dead-cst 0.9.0

Python dead code analysis using libcst

dead-cst

Python dead code analysis using libcst.

dead-cst builds a full symbol graph of your Python codebase, walks from your entrypoints, and reports (or removes) anything unreachable.

Pre-release software. dead-cst is in early alpha. APIs, CLI flags, and output formats may change without notice, and bugs are expected. dead-cst remove itself is non-destructive — it emits a patch to stdout — but apply the patch on a clean working tree so you can inspect (and easily revert) the result.

Installation

pip install dead-cst

Or with uv:

uv add dead-cst

Quick start

# Find dead code in your project
dead-cst analyze ./src -e "re:.*__main__\.py"

# See why a symbol is kept alive
dead-cst why-alive ./src mypackage.some_module.some_function

# Generate a patch that removes dead code, then apply it
dead-cst remove ./src -e "re:.*__main__\.py" | git apply

# List third-party dependencies imported by the codebase
dead-cst dependencies ./src

CLI reference

dead-cst analyze

Analyze a Python codebase for dead code.

dead-cst analyze ROOT -e ENTRYPOINT [OPTIONS]

Option	Description
-e, --entrypoint	Entrypoint: file path, FQN, or re:pattern for regex (repeatable)
-p, --path	Search path spec: package:dep1,dep2 or package (repeatable)
--resolver	Path resolver to run, e.g. uv (mutually exclusive with -p)
--plugin	Edge plugin to run, e.g. main_block, project_scripts (repeatable)
--format	Output format: text or json
-v, --verbose	Enable verbose logging
--no-cache	Bypass the per-file VisitorPayload cache
-j, --workers	Run cache-miss visitor passes in this many worker processes (>=2 enables it)

Exit code 1 if dead code is found, 0 otherwise.

dead-cst why-alive

Show why a symbol is considered alive by printing its predecessor chain.

dead-cst why-alive ROOT FQNAME [OPTIONS]

Option	Description
-p, --path	Search path spec: package:dep1,dep2 or package (repeatable)
--resolver	Path resolver to run, e.g. uv (mutually exclusive with -p)
--plugin	Edge plugin to run, e.g. main_block, project_scripts (repeatable)
-v, --verbose	Enable verbose logging
--no-cache	Bypass the per-file VisitorPayload cache
-j, --workers	Run cache-miss visitor passes in this many worker processes (>=2 enables it)

dead-cst unused-exports

Report __all__ entries whose targets are only alive because of __all__. Useful in closed-world / monorepo settings to prune the public surface.

dead-cst unused-exports ROOT -e ENTRYPOINT [OPTIONS]

Option	Description
-e, --entrypoint	Entrypoint: file path, FQN, or re:pattern for regex (repeatable)
-p, --path	Search path spec: package:dep1,dep2 or package (repeatable)
--resolver	Path resolver to run, e.g. uv (mutually exclusive with -p)
--plugin	Edge plugin to run, e.g. main_block, project_scripts (repeatable)
-v, --verbose	Enable verbose logging
--no-cache	Bypass the per-file VisitorPayload cache
-j, --workers	Run cache-miss visitor passes in this many worker processes (>=2 enables it)

dead-cst dependencies

List third-party dependencies imported by the codebase. Each package gets its own section. Distributions are reported as [external dist] <name>; files resolved inside site-packages without a matching distribution are reported as [external file] <name>.

dead-cst dependencies ROOT [OPTIONS]

Option	Description
-p, --path	Search path spec: package:dep1,dep2 or package (repeatable)
--resolver	Path resolver to run, e.g. uv (mutually exclusive with -p)
--format	Output format: text or json
-v, --verbose	Enable verbose logging
--no-cache	Bypass the per-file VisitorPayload cache
-j, --workers	Run cache-miss visitor passes in this many worker processes (>=2 enables it)

dead-cst remove

Emit a git apply-compatible unified diff that removes the dead code. The command never touches source files itself — pipe the patch into git apply (or write it to a file with -o and apply later).

