What are Assignment Expressions and How Do They Work in Python 3.8?

Assignment Expressions with the "Walrus" Operator: A Python 3.8 Feature

Python 3.8 introduced a new concept known as "assignment expressions" using the ":=" operator, commonly referred to as the "walrus" operator.

Syntax, Semantics, and Grammar

An assignment expression takes the form name := expr, where expr is any valid Python expression and name is an identifier. The semantics are that the expression's value is assigned to name, and the value of the assignment expression is also the same as expr.

Rationale

The primary motivation for introducing assignment expressions was to enable assignments within constructs like list comprehensions and lambda functions that previously forbade them. It also facilitates interactive debugging without the need for code refactoring.

Use Cases

a) Getting Conditional Values:

• Before:

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

• After:

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

b) Simplifying List Comprehensions:

• Before:

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

• After:

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

Differences from Regular Assignments

Assignment expressions differ from regular assignments in several ways:

• They are expressions rather than statements.
• They have right-to-left precedence.
• They differ in priority around commas.
• They do not support multiple targets, assignments to non-single names, iterable packing/unpacking, inline type annotations, or augmented assignments.

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