a != b != c as “all three differ”. It means a != b and b != c, so a and c may still be equal.
• Using or for defaults when 0 is valid. count or 10 turns a real 0 into 10.

print(3 + 4 * 2 ** 2)
print(-3 ** 2)
print(2 * 3 % 4)
print(7 // 2 * 2)
print(not True or True)
print(1 < 2 < 3 < 3)
print(0 or "" or "x")
print(10 / 4 * 2)
print(2 + 3 > 4 and 1 + 1 == 2)
print("a" * 2 + "b" * 3)

19
-9
2
6
True
False
x
5.0
True
aabbb

x = 3
print(x == 1 or 2)
print(x == 1 or x == 2)
print(1 < x < 5)

2
False
True

Summary

• ** binds tighter than a unary minus, and ** groups right to left. Use parentheses when unsure.
• a < b < c is a chained comparison. and and or return one of their operands.
• := names a value inside an expression and has existed since Python 3.8.

Next: strings.