dead-cst remove ROOT -e ENTRYPOINT [OPTIONS]

dead-cst remove ./src -e mypkg.__main__ | git apply
# or
dead-cst remove ./src -e mypkg.__main__ -o dead.patch && git apply dead.patch

Option	Description
-e, --entrypoint	Entrypoint: file path, FQN, or re:pattern for regex (repeatable)
-p, --path	Search path spec: package:dep1,dep2 or package (repeatable)
--resolver	Path resolver to run, e.g. uv (mutually exclusive with -p)
--plugin	Edge plugin to run, e.g. main_block, project_scripts (repeatable)
-v, --verbose	Enable verbose logging
-o, --output	Write the patch to this file instead of stdout
--no-cache	Bypass the per-file VisitorPayload cache
-j, --workers	Run cache-miss visitor passes in this many worker processes (>=2 enables it)

dead-cst cache clear

Delete the on-disk VisitorPayload cache (<root>/.dead-cst-cache/) for a project. Each row is keyed by an analysis-wide fingerprint over the visitor / plugin / unreachable-region-detector (name, version) triple, schema version, and Python version, so most analyzer-version changes invalidate it automatically; this command is for force-clearing when needed. Resolvers, search paths, and the package layout deliberately do not enter the fingerprint — import resolution runs unconditionally on every analysis, so resolver / search-path / package-layout swaps re-stitch edges without re-running the visitor.

dead-cst cache clear [ROOT]

ROOT defaults to the current directory.

Python API

import re
from pathlib import Path
from dead_cst import Analysis
from dead_cst.plugins import ExplicitEntrypointPlugin, MainBlockPlugin
from dead_cst.resolvers import ManualResolver

root = Path("./src")
analysis = Analysis(
    root,
    resolver=ManualResolver(specs=["."]),
    plugins=[
        MainBlockPlugin(),
        ExplicitEntrypointPlugin(specs=[re.compile(r".*__main__\.py")]),
    ],
)

# Whole-project queries: cheap to construct, lazy to materialize.
for node in analysis.dead():
    print(f"dead: {node.fqname} ({node.type}) at {node.path}")

# Per-package queries scope work to the smallest package set that gives
# correct reachability answers. Local queries (modules, declarations)
# never materialize cross-package state.
pkg = analysis.package(root)
print(sum(1 for _ in pkg.modules()), "modules")
pkg.remove_dead_code()  # codemod, scoped to this package

For non-destructive review, dead_cst.codemod.generate_patch(G, root) returns the same removal as a git apply-compatible unified diff. Selection is driven entirely by G.nodes, so you can pass any subgraph slice — e.g. one strongly-connected component at a time — to review a large codebase as a series of focused patches.

Analysis(...).materialize_all() returns the full networkx.MultiDiGraph if you need raw access; analysis.package(path).graph() returns the closure-scoped subgraph for one package.

Edge plugins and the unreachable-region detector share a single Cacheable protocol (name: str, version: int). The core SymbolVisitor carries the same pair, so visitor-level changes get an explicit knob too. The visitor / plugin / detector triple feeds the per-file cache fingerprint — bumping any of those versions invalidates stale payloads automatically. Path resolvers do not implement Cacheable: their output flows through the (uncached) edge-stitching pass, so swapping or upgrading a resolver re-stitches edges without invalidating cached payloads (and there is no version to bump). The package __version__ is intentionally not in the fingerprint: every component whose output can shift between releases owns a dedicated version, and folding in __version__ would let unbumped components ride for free on a release bump.

