55_assignment_expressions.md

55. Assignment expressions (:=)

Source: src/csharp_to_python_learning/concepts/11_additional_language_features/topic_55_assignment_expressions.py

What C# developers usually expect C# developers usually declare a variable before condition checks.

C# example Simple equivalent:

var items = new[] { "a", "b", "c", "d" };
if ((items.Length) is var size && size > 2) Console.WriteLine($"size:{size}");

Advanced equivalent:

var raw = new[] { " A ", " ", "B", "", " C" };
var cleaned = raw.Select(item => item.Trim()).Where(item => item.Length > 0).ToList();
Console.WriteLine($"[{string.Join(", ", cleaned)}]");

Simple Python example from this file

items = ["a", "b", "c", "d"]
if (size := len(items)) > 2:
    print(f"size:{size}")

Advanced Python example from this file

raw = [" A ", " ", "B", "", " C"]
cleaned = [item for raw_item in raw if (item := raw_item.strip())]
print(cleaned)

Python equivalent Python's walrus operator (:=) allows assignment inside expressions when it improves clarity.

Detailed explanation for C# developers Use this feature sparingly for readable pipelines, loop conditions, and lightweight parsing.

Common mistakes for C# developers

1. Overusing walrus in dense expressions.
2. Hiding important state changes inside nested conditions.

Exercises

1. Refactor one parsing loop to use a single walrus assignment.
2. Compare readability with and without walrus in a code review.

Expected output

• size:4
• ['A', 'B', 'C']