11_closures.md

11. Closures

Source: src/csharp_to_python_learning/concepts/03_functions_and_functional_tools/topic_11_closures.py

What C# developers usually expect C# developers usually expect explicit type declarations, predictable object lifetimes, and compile-time guidance.

C# example Simple equivalent:

var count = 0;
int Inc() => ++count;
Console.WriteLine($"{Inc()} {Inc()}");

Advanced equivalent:

var cache = new Dictionary<int, int>();
int Square(int value)
{
    if (!cache.ContainsKey(value)) cache[value] = value * value;
    return cache[value];
}
Console.WriteLine($"{Square(12)} {Square(12)}");

Simple Python example from this file

def make_counter():
    count = 0

    def inc():
        nonlocal count
        count += 1
        return count

    return inc

counter = make_counter()
print(counter(), counter())

Advanced Python example from this file

def memoized_square():
    cache: dict[int, int] = {}

    def run(value: int) -> int:
        if value not in cache:
            cache[value] = value * value
        return cache[value]

    return run

square = memoized_square()
print(square(12), square(12))

Python equivalent Python approaches this concept with less ceremony: Closures capture surrounding state, similar to C# captured variables in local functions/lambdas. You still keep production discipline through tests, typing, and tooling.

Detailed explanation for C# developers Closures capture surrounding state, similar to C# captured variables in local functions/lambdas. In practice, combine this with logging, tests, and type hints so the flexibility does not turn into ambiguity. Prefer small functions, clear data boundaries, and explicit contracts where behavior matters.

Common mistakes for C# developers

1. Assuming C# defaults apply directly in Closures without checking Python runtime behavior.
2. Skipping tests because dynamic code feels simpler at first; regressions grow quickly without guardrails.

Exercises

1. Rewrite one recent C# snippet in Python using this concept: Closures.
2. Add input validation, type hints, and one pytest test for the rewritten snippet.

Expected output

• 1 2
• 144 144