astero

The middle end, without writing it.

Parser generators took the front end fifty years ago; lexers are no longer written by hand. The middle never got the same treatment. Every compiler and every analyser still rolls its own symbol table, binder, renamer, fresh-name supply, capture-avoiding substitution and bracketing, each working out the same facts about the language to do it.

You declare which positions bind a name, which read one, and which open a scope. The passes above follow from that. For Python you declare nothing, because the grammar ships with the library.

The library ships the part that costs weeks to find out. That comprehensions stopped opening a scope in 3.12. That PEP 695 blocks are spelled type parameter on 3.12 and type parameters from 3.13. That 3.14 adds annotation blocks unless the module carries from __future__ import annotations. You find each one by hitting it, bisecting it, and reading a PEP.

pip install abilian-astero        # or: uv add abilian-astero

The distribution is called abilian-astero, because PyPI already had astero. The import name is astero:

from astero.python import PY, VARS

Python 3.11 through 3.15. No runtime dependencies. Apache-2.0.

The problem it solves

Twenty-two positions in Python bind a variable name (eighteen on 3.11, before PEP 695 added four). Here are four of them:

def f(x):                      # a parameter
    import os as x             # an import alias
    try:
        pass
    except E as x:             # an exception name
        pass
    return [x for x in xs]     # a comprehension target

A renamer written with ast.NodeTransformer and a visit_Name method reaches one. That is not a hypothetical: it is the shape of a defect found in three separate compilers. None of the three crashed. One never bound **kwargs, one inlined a call so that [99 for 99 in xs] came out, one set an assignment context by hand and produced JavaScript that computed NaN where Python gave 5.

With astero the list is a query, which serves every pass that needs it:

from astero.python.hygiene import rename
from astero.python import PY, VARS

rename(tree, {"x": "y"}, PY, VARS)      # reaches all of them

Python releases change the set: PEP 695 added four positions in 3.12. Code written against the query follows the language without being edited.

What it is not

astero does not parse, and does not replace ast. CPython already declares the shape of every node, which astero reads: Assign.targets is list[expr] and Assign.value is expr, both straight from the interpreter.

ast cannot tell you that the first of those two introduces a name and the second does not. To ast they are both expr. That fact lives in the language reference and in the head of whoever writes the pass. astero ships it: of Python's 176 fields across 124 productions on 3.13, 138 are plain subtrees with nothing to say, while the other 38 carry a role. Those counts move with the language (3.11 has 163 fields and 3.15 has 183), which is why astero reads them off the interpreter.

What you get

Declare a role for each field of each production: whether it introduces a name, refers to one, or holds a subtree. Six modules read that declaration.

module	what it gives you
grammar	which fields bind a name, which read one, which hold a bare identifier, which a traversal must enter, which productions can appear
rewriting	tree rewriting from pattern => result rules, with every Name's context recomputed for you
scopes	the scope tree of a module, and the names each scope binds
hygiene	renaming, and substitution that cannot capture a name
emit_rules	a code generator written as templates, with brackets derived from a precedence table
coverage	a test that fails when your dispatch table misses a production

Add a binding form to your language and edit the declaration: all six follow.

Rewriting, without touching contexts

A rule is one string with => between pattern and replacement:

from astero.python import Pass, rules

fold = Pass("fold", rules("""
    _x + 0 => _x
    _x * 1 => _x
"""))

Identifiers spelled _name are metavariables, _ matches anything without binding, and *_xs splices the rest of a list. Rules never mention ctx or source positions, because both are recomputed from the shape of the result. That is what makes the AugAssign defect above impossible to write.

Your own IR, as well as Python's

A grammar carries each production's constructor, so a rewrite builds your node classes. If your AST is generated from CPython's, the whole declaration is one line:

PRESCRYPT = astero.python.build(module=prescrypt.front.ast.ast, name="prescrypt")

If your IR is annotated dataclasses, from_dataclasses reads the fields and sorts off the annotations, leaving you only the roles to write. The PL/0 tutorial builds a complete compiler that way, for a language with no relationship to Python.

Why you can rely on it

Every derivation is compared against an independent source of truth, over the whole Python standard library, on Python 3.11 through 3.15.

derived	compared against	result
assignment contexts	what CPython's parser produces	every position in the standard library, no disagreements
scopes and their names	CPython's symtable	99.98% of blocks agree
emitted source	reparsing the emitted text	1,797 modules, 2,266,043 expressions
operand positions of an SSA IR	that compiler's own declaration	exact match

For assignment contexts that is ten authored role entries reconstructing all 1,457,931 positions across the 1,761 files of the 3.13 standard library. The tests hold the ratio as measured. A regression fails; an improvement does not have to be chased.

The rewrite engine was first validated by reimplementing all six of latexify_py's tree transformations as rule sets. Differential testing against the originals found 121 output differences, each of them a latent bug in the hand-written version.

Three compilers are built on it: a Python-to-C compiler, a Python-to-WebAssembly compiler, and a Python-to-JavaScript transpiler.

What astero is not

• Not a parser. Bring your own AST. astero starts from a tree.
• Not a code generator you run. Nothing is written to disk; there is no build step. Every entry point is a function call at run time.
• Not a framework. It does not own your pipeline, your IR, or your main. Adopt one query in one function and leave the rest alone.

Lowering and cost models stay yours. astero answers questions about a declaration; it does not decide what your compiler should do with the answers.

Documentation

• Getting started: install it and run three queries against real code.
• TinyPy tutorial: a working compiler for a Python subset with two back ends, in 357 lines of which 266 are code: the compiler is 189, each back end under 100.
• PL/0 tutorial: the same, for a language that is not Python.
• User guide: every module, its API, and when to reach for it.
• Adopting astero: fitting it into a compiler you already have.
• API reference: generated from the source.

Status

Version 0.2.x. The API may still change between minor versions; the changelog says what moved. 0.2.0 split the package by language: everything Python-specific is under astero.python, with no shims. Every derivation is checked against CPython on five interpreters. Three compilers are built on it.

Development

make test     # pytest
make lint     # ruff, ruff format, ty, pyrefly, zuban, mypy
make docs     # build the documentation site

This code is version-sensitive, so a green run on one interpreter says little about the others. nox runs the suite on each:

nox -s tests            # 3.11 through 3.15
nox -s check            # everything `make lint` runs

Contributions are welcome. docs/src/guides/developer-guide.md describes how the library is organised and what a new derivation has to prove before it lands.