Walrus Operator

Python · Reference cheat sheet

📋 Overview

The walrus operator := (PEP 572, 3.8+) assigns inside an expression. Use it to avoid double computation or awkward control-flow temporaries — sparingly, for clarity.

🔧 Core concepts

Cannot replace plain statement assignment everywhere; illegal in some syntactic positions (e.g. unparenthesized in some tuple contexts historically — prefer parentheses when unsure).

💡 Examples

if / while:

from pathlib import Path

path = Path("data.txt")
if (text := path.read_text(encoding="utf-8")).strip():
    print(text[:20])

while (line := input("cmd> ")) != "quit":
    print("got", line)

Reuse match result:

import re

pattern = re.compile(r"(\d+)")
if m := pattern.search("id=42"):
    print(int(m.group(1)))

Comprehension:

def costly(x: int) -> int:
    return x * x

nums = [1, 2, 3, 4]
# compute once per item
big = [y for x in nums if (y := costly(x)) > 5]
print(big)  # [9, 16]

Chunked read:

from pathlib import Path

with Path("blob.bin").open("rb") as f:
    while chunk := f.read(4096):
        print(len(chunk))

⚠️ Pitfalls

• Overuse hurts readability — if the name is important, use a normal assignment.
• In comprehensions, the bound name's scope can surprise readers (leaks in some contexts historically; in 3.x comps have isolated scopes for for targets, but := targets in the comprehension scope).
• Don't hide side effects inside walrus expressions.
• Not valid as a standalone replacement for all = uses.
• Combine carefully with and/or — parentheses help.

🔗 Related

• Conditionals
• Loops
• Comprehensions
• Files I/O
• Regex
• Operators