The walrus operator in Python

Assign inside an expression, when the value is needed twice.

lines = iter(["first", "second", "", "third"])

while (line := next(lines, None)) is not None:
    if not line:
        print("blank line, stopping")
        break
    print(line)

values = [1, 2, 3, 4, 5, 6]
if (total := sum(values)) > 20:
    print(f"total {total} is large")

print([squared for n in values if (squared := n * n) % 2 == 0])

How it works

1. := binds and evaluates in one place.
2. It shines in a while loop reading until a sentinel.
3. Used anywhere else it usually hurts readability.

Keywords and builtins used here

• break
• for
• if
• iter
• next
• print
• sum
• while

The run, in numbers

Lines

13

Characters to type

314

Tokens

111

Three-star pace

100 tpm

At the three-star pace of 100 tokens a minute, this run takes about 67 seconds.

Type this snippet

Step 1 of 3 in Expressions, step 65 of 72 in Language basics.

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