Entrypoint detection is fully plugin-driven. Builtins:

Plugin	Purpose
MainBlockPlugin	Mark modules containing if __name__ == "__main__": as entrypoints
ProjectScriptsPlugin	Read pyproject.toml [project.scripts] and mark each target as an entrypoint
ExplicitEntrypointPlugin	Match user-supplied file paths / FQNs / regexes (powers the -e flag)
ModuleDundersPlugin	Keep top-level dunder variables (__all__, __version__, etc.) alive (always on)
PytestPlugin	Keep pytest-discovered tests, conftest.py decls, and @pytest.fixture functions alive (--plugin pytest)
UnittestPlugin	Keep stdlib unittest.TestCase / IsolatedAsyncioTestCase subclasses and setUpModule / tearDownModule / load_tests hooks alive. Transitive: a class extending a project-local TestCase mixin or a re-exported TestCase is detected (--plugin unittest)
MockPatchPlugin	Resolve string-fqname patch targets so symbols whose only consumers are tests stay alive. Recognizes unittest.mock.patch / mock.patch (decorator and context-manager forms, plus aliased imports), pytest-mock's mocker.patch, and pytest's monkeypatch.setattr("X.Y", v) / monkeypatch.delattr("X.Y") (--plugin mock_patch)
FastAPIPlugin	Detect top-level FastAPI() / APIRouter() instances; mark FastAPI apps as entrypoints and add instance -> handler edges for every @app.get(...)-style decorator (HTTP methods, websockets, middleware, exception handlers, on_event). Routers stay pass-through, so an APIRouter that's never include_router'd remains dead (--plugin fastapi)
FlaskPlugin	Detect top-level Flask() / Blueprint() instances; mark Flask apps as entrypoints and add instance -> handler edges for every @app.route(...) / @app.get(...) / lifecycle / errorhandler / template-helper / URL-processor decorator. Blueprints stay pass-through, so a Blueprint that's never register_blueprint'd remains dead (--plugin flask)
TyperPlugin	Detect top-level Typer() instances and add instance -> handler edges for every @app.command(...) / @app.callback(...) decorator. Typer apps are pass-through (reach them via [project.scripts] or if __name__ == "__main__": app()), so a sub-typer that's never add_typer'd stays dead (--plugin typer)
ClickPlugin	Detect top-level Click Group instances (functions decorated @click.group(...) or X = click.Group(...)) and add instance -> handler edges for every @cli.command(...) / @cli.group(...) / @cli.result_callback(...) decorator. Groups are pass-through (reach them via [project.scripts] or a __main__ block), so a sub-group that's never add_command'd stays dead (--plugin click)
CycloptsPlugin	Detect top-level cyclopts.App() instances and add instance -> handler edges for every @app.command(...) / @app.default(...) decorator. Apps are pass-through (reach them via [project.scripts] or a __main__ block), mirroring TyperPlugin (--plugin cyclopts)
InitSubclassPlugin	Detect classes that define __init_subclass__ and add parent -> subclass edges for every (transitive) first-party subclass. Parents stay pass-through, so a registry base class only keeps subclasses alive once something else (an entrypoint, an import) keeps the parent alive (--plugin init_subclass)

For project-specific dynamic-import patterns, three abstract bases ship as scaffolding that subclasses configure in 4-5 lines:

Abstract base	Use it for
DecoratedDeclPlugin	"Find decorated decls in files matching a search path." Subclass with package_prefix, decorator_module, decorator_names, constructor_names. Pure observe-time.
LiteralListPlugin	"Read <owner>.<var> = ['fqn', ...] and treat each entry as alive." Subclass with owner_fqname, variable_name. observe parses and caches; finalize only does graph lookups.
DispatchAppPlugin	"Wire @<instance>.<reg>(...) handlers to a CLI app instance." Subclass with app_module, constructor_targets, registration_decorators. Powers TyperPlugin and CycloptsPlugin; reuse for any framework with the X = App(); @X.command(...) shape.

All three bases require subclasses to set name (a unique identifier for the cache namespace) and version (a Unix epoch int — bump it to the current epoch when the subclass's config changes). For example:

from dataclasses import dataclass
from dead_cst.plugins import LiteralListPlugin

@dataclass(kw_only=True)
class MyInternalModulesPlugin(LiteralListPlugin):
    owner_fqname: str = "myapp.config"
    variable_name: str = "INTERNAL_MODULES"
    name: str = "my_internal_modules"
    version: int = 1700000000

Write your own from scratch by implementing the EdgePlugin protocol (name, version, observe, finalize); register under the dead_cst.plugins entry-point group for CLI discovery.

