What are Assignment Expressions and Why Were They Introduced to Python 3.8?

Understanding Assignment Expressions in Python 3.8

Since Python 3.8, the "walrus" operator (:=) has introduced assignment expressions to the language. This new feature allows programmers to assign values within comprehensions and lambda functions, which were previously restricted to regular assignments.

Rationale for Assignment Expressions

The primary motivation behind assignment expressions is to enable more concise and efficient code in scenarios where traditional assignments are not supported. For instance, assignment expressions facilitate:

• Conditional Value Acquisition: Conditional values can be obtained more succinctly, as demonstrated by the interactive debugging example provided in the Python documentation.
• Simplified List Comprehensions: Complex list comprehensions can be simplified by assigning intermediate values to variables within the comprehension, making the code more readable and manageable.

Syntax, Semantics, and Grammar

Assignment expressions follow a specific syntax: name := expr, where name is an identifier and expr is any valid Python expression. The value of the assignment expression is the same as the expression expr, but an additional side effect assigns the value to the target name.

Assignment expressions differ from regular assignment statements in several key aspects:

• They are expressions, not statements, and therefore can appear in contexts where expressions are expected.
• They have a different evaluation order (right-to-left) and a different precedence compared to regular assignments.
• They do not support multiple targets, assignments to non-single names, iterable packing/unpacking, inline type annotations, or augmented assignment operations.

Reasons for the Introduction of Assignment Expressions

Despite the withdrawal of PEP 379, which proposed a similar concept, PEP 572 introduced assignment expressions to Python for several reasons:

• To enhance code readability and maintainability, especially within comprehensions and lambdas.
• To enable interactive debugging without the need for code refactoring.
• To provide a more efficient way of assigning conditional values and simplifying list comprehensions.
• To align Python's syntax with other modern programming languages that support assignment expressions.

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