pythonnet clr: how to get the signature of a function/method?

Question

When importing a .NET library in Python with:

import clr
clr.AddReference(r"C:\foo.dll")
from foo import *

my_foo = Foo()
print(my_foo.my_method)
# <bound method 'my_method'>

I'd like to know the signature of the function (its parameters). This doesn't work:

from inspect import signature
print(signature(my_foo.my_method))

It fails with:

ValueError: callable <bound method 'GroupStatusGet'> is not supported by signature

Question: how to get the signature of a function on Python + .NET ?

Answer 1

Python's built-in tools like inspect cannot be used to examine the signature of methods from .NET assemblies. This is because Python's introspection features work primarily with Python objects and their associated metadata, and .NET assemblies are fundamentally different types of objects. Therefore, inspect.signature will fail with a ValueError if you try to apply it to a .NET method.

You can use .NET's own reflection capabilities to inspect the signatures of its methods. Here's how you might do this using the System.Reflection namespace in .NET:

import clr
clr.AddReference(r"C:\foo.dll")

from foo import *
from System.Reflection import BindingFlags

my_foo = Foo()

method_info = Foo.GetMethod('my_method', BindingFlags.Public | BindingFlags.Instance)

if method_info is None:
    print("Method 'my_method' not found.")
else:
    print(f"Found method 'my_method' in class 'Foo'.")
    params = method_info.GetParameters()
    if not params:
        print("The method 'my_method' does not take any parameters.")
    else:
        print(f"The method 'my_method' takes the following parameters:")
        for param in params:
            print(f"  Parameter name: {param.Name}")
            print(f"  Parameter type: {param.ParameterType}")
            print(f"  Parameter position: {param.Position}")
            print()

Edit #1

The code example will throw AttributeError as mentioned in the comments. The issue here is related to the reflection API. The method GetMethod is not an instance method, but a method from the .NET's Type class.

I've fixed the code (validated locally):

import clr
from System.Reflection import BindingFlags
from System import String
from System import Type

my_string = String("Hello, World!")
my_string_type = my_string.GetType()

method_info = my_string_type\
    .GetMethod('ToLower', BindingFlags.Public | BindingFlags.Instance, None, Type.EmptyTypes, None)

if method_info is None:
    print("Method 'ToLower' not found.")
else:
    print(f"Found method 'ToLower' in class '{my_string_type}'.")
    params = method_info.GetParameters()
    if not params:
        print("The method 'ToLower' does not take any parameters.")
    else:
        print(f"The method 'ToLower' takes the following parameters:")
        for param in params:
            print(f"  Parameter name: {param.Name}")
            print(f"  Parameter type: {param.ParameterType}")
            print(f"  Parameter position: {param.Position}")
            print()