lambda, map, and filter

Scenario: A small one-use transformation must be passed into another function.

New words, in plain English

• First-class function - a function that can be stored and passed like data
• lambda - a small anonymous single-expression function
• map - a lazy transformation of every item
• filter - a lazy selection of matching items

map(fn, values) and filter(test, values) return iterators. Comprehensions are often clearer; lambdas shine as short callback functions.

Analogy: A Python program is like a clear set of instructions for a careful helper: names label things, indentation groups steps, and errors explain where the helper became confused.

A small, real example

ports = [443, 80, 5432]
labels = list(map(lambda p: f"tcp/{p}", ports))
privileged = list(filter(lambda p: p < 1024, ports))

Tip: Name a function with def if its logic is reused or deserves documentation.

Goal: Practice this idea in the py-list-comprehension challenge at /labs/python. Fix the broken program, run it yourself, then use check to prove the real end state.