COM servers with `comtypes`

The comtypes package makes it easy to access and implement both custom and dispatch based COM interfaces.

Implementing a simple COM object 

To implement a COM server object in comtypes you need to write a type library describing the coclass, the interface(s) that the object implements, and (optional) the event interface that the object supports. Also you have to write a Python module that defines a class which implements the object itself.

We will present a short example here that does actually work.

Define the COM interface 

Start writing an IDL file. It is a good idea to define dual interfaces, and only use automation compatible data types.

import "oaidl.idl";
import "ocidl.idl";

[
        uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx),
        dual,
        oleautomation
]
interface IMyInterface : IDispatch {
        HRESULT MyMethod([in] INT a, [in] INT b, [out, retval] INT *presult);
}

[
  uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)
]
library MyTypeLib
{
        importlib("stdole2.tlb");

        [uuid(xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)]
    coclass MyObject {
    [default] interface IMyInterface;
        };
};

Please note that you must replace the ‘xxxx’ placeholders in the section above with separate GUIDs that you must generate yourself. You can use Python to generate unique GUIDs by running this in a windows console:

C:\> python -c "from comtypes import GUID; print(GUID.create_new())"
{26F87CEB-603A-4FFE-8865-DB67A9E3A308}

The IDL file should now be compiled with the Microsoft MIDL compiler to a TLB type library file.

Implement the class 

Generate and import the wrapper module (which is named after the library statement in the IDL file), and create a subclass of the MyObject coclass.

Most required class attributes are already defined in the typelib wrapper file. You must at least add attributes for registration that are not in the type library.

import comtypes
import comtypes.server.localserver
from comtypes.client import GetModule

# generate wrapper code for the type library, this needs
# to be done only once (but also each time the IDL file changes)
GetModule('path\\to\\mytypelib.tlb')
from comtypes.gen.MyTypeLib import MyObject

class MyObjectImpl(MyObject):
    # registry entries
    _reg_threading_ = "Both"
    _reg_progid_ = "MyTypeLib.MyObject.1"
    _reg_novers_progid_ = "MyTypeLib.MyObject"
    _reg_desc_ = "Simple COM server for testing"
    _reg_clsctx_ = comtypes.CLSCTX_INPROC_SERVER | comtypes.CLSCTX_LOCAL_SERVER
    _regcls_ = comtypes.server.localserver.REGCLS_MULTIPLEUSE

The meaning of the attributes:

_reg_threading_

Must be set to “Both”, “Free”, or “Apartment”. It specifies the apartment model in which the server runs.

_reg_progid_

_reg_novers_progid_

(optional) The short names that can later be used to specify your object, instead of the CLSID in type library. Typically the type library name plus the coclass name plus a version number are combined to form the progid, and the type library name plus the coclass name are combined to form the version independend progid.

_reg_desc_

(optional) The name of the coclass.

_reg_clsctx_

The constant specifies in which contexts the COM server can operate.

_regcls_

(optional) The constant is only used for com objects that run in their own process, see the MSDN docs for more info. In comtypes, several REGCLS values are defined in the comtyper.server.localserver module.

You do not yet implement any methods on the class, because basic functionality is already present.

Register and run the object for the first time 

A COM object must by registered with Windows, and will also be started at runtime by Windows. This magic, on the comtypes side, is done by the comtypes.server.register.UseCommandLine function. You should call it in the if __name__ == "__main__" block of your script, with the MyObjectImpl class:

if __name__ == "__main__":
    from comtypes.server.register import UseCommandLine
    UseCommandLine(MyObjectImpl)

You should now run your script with a /regserver command line option, this will write information about your object into the Windows registry:

C:\> python myserver.py /regserver

If you have the Microsoft OLEVIEW utility, you can now open the “All Objects” item, and look for the “Simple COM server for testing” object. If everything works well, you can even create an instance of your COM object by double clicking the entry, and you will see that the object implements quite some interfaces already.

