astichi 0.1.0

AST composition helpers: markers, hygiene, lifting, and lowering for composable Python code generation

astichi

AST-level composition and stitching for Python code generation.

Astichi takes small, marker-bearing Python snippets, composes them into one coherent program, and emits runnable Python. It is built for generators that need low-overhead output without falling back to brittle string templates or runtime abstraction in hot paths.

Astichi is a fit when you want to:

• describe codegen intent in Python-shaped snippets
• stitch block and expression fragments at named composition sites
• bind compile-time values into source before final lowering
• unroll compile-time loops into straight-line Python
• synthesize managed imports that participate in hygiene
• inspect composables with descriptors before wiring them
• emit inspectable source with provenance instead of opaque runtime machinery

It is not a general macro system and not a generic codemod framework. It is a focused library for generators that need a reliable AST stitcher.

Why Astichi

Code generators often hit the same wall: the desired output is simple, specialized Python, but the implementation ends up split between string concatenation, ad hoc templates, and fragile scope management.

Astichi handles the parts that usually go wrong:

• valid Python ASTs instead of fragile template fragments
• deterministic insertion order for stitched code
• compile-time binding and loop unrolling before emission
• specialized straight-line Python instead of runtime dispatch layers
• emitted source you can diff, test, and round-trip

Marker mental model

Astichi is marker-bearing Python source plus a small build pipeline.

• Markers are recognized from authored Python source.
• Marker meaning comes from AST position, not string matching alone.
• compile(...) parses marker-bearing source into a Composable.
• build() wires composables together.
• describe() exposes holes, binds, ports, and builder target addresses for data-driven composition.
• materialize() resolves inserts, bindings, and hygiene, then produces real Python.

The core markers are:

• astichi_hole(name) -> insertion site
• astichi_keep(name) -> hygiene-preserved name in expression / statement source
• name__astichi_keep__ -> hygiene-preserved name in identifier position
• name__astichi_arg__ -> identifier demand
• name__astichi_param_hole__ -> function-parameter insertion target
• astichi_funcargs(...) -> call-argument payload
• astichi_bind_external(name) -> external/literal value slot
• astichi_ref(path) -> compile-time reducible identifier / attribute path
• astichi_pyimport(module=..., names=(...)) -> managed Python import
• astichi_comment("...") -> final-output source comment
• astichi_pass(name, outer_bind=True) -> explicit same-name boundary read
• astichi_import(name) -> explicit whole-scope boundary import
• astichi_export(name) -> explicit outward supply
• astichi_insert(...) -> internal emitted metadata, not general authored API

Comment marker note:

• astichi_comment("...") is statement-only. Ordinary materialize() strips it for executable output; emit_commented() renders it as real # comments.
• Multi-line payloads keep the marker statement's indentation, and only exact {__file__} / {__line__} substrings are expanded.

Value-form target note:

• astichi_ref(...) and astichi_pass(...) are ordinary value-form surfaces in expressions.
• If the marker result itself must occupy an Assign / AugAssign / Delete target position, append ._ or .astichi_v: astichi_ref("self.f0")._ = 1, astichi_pass(counter).astichi_v = 1.
• If you immediately continue to a real attribute, plain Python target syntax already works: astichi_pass(obj).field = 1.

The one rule that matters most is scope:

• astichi_insert is the basic Astichi boundary.
• Each inserted composable lives in its own Astichi scope.
• There is no implicit capture across that boundary.
• If a name crosses the boundary, make it explicit with keep, pass, import, or export.
• Function parameters are the pinned exception: parameter names and uses in the function scope stay attached to that parameter binding.

Small example:

import astichi

builder = astichi.build()
builder.add.Root(
    astichi.compile(
        """
items = []
astichi_hole(body)
result = tuple(items)
"""
    )
)
builder.add.Step(
    astichi.compile(
        """
astichi_pass(items, outer_bind=True).append("x")
"""
    )
)
builder.Root.body.add.Step(order=0)

materialized = builder.build().materialize()
print(materialized.emit(provenance=False))

Emitted Python:

items = []
items.append("x")
result = tuple(items)

Without astichi_pass(items, outer_bind=True), the inner snippet does not get to reuse items just because the spelling matches. That is deliberate. Astichi defaults to isolated scopes and only crosses them when the source says so.

The fluent builder is also available as a data-driven named API. Descriptor target data can feed that API directly:

hole = root.describe().single_hole_named("body")

builder = astichi.build()
builder.add("Root", root)
builder.add("Step", astichi.compile("value = 1\n"))
builder.target(hole.with_root_instance("Root")).add("Step")

That builder.target(...) call uses the same target address as builder.Root.body.add.Step(), but the address came from describe() instead of a Python attribute chain.

Example: schema-specialized row projector

Suppose an ingestion pipeline knows its event schema at build time, and each field needs its own normalization step. A runtime loop or dispatch table adds overhead to every row. String templating works until ordering, scope, and correctness start fighting each other.

Astichi lets you define the skeleton once, stitch in field-specific steps, and emit the straight-line Python you actually want to run.

import astichi

root = astichi.compile(
    """
astichi_bind_external(FIELDS)

def project_row(row):
    out = {}
    for field in astichi_for(FIELDS):
        astichi_hole(step)
    return out
"""
).bind(FIELDS=("user_id", "total_cents", "created_at"))

builder = astichi.build()
builder.add.Root(root)

builder.add.UserId(
    astichi.compile("out['user_id'] = int(row['user_id'])\n")
)
builder.add.TotalCents(
    astichi.compile("out['total_cents'] = int(row['total_cents'])\n")
)
builder.add.CreatedAt(
    astichi.compile("out['created_at'] = row['created_at'][:10]\n")
)

builder.Root.step[0].add.UserId()
builder.Root.step[1].add.TotalCents()
builder.Root.step[2].add.CreatedAt()

projector = builder.build().materialize()
print(projector.emit(provenance=False))

Emitted Python:

def project_row(row):
    out = {}
    out["user_id"] = int(row["user_id"])
    out["total_cents"] = int(row["total_cents"])
    out["created_at"] = row["created_at"][:10]
    return out

That is the point: no runtime field loop, no dispatch registry, no handwritten template surgery. The generated function is plain Python, specialized to the known schema, and suitable for hot-path use.

This is exactly the class of problem where a reliable AST stitcher matters:

• block fragments must land in the right lexical scope
• per-field steps must keep deterministic order
• compile-time schema data must become literal Python
• the final output must still be valid, inspectable source

Current surface

Astichi currently provides:

• astichi.compile(source, file_name=None, line_number=1, offset=0)
• astichi.build() for builder-based composition
• concrete composables with .bind(...), .describe(), .materialize(), and .emit(...) / .emit_commented()
• data-driven builder calls such as builder.add("Root", root) and builder.target(hole.with_root_instance("Root")).add("Step")
• provenance helpers in astichi.emit

Supported pieces today include block holes, expression inserts, external binding, managed Python imports, materialization, emission, and builder-driven loop unrolling.

Layout

Path	Role
src/astichi/	Library code
docs/	User-facing docs
tests/	Pytest suite
dev-docs/	Design notes, active summary, and requirements
scratch/	Throwaway experiments (not shipped)

Development

python -m venv .venv && source .venv/bin/activate
pip install -e ".[dev]"
PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest

Status

Early development (0.1.0), but already useful for controlled codegen pipelines.

Start with:

• docs/ for the user-facing surface
• dev-docs/AstichiSingleSourceSummary.md for the current implementation snapshot and known gaps

License

MIT. See LICENSE.