interpreter

id	behavioral/interpreter
name	Interpreter
aliases	little-language expression-tree
guide_url
problem	Represent a small language's grammar as data and evaluate sentences in it.
symptoms	mini query language user-supplied formulas rules engine evaluate expressions safely
verdict	prefer-alternative
caveats	Before inventing a language, check whether Python is the language: ast.literal_eval for data, a vetted ast walk for arithmetic, a real parser library beyond that. Never eval() user input — the safe version of this pattern exists precisely to avoid that.
stdlib_sightings	ast.literal_eval ast.NodeVisitor re

Interpreter

Represent a tiny language's grammar as data and evaluate sentences safely. Verdict: prefer an alternative — Python's own parsers (ast, re) cover most little-language needs; grammar-as-data covers the rest.

Where	What
pattern/	The importable code: Interpreter (tuple-tree evaluator), hardened safe_eval
docs/	Fundamentals · Implementation guide · External examples
examples/flag_rules/	Mini-project: feature-flag rules engine built on pattern/
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.behavioral.interpreter.examples.flag_rules.main