Improve performance of is_function

src/qcodes/utils/function_helpers.py

@@ -1,5 +1,5 @@
 from asyncio import iscoroutinefunction
-from inspect import signature
+from inspect import CO_VARARGS, signature
 
 
 def is_function(f: object, arg_count: int, coroutine: bool = False) -> bool:
@@ -29,6 +29,26 @@ def is_function(f: object, arg_count: int, coroutine: bool = False) -> bool:
         # otherwise the user should make an explicit function.
         return arg_count == 1
 
+    if func_code := getattr(f, "__code__", None):
+        # handle objects like functools.partial(f, ...)
+        func_defaults = getattr(f, "__defaults__", None)
+        number_of_defaults = len(func_defaults) if func_defaults is not None else 0
+
+        if getattr(f, "__self__", None) is not None:
+            # bound method
+            min_positional = func_code.co_argcount - 1 - number_of_defaults
+            max_positional = func_code.co_argcount - 1
+        else:
+            min_positional = func_code.co_argcount - number_of_defaults
+            max_positional = func_code.co_argcount
+
+        if func_code.co_flags & CO_VARARGS:
+            # we have *args
+            max_positional = 10e10
+
+        ev = min_positional <= arg_count <= max_positional
+        return ev
+
     try:
         sig = signature(f)
     except ValueError:

tests/utils/test_isfunction.py

@@ -1,3 +1,4 @@
+from functools import partial
 from typing import NoReturn
 
 import pytest
@@ -36,6 +37,32 @@ def f2(a: object, b: object) -> NoReturn:
         is_function(f0, -1)
 
 
+def test_function_partial() -> None:
+    def f0(one_arg: int) -> int:
+        return one_arg
+
+    f = partial(f0, 1)
+    assert is_function(f, 0)
+    assert not is_function(f, 1)
+
+
+def test_function_varargs() -> None:
+    def f(*args) -> None:
+        return None
+
+    assert is_function(f, 0)
+    assert is_function(f, 1)
+    assert is_function(f, 100)
+
+    def g(a, b=1, *args) -> None:
+        return None
+
+    assert not is_function(g, 0)
+    assert is_function(g, 1)
+    assert is_function(g, 2)
+    assert is_function(g, 100)
+
+
 class AClass:
     def method_a(self) -> NoReturn:
         raise RuntimeError("function should not get called")