Drop comtypes requirement

[wkschwartz]

On Windows, xlwings has required comtypes since at least version v0.9.0 (588f0f5) until at least the current master (dceeedd). After looking at it carefully, I think this may be unnecessary. The cost of requiring an unnecessary library is added time in installation under normal set ups and added complexity and size in packaging and distribution (see, e.g., #625, which is quite similar to my use case). Very shortly I will send a PR to accomplish my recommended changes.

The only imports from comtypes are IUnknown and IDispatch:

xlwings/xlwings/_xlwindows.py

Lines 29 to 30 in dceeedd

	from comtypes import IUnknown
	from comtypes.automation import IDispatch

IUnknown

IUnknown is only used in comtypes_to_pywin() in case the interface argument is not set:

xlwings/xlwings/_xlwindows.py

Lines 196 to 202 in dceeedd

	def comtypes_to_pywin(ptr, interface=None):
	_PyCom_PyObjectFromIUnknown = PyDLL(pythoncom.__file__).PyCom_PyObjectFromIUnknown
	_PyCom_PyObjectFromIUnknown.restype = py_object
	if interface is None:
	interface = IUnknown
	return _PyCom_PyObjectFromIUnknown(ptr, byref(interface._iid_), True)

But comtypes_to_pywin is only called once, in get_xl_app_from_hwnd(), with the interface argument set to IDispatch.

xlwings/xlwings/_xlwindows.py

Lines 217 to 225 in dceeedd

	def get_xl_app_from_hwnd(hwnd):
	pythoncom.CoInitialize()
	child_hwnd = win32gui.FindWindowEx(hwnd, 0, 'XLDESK', None)
	child_hwnd = win32gui.FindWindowEx(child_hwnd, 0, 'EXCEL7', None)
	ptr = accessible_object_from_window(child_hwnd)
	p = comtypes_to_pywin(ptr, interface=IDispatch)
	disp = COMRetryObjectWrapper(Dispatch(p))
	return disp.Application

Thus

xlwings/xlwings/_xlwindows.py

Lines 200 to 202 in dceeedd

	if interface is None:
	interface = IUnknown
	return _PyCom_PyObjectFromIUnknown(ptr, byref(interface._iid_), True)

could be replaced as

    return _PyCom_PyObjectFromIUnknown(ptr, byref(IDispatch._iid_), True)

with comtypes_to_pywin no longer taking an interface argument.

So the import of IUnknown can be removed completely without changing a thing.

IDispatch

IDispatch is used only in get_xl_app_from_hwnd() as shown above and from accessible_object_from_window():

xlwings/xlwings/_xlwindows.py

Lines 188 to 193 in dceeedd

	def accessible_object_from_window(hwnd):
	ptr = POINTER(IDispatch)()
	res = oledll.oleacc.AccessibleObjectFromWindow(
	hwnd, OBJID_NATIVEOM,
	byref(IDispatch._iid_), byref(ptr))
	return ptr

(accessible_object_from_window() is called only from get_xl_app_from_hwnd() as shown above and from is_hwnd_xl_app(), where nothing is done with the returned pointer.)

So it looks like there is one use of ctypes.POINTER(IDispatch) and there are two uses of ctypes.byref(IDispatch._iid_).

ctypes.POINTER(IDispatch)

Correct me if I'm wrong, but I believe this declares a pointer to a structure, and, from the code defining IDispatch, it looks like the structure has no _fields_. IDispatch inherits from our friend IUnknown, which also defines no _fields_. (The metaclass for IUnknown is defined here; notice that IUnknown._case_insensitive_ is set to False.)

Moreover, the pointer produced by POINTER(IDispatch) is only ever passed directly to oleacc.AccessibleObjectFromWindow (shown above) and to pythoncom.PyCom_PyObjectFromIUnknown. Being C++ functions and being called through ctypes suggests to me that AccessibleObjectFromWindow and PyCom_PyObjectFromIUnknown really only care about the memory address contained in the pointer and, for the byref use, the memory address at which it is stored, not any of its Python metadata it might gain by coming from POINTER(IDispatch) rather than ctypes.c_void_p. I.e., it's not like there's any type checking happening.

So I would argue that POINTER(IDispatch)() can be replaced with ctypes.c_void_p() and a comment linking to IDispatch's documentation.

ctypes.byref(IDispatch._iid_)

IDispatch._iid_ is defined here. Can this be replaced with the following? It's just the essentials from here.

import ctypes

class _GUID(ctypes.Structure):
    # https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-dtyp/49e490b8-f972-45d6-a3a4-99f924998d97
    _fields_ = [
        ("Data1", ctypes.c_ulong),
        ("Data2", ctypes.c_ushort),
        ("Data3", ctypes.c_ushort),
        ("Data4", ctypes.c_byte * 8)]

_IDISPATCH_GUID = GUID()
ctypes.oledll.ole32.CLSIDFromString("{00020400-0000-0000-C000-000000000046}", ctypes.byref(_IDISPATCH_GUID))

Then the two instances of byref(IDispatch._iid_) could be replaced with byref(_IDISPATCH_GUID).