Python Lambda

Python Lambda Functions: Anonymous One-Liners & Higher-Order Functions

Lambda functions are small, anonymous functions declared inline using the lambda keyword. They are restricted to a single expression whose evaluation result is automatically returned.

Lambda Architecture vs Standard Function

flowchart LR
    subgraph Standard Function
        def["def add(x, y):"] --> ret["return x + y"]
    end
    subgraph Lambda Expression
        lam["lambda x, y: x + y"]
    end

Lambda Syntax: lambda arguments: expression

• Single Expression Only: Cannot contain multi-line code, assignments, or explicit return statements.
• Used with Higher-Order Functions: Commonly passed into sorted(), map(), filter(), or min()/max().

Practical Code Example

from typing import List, Dict, Any

def Higher_order_demo() -> None:
    users: List[Dict[str, Any]] = [
        {"name": "Alice", "age": 28, "score": 92},
        {"name": "Bob", "age": 34, "score": 85},
        {"name": "Charlie", "age": 22, "score": 98},
    ]

    # Sort users by age using lambda
    users_by_age = sorted(users, key=lambda u: u["age"])
    print("Sorted by Age:", [u["name"] for u in users_by_age])

    # Sort users by score descending using lambda
    users_by_score = sorted(users, key=lambda u: u["score"], reverse=True)
    print("Top Scorers:", [f"{u['name']} ({u['score']})" for u in users_by_score])

    # Filtering with lambda and filter()
    high_scorers = list(filter(lambda u: u["score"] > 90, users))
    print("High Scorers (> 90):", [u["name"] for u in high_scorers])

if __name__ == "__main__":
    Higher_order_demo()

Best Practices & Gotchas

• Do Not Assign Lambdas to Variables: Avoid add = lambda x, y: x + y. Use standard def add(x, y): return x + y for better stack trace debugging.
• Prefer List Comprehensions over map()/filter(): [u for u in users if u['score'] > 90] is cleaner than list(filter(lambda u: ..., users)).
• Keep Expressions Simple: If a lambda expression becomes hard to read, refactor it into a named function.

Self-Check Challenge

Sort a list of strings ["apple", "banana", "kiwi", "fig"] by string length using sorted() and a lambda key function.