How to introspect a function defined in a Cython C extension module

Question

Python's inspect module doesn't seem to be able to inspect the signatures of "built-in" functions, which include functions defined in C extension modules, like those defined by Cython. Is there any way to get the signature of a Python function you have defined in such a module, and specifically in Cython? I am looking to be able to find the available keyword arguments.

MWE:

# mwe.pyx
def example(a, b=None):                                                                                                                                                       
    pass       

and

import pyximport; pyximport.install()                                                                                                                                         
import mwe                                                                                                                                                                    
import inspect                                                                                                                                                                

inspect.signature(mwe.example)   

yields:

Traceback (most recent call last):                                                                                                                                           
  File "mwe_py.py", line 5, in <module>                                                                                                                                      
    inspect.signature(mwe.example)                                                                                                                                           
  File "/nix/store/134l79vxb91w8mhxxkj6kb5llf7dmwpm-python3-3.4.5/lib/python3.4/inspect.py", line 2063, in signature                                                         
    return _signature_internal(obj)                                                                                                                                          
  File "/nix/store/134l79vxb91w8mhxxkj6kb5llf7dmwpm-python3-3.4.5/lib/python3.4/inspect.py", line 1965, in _signature_internal                                               
    skip_bound_arg=skip_bound_arg)                                                                                                                                           
  File "/nix/store/134l79vxb91w8mhxxkj6kb5llf7dmwpm-python3-3.4.5/lib/python3.4/inspect.py", line 1890, in _signature_from_builtin                                           
    raise ValueError("no signature found for builtin {!r}".format(func))                                                                                                     
ValueError: no signature found for builtin <built-in function example>    

In Python 3.4.5 and Cython 0.24.1

Answer 1

I've retracted my duplicate suggestion (saying that it was impossible...) having investigated further. It seems to work fine with reasonably recent versions of Cython (v0.23.4) and Python 3.4.4.

import cython
import inspect
scope = cython.inline("""def f(a,*args,b=False): pass """)
print(inspect.getfullargspec(scope['f']))

gives the output

FullArgSpec(args=['a'], varargs='args', varkw=None, defaults=None, kwonlyargs=['b'], kwonlydefaults={'b': False}, annotations={})

---

Also mentioned in the documentation is the compilation option "binding" which apparently makes this detail more accessible (although I didn't need it).

---

I have a feeling that this may depend on improvements to inspect made relatively recently (possibly this fix) so if you're using Python 2 you're probably out of luck.

---

Edit: your example works if you using the binding compilation option:

import cython
@cython.binding(True)
def example(a, b=None):                                                                                                                                                       
    pass

I suspect that inline adds it automatically (but the code to do inline is sufficiently convoluted that I can't find proof of that either way). You can also set it as a file-level option.

Answer 2

The answer above using the binding decorator works for me when running code that has been cythonized. But, when I was running that same code within a Django 2.2 app, the application would fail on start with an error that cython has no attribute 'binding'. To avoid this I have added this "special cython header" at the top of my file containing the cythonized function as documented here to achieve the same results.

# cython: binding=True

def example(a, b=None):                                                                                                                                                       
    pass