+++ slug = "assignment-expressions" title = "Assignment Expressions" section = "Control Flow" summary = "The walrus operator assigns a value inside an expression." doc_path = "/reference/expressions.html#assignment-expressions" see_also = [ "conditionals", "while-loops", "variables", ] +++

The assignment expression operator := assigns a name while evaluating an expression. It is often called the walrus operator.

Use it when computing a value and testing it are naturally one step. Avoid it when a separate assignment would make the code easier to read.

The boundary is readability: the walrus operator can remove duplication, but it should not hide important state changes.

:::program

messages = ["hello", "", "python"]

for message in messages:
    if length := len(message):
        print(message, length)

queue = ["retry", "ok"]
while (status := queue.pop(0)) != "ok":
    print(status)
print(status)

:::

:::cell An assignment expression can name a computed value while a condition tests it. Here empty strings are skipped because their length is zero.

messages = ["hello", "", "python"]

for message in messages:
    if length := len(message):
        print(message, length)

hello 5
python 6

:::

:::cell The same idea works in loops that read state until a sentinel appears. The assignment and comparison stay together.

queue = ["retry", "ok"]
while (status := queue.pop(0)) != "ok":
    print(status)
print(status)

retry
ok

:::

:::note

• name := expression assigns and evaluates to the assigned value.
• Use it to avoid computing the same value twice.
• Prefer a normal assignment when the expression becomes hard to scan. :::