Get parameter value (arg) of built-in functions in Python

Question

I wrote my own c-module for Python and for the user table in the documentation I need the number of parameters of the built-in functions at runtime .

Python 2 has functions, such as inspect.getargspec, or functions in Python 3, such as inspect.signature , that support normal Python functions, but leave the built-in functions unsupported.

There are still two more community solutions:

• Text string analysis
• Parsing the source * .c file
• See answer for third approach

In some cases, docstrings are outdated and / or it is difficult to extract the argument counter, since docstring can be any simple string. Parsing the * .c source file is also a good approach, but you may not have access to it.

Answer 1

In the following case, this is a working solution that I came up with for Python 2 and 3.

What is he doing?

99 None . PyArg_ParseTuple , - . , , , .

:

? , , . C :

static PyObject* example(PyObject *self, PyObject *args)
{
    int myFirstParam;
    if(!PyArg_ParseTuple(args, "i", &myFirstParam))
        return NULL;
    ...
}

:

import inspect
import time
import re
import types
import sys


def get_parameter_count(func):
    """Count parameter of a function.

    Supports Python functions (and built-in functions).
    If a function takes *args, then -1 is returned

    Example:
        import os
        arg = get_parameter_count(os.chdir)
        print(arg)  # Output: 1

    -- For C devs:
    In CPython, some built-in functions defined in C provide
    no metadata about their arguments. That why we pass a
    list with 999 None objects (randomly choosen) to it and
    expect the underlying PyArg_ParseTuple fails with a
    corresponding error message.
    """

    # If the function is a builtin function we use our
    # approach. If it an ordinary Python function we
    # fallback by using the the built-in extraction
    # functions (see else case), otherwise
    if isinstance(func, types.BuiltinFunctionType):
        try:
            arg_test = 999
            s = [None] * arg_test
            func(*s)
        except TypeError as e:
            message = str(e)
            found = re.match(
                r"[\w]+\(\) takes ([0-9]{1,3}) positional argument[s]* but " +
                str(arg_test) + " were given", message)
            if found:
                return int(found.group(1))

            if "takes no arguments" in message:
                return 0
            elif "takes at most" in message:
                found = re.match(
                    r"[\w]+\(\) takes at most ([0-9]{1,3}).+", message)
                if found:
                    return int(found.group(1))
            elif "takes exactly" in message:
                # string can contain 'takes 1' or 'takes one',
                # depending on the Python version
                found = re.match(
                    r"[\w]+\(\) takes exactly ([0-9]{1,3}|[\w]+).+", message)
                if found:
                    return 1 if found.group(1) == "one" \
                            else int(found.group(1))
        return -1  # *args
    else:
        try:
            if (sys.version_info > (3, 0)):
                argspec = inspect.getfullargspec(func)
            else:
                argspec = inspect.getargspec(func)
        except:
            raise TypeError("unable to determine parameter count")

        return -1 if argspec.varargs else len(argspec.args)



def print_get_parameter_count(mod):
    for x in dir(mod):
        e = mod.__dict__.get(x)
        if isinstance(e, types.BuiltinFunctionType):
            print("{}.{} takes {} argument(s)".format(mod.__name__, e.__name__, get_parameter_count(e)))

import os
print_get_parameter_count(os)

:

os._exit takes 1 argument(s)
os.abort takes 0 argument(s)
os.access takes 2 argument(s)
os.chdir takes 1 argument(s)
os.chmod takes 2 argument(s)
os.close takes 1 argument(s)
...