Types like size_t/uint32_t are represented as int when negative integer value is used or calling help()

[mpatykx]

Hi, I will start with an example:

// main.cpp
#include <pybind11/pybind11.h>

PYBIND11_MODULE(example, m)
{
    m.def("get", [](const size_t index) { return index; }, pybind11::arg("index"), "A function that returns a number");
}

I build it with:

c++ -O3 -Wall -shared -std=c++11 -fPIC $(python3 -m pybind11 --includes) main.cpp -o example$(python3-config --extension-suffix)

And then I have Python script:

# main.py
import example

for number in [1, 0, -1]:
    print(type(number), ' -> ', example.get(number))

Which when run (python3 main.py) gives output:

<class 'int'>  ->  1
<class 'int'>  ->  0
Traceback (most recent call last):
  File "main.py", line 5, in <module>
    print(type(number), ' -> ', example.get(number))
TypeError: get(): incompatible function arguments. The following argument types are supported:
    1. (index: int) -> int

Invoked with: -1

Which is very confusable for the users of such module. You pass int and the console says that provided argument is not an int but int is supported:

TypeError: get(): incompatible function arguments. The following argument types are supported: 1. (index: int) -> int, Invoked with: -1

The same misleading information can be obtained when called using help(example.get):

get(...) method of builtins.PyCapsule instance
    get(index: int) -> int
    
    A function that returns a number

What can I do to make it more explicit in such cases that size_t (or any other unsigned integer) is the supported argument but signed integer is not?