inspect.signature fails on compiled functions

[twoertwein]

signature.py

import inspect

import compiled

inspect.signature(compiled.fun)

compiled.py

def fun(test: str = "test") -> int:
    return 1

$ mypyc compiled.py
$ python signature.py
Traceback (most recent call last):
  File "/pool01/home/twoertwe/git/python-tools/signature.py", line 5, in <module>
    inspect.signature(compiled.fun)
  File "/home/twoertwe/local/lib/python3.9/inspect.py", line 3130, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
  File "/home/twoertwe/local/lib/python3.9/inspect.py", line 2879, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "/home/twoertwe/local/lib/python3.9/inspect.py", line 2334, in _signature_from_callable
    return _signature_from_builtin(sigcls, obj,
  File "/home/twoertwe/local/lib/python3.9/inspect.py", line 2145, in _signature_from_builtin
    raise ValueError("no signature found for builtin {!r}".format(func))
ValueError: no signature found for builtin <built-in function fun>

I encountered this issue when using a compiled function with client.register_worker_callbacks in dask https://docs.dask.org/en/latest/futures.html?highlight=register_worker_callbacks#distributed.Client.register_worker_callbacks

[JelleZijlstra]

This would be fixable by adding __text_signature__ attribute to the compiled function object.

[insilications]

@JelleZijlstra is there a dynamic way of doing it? Besides manually editing the sources generated by mypyc...

[insilications]

Looking into #437 ...

Maybe this would be the starting point to make mypyc utilize docstrings in a useful manner. I'm trying to understand mypy AST and IR generation to see if this can be easily implemented

[JukkaL]

This would need to happen during IR building. I don't expect it to be very difficult to implement but not quite trivial either.

[insilications]

I'll look into it. I know advanced C/C++, but very little experience with complex python. I will try this as a study target.

[dosisod]

@JelleZijlstra and @JukkaL I can take this on if no one is working on it, since this is preventing me from using Mypyc to compile one of my projects.

I am still looking into things, but from what I can tell, __text_signature__ is not officially documented, and has to be stuffed in via the __doc__ field of the compiled function, is that correct? I tried setting the __text_signature__ attribute on the function, but kept getting write-protected errors.

[graipher]

This also affects compiling any code that contains a FastAPI route. A minimal app.py file like this:

from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root() -> None:
    return

followed by mypyc app.py and python -c 'from app import app' leads to an error, since FastAPI internally uses inspect.signature to figure out the inputs needed for an endpoint.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    from main import app
  File "main.py", line 7, in <module>
    @app.get("/")
  File "/home/user/code/test_mypyc/.venv/lib/python3.13/site-packages/fastapi/routing.py", line 994, in decorator
    self.add_api_route(
    ~~~~~~~~~~~~~~~~~~^
        path,
        ^^^^^
    ...<23 lines>...
        generate_unique_id_function=generate_unique_id_function,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/user/code/test_mypyc/.venv/lib/python3.13/site-packages/fastapi/routing.py", line 933, in add_api_route
    route = route_class(
        self.prefix + path,
    ...<24 lines>...
        generate_unique_id_function=current_generate_unique_id,
    )
  File "/home/user/code/test_mypyc/.venv/lib/python3.13/site-packages/fastapi/routing.py", line 466, in __init__
    return_annotation = get_typed_return_annotation(endpoint)
  File "/home/user/code/test_mypyc/.venv/lib/python3.13/site-packages/fastapi/dependencies/utils.py", line 255, in get_typed_return_annotation
    signature = inspect.signature(call)
  File "/home/user/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/inspect.py", line 3344, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                                   globals=globals, locals=locals, eval_str=eval_str)
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/inspect.py", line 3070, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
                                    follow_wrapper_chains=follow_wrapped,
                                    globals=globals, locals=locals, eval_str=eval_str)
  File "/home/user/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/inspect.py", line 2582, in _signature_from_callable
    return _signature_from_builtin(sigcls, obj,
                                   skip_bound_arg=skip_bound_arg)
  File "/home/user/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/inspect.py", line 2368, in _signature_from_builtin
    raise ValueError("no signature found for builtin {!r}".format(func))
ValueError: no signature found for builtin <built-in function read_root>