chapter_07_functions_first_class

Chapter 7 — Functions as First-Class Objects

Theme: Functions in Python are real, tangible objects. They can be created, assigned, passed, returned, and inspected just like an integer or a dictionary.

What You'll Learn

Topic	Key insight
First-Class Functions	Functions are objects of the function class.
Higher-Order Functions	Functions that take or return other functions (map, filter, sorted).
__call__ method	Any object can act like a function if it implements __call__.
lambda functions	Restricted to a single pure expression. Often abused.
Modern Replacements	List comprehensions are usually faster/clearer than map/filter.
Parameter Signatures	/ for positional-only, * for keyword-only arguments.
Functional Primitives	operator module and functools.partial replace trivial lambdas.

Key Files

• examples.py — Core implementations of signatures, __call__, and operator.
• exercises.py — 5 original exercises including Fredrik Lundh's lambda refactoring.
• mini_project.py — Functional pluggable rule engine for password validation.
• benchmarks.py — Performance: itemgetter vs lambda, listcomp vs map.
• notes.md — Reference tables for callables and parameters.
• pitfalls.md — Production traps: map consumption, lambda loop binding.
• interview_questions.md — L3 to L6 interview prep.
• architecture_notes.md — CPython's PyFunctionObject and METH_FASTCALL.

30-Second Rules

# Rule 1: Use comprehensions over map/filter
bad = list(map(upper, filter(is_even, data)))
good = [upper(x) for x in data if is_even(x)]

# Rule 2: Use operator module over trivial lambdas
bad = sorted(data, key=lambda x: x[1])
good = sorted(data, key=itemgetter(1))

# Rule 3: Use functools.partial to adapt signatures
bad = button.on_click(lambda: do_action("save"))
good = button.on_click(partial(do_action, "save"))

Reference: Fluent Python 2nd ed., Chapter 7 — pages 231–252