What are Assignment Expressions and How Do They Work in Python?

Assignment Expressions in Python: The "Walrus" Operator (=:)

Introduced in Python 3.8, assignment expressions utilizing the "walrus" operator (:=) provide a significant language enhancement, enabling assignments within comprehensions and lambdas.

Syntax and Semantics

An assignment expression is a named expression of the form name := expr, where name is an identifier and expr is any valid expression. The expression evaluates to the value of expr, while simultaneously assigning that value to name.

Rationale for Introduction

The primary motivation for adding assignment expressions was to:

• Permit assignments in contexts where traditional assignments were prohibited, such as list comprehensions and lambdas.
• Enhance interactive debugging by facilitating easy value assignment.

Usage Examples

a) Obtaining Conditional Values

Instead of:

<code class="python">command = input("> ")
while command != "quit":
    print("You entered:", command)
    command = input("> ")</code>

Assignment expressions allow:

<code class="python">while (command := input("> ")) != "quit":
    print("You entered:", command)</code>

b) Simplifying List Comprehensions

Example:

<code class="python">[(lambda y: [y, x/y])(x+1) for x in range(5)]</code>

Can be simplified to:

<code class="python">[[y := x+1, x/y] for x in range(5)]</code>

Key Differences from Regular Assignments

Assignment expressions differ from regular assignments in several aspects:

• They are evaluated right-to-left.
• They have a different precedence than commas.
• They do not support multiple targets, assignments to non-single names, or iterable packing/unpacking.
• Inline type annotations and augmented assignments are also unsupported.

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