Path resolution is similarly pluggable. PathResolver implementations return a tuple of Package records (path, name, exported, deps) to feed Analysis. Builtins: ManualResolver (explicit package:dep specs from -p) and UvResolver (parses uv.lock to discover workspace members and their inter-member dep edges). Third-party resolvers register under dead_cst.resolvers.

Unreachable-code detection is pluggable through the UnreachableRegionDetector protocol. Analysis accepts an unreachable_detector whose find_regions(wrapper) -> list[CodeRange] is invoked once per file. The built-in DefaultUnreachableRegionDetector covers three things out of the box:

• Literal truthiness on every if / while test (e.g. if False: always-dead body, if True: ... else: ... always-dead else).
• Flow-sensitive name resolution over simple Name = literal (and Name: T = literal) assignments. Chains like foo = False; bar = foo or False; if bar: ... resolve to dead because the goal-directed TruthinessResolver recursively evaluates each binding's RHS on demand and memoizes the result. Mutable container literals (x = [], x = {}, comprehensions) are deliberately not folded — .append / item assignment between binding and use is invisible to the binding-only flow walk, so their truthiness stays unknown.
• Post-terminator regions inside every suite. Statements after an unconditional return / raise / break / continue / assert <statically-falsy> are marked dead, and so are the statements after a compound if / with / try whose every reachable branch itself terminates — so if True: return (and constant-folded variants like if FLAG: return with FLAG = True) kills the rest of its enclosing suite. Suite-relative, so a raise in a try body kills only the rest of the try body — the except handler still runs on its own path.

To layer in domain knowledge — e.g. config flags whose values are fixed in production — subclass and override resolve(self, expr) -> bool | None. The override gets first crack at every non-keyword expression routed through the resolver chain; returning None defers to the built-in literal handling and name lookup. Constants resolved this way compose with name resolution, so a single high-level decision propagates through chains:

from dataclasses import dataclass

import libcst as cst
from dead_cst import Analysis
from dead_cst.branches import DefaultUnreachableRegionDetector

@dataclass(frozen=True)
class FlagAwareDetector(DefaultUnreachableRegionDetector):
    # name/version satisfy the Cacheable contract -- bump version when
    # the override's logic changes so stale per-file payloads rebuild
    # automatically.
    name: str = "flag_aware"
    version: int = 1700000000

    def resolve(self, expr: cst.BaseExpression) -> bool | None:
        # The override is consulted recursively, so guard with an early
        # isinstance check to keep it cheap.
        if (
            isinstance(expr, cst.Call)
            and isinstance(expr.func, cst.Name)
            and expr.func.value == "check_flag"
            and expr.args
            and isinstance(expr.args[0].value, cst.SimpleString)
        ):
            return MIGRATIONS[expr.args[0].value.evaluated_value]
        return None

graph = Analysis(
    root,
    resolver=ManualResolver(specs=["."]),
    unreachable_detector=FlagAwareDetector(),
).materialize_all()

With the override above, if check_flag("migration-abc"): ... and flag = check_flag("migration-abc"); if flag: ... both resolve to a known truthiness, and the unreachable suite is flagged just like a literal if False: would be.

For detectors that don't fit the constant-folding model at all, write a fresh class that implements find_regions(wrapper) -> list[CodeRange] directly — the protocol requires nothing else beyond the Cacheable (name, version) pair.

Graph model

The graph has one node per top-level declaration plus a synthetic module node per file. Edges run from a declaration to each symbol it references, and from every submodule to its parent package so __init__.py stays alive as long as anything in the package does. Entrypoints seed the reachability walk; every node not reached is reported as dead.

A module-level import / from ... import ... is itself a declaration of type "import" in the current module. Uses of the imported name inside the file are wired through that local import node, and the import node in turn points at the upstream module (and, when applicable, at the specific imported symbol). Removing the last local use therefore makes the import itself dead, which is how dead-cst remove knows to drop now-unused import lines.