You can also create an instance of the object with comtypes:

>>> from comtypes.client import CreateObject
>>> x = CreateObject("MyTypelib.MyObject")
>>> x  
<POINTER(IMyInterface) ptr=... at ...>

Of course, calling a method does not yet work if it is not implemented in the server script. In such cases, a COMError is raised with an hresult of -2147467263 (E_NOTIMPL, '0x80004001' in signed-32bit hex):

>>> x.MyMethod(1, 2)  
Traceback (most recent call last):
  ...
_ctypes.COMError: (-2147467263, ..., (None, None, None, 0, None))

Implementing COM methods 

NOTE: The documentation in this section is also valid for writing COM event handlers!

In the IDL file, the method signature is defined like this:

HRESULT MyMethod([in] INT a, [in] INT b, [out, retval] INT *presult);

So, this method takes two integers and returns a third one, writing the latter into a pointer.

You must add e Python method to the class MyObject that implements this behaviour.

Determining the method name 

The method implementing the IMyInterface.MyMethod can either be named IMyInterface_MyMethod or MyMethod. Choose a name that does not conflict with other methods of the class, and that serves your personal naming conventions.

In comtypes, there are two ways to implement COM server methods. You can choose between a ‘low level’ and a ‘high level’ implementation strategy, on a method by method basis (the names ‘Low level’ and ‘high level’ are probably misleading a bit, suggestions for better names would be welcomed). comtypes uses different calling conventions for ‘low level’ and ‘high level’ method implementations.

comtypes inspects the method for the name of the second parameter, just after the self parameter:

If the second parameter is present and is named this, then the low level calling convention is used.
If the second parameter is not present, or is not named this, then the high level calling convention is used.

Low level implementation 

A low-level method implementation is called with the following arguments:

the usual self argument
for the this argument either None is passed, or the address of the COM object itself as an integer. The value of it can usually and should be ignored.
any other arguments listed in the IDL method signature.

[in] parameters from the method signature are usually converted to native Python objects, if possible. For [out] or [out, retval] parameters ctypes pointer instances are passed, you are required to put the result value into the pointer(s).

A low level method implementation must return a numerical HRESULT value, which specifies a success or failure code for the operation. The usual S_OK success code has a value of zero, but for convenience you can also return None instead.

So, a sample low-level implementation for MyMethod for our object would be this, assuming we want to return the sum of the two [in] parameters:

class MyObjectImpl(MyObject):
    # NOTE: the 'this' second parameter is required
    def MyMethod(self, this, a, b, presult):
        presult[0] = a + b
        return 0

High level implementation 

A high-level method implementation is called with the following parameters:

the usual self argument
the [in] parameters from the IDL method signature.

If there is a single [out] or [out, retval] parameter, then the method must return the result value; if there are more than one [out] or [out, retval] parameters, then a tuple containing the correct number must be returned. If there are no [out] or [out, retval] parameters, the return value does not matter and is ignored.

A sample high-level implementation for MyMethod is this:

class MyObjectImpl(MyObject):
    # NOTE: NO second 'this' parameter
    def MyMethod(self, a, b):
        return a + b

Choosing between low-level or high-level implementation 

Both implementation strategies have their own advantages and disadvantages, so you should choose between them on a case by case basis:

Low-level makes it easy to return special HRESULT values in the case that your object requires it.

High-level is usually easier to write, and is compatible with the normal calling convention that Python also chooses. However, it is more difficult to specify the HRESULT value to return in case you want to communicate error codes to the caller.

Run the object again and test the method 

We can now create the object and test the implemented method:

>>> from comtypes.client import CreateObject
>>> myobj = CreateObject("MyTypelib.MyObject")
>>> myobj  
<POINTER(IMyInterface) ptr=... at ...>
>>> myobj.MyMethod(42, 5)
47

More details on COM objects 

To be written…