metapathology 0.2.3

Diagnose which finders handle imports and how sys.meta_path changes

metapathology

Diagnose Python import hooks without changing import outcomes.

[!IMPORTANT] This project is mostly AI generated (for now). I do understand what it does and how it works, but I only skimmed the code. I can attest the explanations in README is accurate though.

metapathology is a stdlib-only diagnostic tool for CPython imports. It reports:

• which finder located each imported module;
• where code changed sys.meta_path, with a stack trace; and
• which modules were found without going through the usual sys.path and sys.path_hooks search.

This is useful when two packages customize imports and one prevents the other from seeing a module. The original example was beartype#556: an editable install finder located a module before beartype.claw's path hook could see it.

Full guides are published at glinte.github.io/metapathology. They cover usage, import-system concepts, report interpretation, the library API, limitations, and development.

Usage

Primary: run your program under observation, no code changes needed —

$ python -m metapathology myscript.py --my-args
$ python -m metapathology -m pytest tests/

A report is printed at exit. An installed metapathology command provides a shorter equivalent:

$ metapathology myscript.py --my-args

Prefer python -m metapathology: it guarantees the hooks land in the same interpreter and venv as the code under investigation.

Library API, for when a wrapper isn't possible (notebooks, embedded interpreters, "I can only touch conftest.py"):

import metapathology

monitor = metapathology.install()  # as early as possible

install() is idempotent and returns the process-wide Monitor. By default, it prints a report to standard error when Python exits. To control when or where the report is written, disable that callback and report explicitly:

import sys

import metapathology

monitor = metapathology.install(report_at_exit=False)
try:
    import package_under_investigation
finally:
    metapathology.report(sys.stdout)
    metapathology.uninstall()

uninstall() is idempotent. It restores a plain sys.meta_path, removes the wrappers from finders, and unregisters the exit report. The CPython audit hook cannot be removed, so it remains installed as an inactive no-op. Recorded events remain available after uninstalling.

For integration with another diagnostic or test harness:

• metapathology.render_report() returns the same report as a string;
• metapathology.report(file=None) writes it to file, or to standard error when omitted;
• metapathology.get_monitor() returns the process-wide monitor, or None before the first call to install(); and
• monitor.events() returns a capture-order snapshot of the structured FindSpecCall, MetaPathMutation, MetaPathReassignment, and InternalError records. Mutating the returned list does not alter the monitor.

Calling report() or render_report() before install() raises RuntimeError. There are no runtime dependencies. See the complete usage guide for CLI behavior, lifecycle details, and integration examples.

Reading the report

The mutation and reassignment sections show how finder precedence changed. Finder attribution groups recorded find_spec() calls by finder and lists the modules each finder claimed. The suspicious-findings section uses these labels:

• [bypass] means a custom finder claimed a source module, but a fresh PathFinder lookup would choose a different loader or origin. Tools attached through sys.path_hooks did not see the import that actually happened.
• [unfindable] means a custom finder claimed a source module that a fresh PathFinder lookup cannot find at all. This is a stronger form of bypass.
• [no-spec] means a new sys.modules entry has neither a __spec__ nor a recorded finder claim. It was probably created manually or loaded through an exec_module()-style path that is invisible to meta-path finders.

These are diagnostic leads, not necessarily defects. Custom finders may bypass the standard path machinery intentionally, and the report replays the current PathFinder state rather than the exact state at import time. The report guide explains every section and finding category.

Resource use

The monitor retains every recorded finder call, mutation, reassignment, and internal error so the final report is exhaustive. Its memory use therefore grows with import activity for as long as monitoring remains enabled; there is currently no event limit or silent dropping policy. For a long-running or import-heavy process, call report() and uninstall() once the behavior of interest has been captured. Stack traces are stored for sys.meta_path changes, which makes mutation records more expensive than finder-call records. At the 400-import point in the reference benchmark matrix, monitoring added a median 3% to the standard-finder workload and 13% when retaining one attributed finder record per import; those records retained about 223 bytes each. Imports already present in sys.modules are cache hits: they do not call finders or create new records. A large application can still resolve thousands of distinct modules during startup, however, and each import can produce records from more than one instrumented finder.

See limitations and resource behavior and the reproducible speed and memory benchmarks before monitoring a long-running process.

How Python finds an imported module

The detailed import walkthrough connects these Python mechanisms to what metapathology can record.

Python first checks the requested module's fully qualified name in sys.modules. If an entry is already there, Python returns that same module object without calling any finder or executing the module again. During a first import, Python adds the new module to this cache before executing its code so recursive imports do not load a second copy. If the name is absent, Python reads sys.meta_path, a list of objects called finders. It asks each finder in order whether it can locate the module by calling the finder's find_spec() method.

A finder returns None when it cannot locate the module, and Python moves on to the next finder. A finder that can locate the module returns a module spec: a small object describing the module and how to load it. Python stops asking other finders once it receives a spec. The Python documentation calls this process the meta path.

One of Python's standard finders is PathFinder. It searches sys.path for a top-level module or a package's __path__ for a submodule. For each path item, it uses a cached path-entry finder or calls the factories in sys.path_hooks to create one. Path hooks therefore do not receive every import themselves. A custom meta-path finder placed before PathFinder can return a spec first, preventing PathFinder and the path-entry finders created by those hooks from seeing the module. See the path-based finder for the full protocol.

How it works

metapathology observes that process in three ways:

1. It registers a sys.addaudithook() callback for CPython's import audit event. This event says that an uncached import is starting; it does not say which finder will succeed. It also lets the monitor recover if less-common code assigns an entirely new list to sys.meta_path.
2. It temporarily replaces sys.meta_path with a compatible list subclass. This records the usual changes as they happen: additions, removals, replacements, clearing, and reordering, with a stack trace showing where each change came from. Newly added finders are prepared for call recording.
3. It wraps each finder's existing find_spec() method. The wrapper records whether the finder returned None or a module spec, then returns the same result. It does not supply a spec of its own or load a module.

The standard BuiltinImporter, FrozenImporter, and PathFinder entries are classes shared by CPython, so metapathology deliberately leaves them unwrapped.

At exit, the report compares the recorded result with what PathFinder.find_spec() finds. If PathFinder cannot find the module or would use a different kind of loader, the report notes that the normal sys.path_hooks route was skipped.

Caveats

• CPython only (relies on the import audit event and import-system internals).
• Monitoring begins when metapathology is installed. Finders added earlier by .pth files are shown in the initial sys.meta_path list, but there can be no stack trace for when they were added because that happened during Python startup.
• The temporary changes to finders and sys.meta_path are reversed by uninstall(). Python does not provide a way to remove an audit hook, so the installed callback remains as an inactive no-op after uninstalling.
• This tool changes sys.meta_path while it is running. Use it for debugging, not as part of an application's normal runtime.

See the complete limitations guide for timing, visibility, replay, cleanup, and memory boundaries.