builder

id	creational/builder
name	Builder
aliases	fluent-builder
guide_url	https://python-patterns.guide/gang-of-four/builder/
problem	Assemble a complex object step by step, so the assembly process is reusable and readable.
symptoms	constructor with ten arguments object needs staged assembly fluent chained construction same steps, different representations
verdict	use-with-care
caveats	Python's keyword arguments with defaults already solve the 'telescoping constructor' problem the GoF Builder exists for. The guide's verdict: the Builder survives in Python mainly as a convenience for callers (e.g. matplotlib's pyplot), not as a construction ceremony.
stdlib_sightings	email.message.EmailMessage configparser.ConfigParser

Builder

Assemble a complex object step by step, with validation at each step and an immutable result. Verdict: use with care — keyword arguments already solve one-shot construction; a builder earns its keep only when assembly is genuinely staged.

Where	What
pattern/	The importable code: SelectBuilder (mutable, fluent) → Query (frozen)
docs/	Fundamentals · Implementation guide · External examples
examples/sql_select_builder/	Mini-project: order analytics on sqlite, every query builder-staged
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.creational.builder.examples.sql_select_builder.main