spintax-core (Python)

A framework-agnostic Spintax engine for Python — parse, render, validate, extract, analyze, and neutralize spintax templates. MIT, zero runtime dependencies, Python 3.10+.

This is the third engine in the Spintax family, and an independent implementation — not a transcription of the others. It is held to the same behavior contract by a shared golden corpus of language-neutral fixtures, which already gates the TypeScript engine and the PHP one. Every one of them passes here, none skipped, none expected to fail.

Install

pip install spintax-core

Use

from spintax_core import render, validate, parse

render("{Hello|Hi} there!")                        # "Hello there!" or "Hi there!"
render("{Hello|Hi} there!", seed=42)               # same seed, same output, every time
render("Hi %name%!", context={"name": "Sam"})      # "Hi Sam!"

# Reuse a template: parse once, render many.
ast = parse('[<sep=", ">fast|cheap|good]')
[render(ast) for _ in range(3)]
# ['Cheap, fast, good', 'Good, cheap, fast', 'Cheap, good, fast']

# Check a template before you ship it.
[d.code for d in validate("{a|b")]                 # -> ['bracket.unclosed']

Rendering is lenient: malformed markup degrades rather than raising, so a template a non-programmer wrote cannot take a page down. Output is also tidied up by default — sentence capitalization, spacing around punctuation, URLs and abbreviations left intact — which is why […fast|cheap|good] above comes back capitalized. Pass post_process=False to turn that off and get the raw pick.

A variable written inside a construct is spliced into it as text before the construct is split, so a pipe-joined value is a list rather than one item:

render('[<sep=", ">%names%]', context={"names": "Ann|Bo|Cy"}, seed=1)  # 'Cy, Bo, Ann' — three elements
render("%names%", context={"names": "Ann|Bo|Cy"})                     # 'Ann|Bo|Cy' — no construct, no split

That is the behaviour of every engine in the family (it comes from the original, which substitutes variables before it reads a bracket). It also means neutralize() does not make a value atomic there — it shields brackets, braces, % and #, and deliberately not |. Strip the pipes yourself if a value must stay one option.

Syntax — enumerations {a|b}, permutations [<sep=", ">a|b], variables %name%, conditionals {?VAR?yes|no}, plural agreement {plural 3: one|few|many}, comments /# … #/, and the #set / #def / #include directives — is documented in full at spintax.net/docs.

• Spec: docs/spec-python-port.md — the parity contract, the API surface, and the decisions behind the port (including where Python's regex dialect is wider than JavaScript's and how each divergence was measured and pinned).
• Sibling engines: @spintax/core (TypeScript) · spintax/core (PHP) · spintax-win (Object Pascal) — all MIT, zero-dep, held to one shared corpus · plus Spintax for WordPress (PHP, GPL, the origin).
• Tracking issue: investblog/spintax-js#43.

Why

The existing PyPI spintax package is GPLv3 and has not shipped since 2018. GPL blocks commercial adoption; this one is MIT and maintained.

License

MIT. The WordPress plugin remains GPL; MIT/Expat is GPL-compatible.

Part of the 301.st toolset. Product home: spintax.net.

Development

The test suite is the shared golden corpus — the same JSON fixtures the TypeScript and PHP engines are tested against, read from a checkout rather than vendored here. A copy would drift, and a drifting contract is not a contract.

git clone https://github.com/investblog/spintax-js ../spintax-js   # once
python -m venv .venv && .venv/bin/pip install -e . pytest
SPINTAX_FIXTURES=../spintax-js/packages/conformance/fixtures pytest

Without the fixtures the suite fails rather than passing an empty run — a green suite that tested nothing is the most expensive kind of green.

The engine is complete, so every corpus case is a real pass: every corpus fixture passes, 0 xfailed, 0 skipped, alongside the port's own local tests. The count is deliberately not written here — the corpus grows, and a number pinned in a file that ships to PyPI immutably goes stale quietly. A skip must never appear — it would mean a case is being neither asserted nor counted. During the build (P0–P3) xfailed was the milestone tracker, one number counting down what the corpus expected and the engine could not yet do; it reached zero when the renderer and analyze landed.

If you mutate a source file to check that a test catches it, delete __pycache__ first. Python invalidates bytecode on (mtime, size). A mutation that preserves both — swapping .*? for .+?, say — leaves the stale .pyc in place, so the next run still imports the mutated module after you have restored the file. That produced a failure pointing at correct code, and the obvious response to it would have been to break the code for real.