Imports whose source line carries a ruff/pyflakes # noqa directive that silences F401 (# noqa, # noqa: F401, multi-rule # noqa: E501, F401, case-variant # NOQA) are pinned alive. File-level # ruff: noqa and # flake8: noqa directives (ruff: / flake8: is matched case-sensitively per ruff; noqa is not) pin every import in the file. This matches ruff's own semantics: an import you have explicitly preserved (re-exports, side-effect imports, TYPE_CHECKING shims guarded by F401) is not surfaced as dead and dead-cst remove will not drop it.

Pinned imports are tagged with NodeFlags.NOQA (in addition to NodeFlags.ENTRYPOINT). The opt-in Analysis.kept_alive_by_flags_only(flags) / PackageView.kept_alive_by_flags_only(flags) query takes any NodeFlags combination and returns the "blast radius" of dropping every entrypoint with those flag bits. Pass NodeFlags.NOQA to audit stale F401 pins ("if I removed every # noqa: F401, what would actually become dead?"), NodeFlags.TESTCASE for "what would die if you dropped the test suite", or NodeFlags.TESTCASE | NodeFlags.NOQA to combine in one pass.

Scope

dead-cst tracks top-level declarations only -- module-level functions, classes, and variables. Nested definitions (inner functions, methods, nested classes) are deliberately not given their own nodes; references made from inside those nested scopes are attributed to the enclosing top-level declaration. Keeping the containing top-level symbol alive keeps its nested source alive with it.

.pyi stub files are ingested for the compiled-extension layout: a binary like mypkg/_native.so shipping next to mypkg/_native.pyi. With no matching .py, the stub is the only Python-discoverable description of those names, so the analyzer parses it under its natural module FQN (mypkg._native) and from mypkg._native import compute resolves to the stub's declaration through the normal import path. A .pyi shipped alongside a real .py is dropped during file enumeration -- the runtime always wins, and the peer stub is invisible to dead-cst.

@typing.overload-decorated functions in .py files are flagged so a dead implementation drags its overloads along during dead-cst remove instead of leaving them behind as orphans.

Limitations

• import * is treated pessimistically: every top-level declaration in the target module is considered used by the importing module.
• __import__('pkg.mod') and importlib.import_module('pkg.mod') are treated the same way when the module name is a string literal -- the call fans out to every top-level decl in the target module so getattr(__import__('pkg.mod'), 'name') keeps pkg.mod.name reachable. Relative names follow the same rules as from .x import *: importlib.import_module('.sub') (or __import__('sub', ..., level=1)) resolves against the file's enclosing package, and an explicit package= literal overrides the anchor. __import__(name, fromlist=[...]) with a literal list/tuple resolves each entry as a possible submodule and fans those out too. Non-literal arguments (name, level, package, fromlist) are skipped with a warning.
• Dynamic attribute access (getattr) and runtime-generated symbols are invisible to static analysis.
• Only first-party code is analysed; third-party dependencies are treated as opaque (they appear as synthetic nodes — see dead-cst dependencies).
• __all__ is followed only when assigned a list/tuple of string literals; dynamic mutation (__all__.append, comprehensions, etc.) is not tracked.
• Files libcst cannot parse are not fatal: the analyser logs a warning and stands in a [unparseable] <module> placeholder so the file stays alive in reachability and importers can still target the module. Declarations inside the file are invisible until parsing succeeds.

Development

git clone https://github.com/lpetre/dead-cst
cd dead-cst
uv sync
uv run pytest
uv run prek run --all-files

See ARCHITECTURE.md for a walkthrough of the analyzer pipeline, CONTRIBUTING.md for the full dev guide, CHANGELOG.md for release notes, and ROADMAP.md for the stack-ranked plan toward 1.0.

TODO

• Host API documentation on Read the Docs.

License

MIT — see LICENSE.