What is the walrus operator (:=) and when is it useful?

The walrus operator (:=), introduced in Python 3.8 (PEP 572), is the assignment expression operator. It assigns a value to a variable as part of a larger expression rather than as a standalone statement. The name comes from its resemblance to a walrus face with tusks.

# Without walrus — evaluate twice
data = fetch_data()
if data:
    process(data)

# With walrus — evaluate once, assign, and test in one expression
if data := fetch_data():
    process(data)

# Classic use: while loop reading chunks from a file
with open('large.bin', 'rb') as f:
    while chunk := f.read(8192):
        process_chunk(chunk)

# Filtering with a computed value — avoid calling the function twice
results = [cleaned for raw in records
           if (cleaned := clean(raw)) is not None]

The walrus operator is most valuable when you need to compute a value, test it, and use it — and calling the computation twice would be wasteful or have side effects. Common patterns: while loops reading from streams, filtering list comprehensions where the filter function is expensive, and reducing nested if-statements.

Avoid overusing it — plain assignment on a separate line is often more readable. The walrus is idiomatic in tight loops and comprehensions; in most other code the conventional two-step (assign then test) is clearer.

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