Introduction to COM

Luca Salvarani — Wed, 02 Nov 2022 15:48:47 +0000

This will be the first post of a series i intend to write in order to challenge myself and share my knowledge about COM automation on Windows.

Useful sources

Early vs Late binding:
- this and this SO answers
- "Python Programming on Win32" book (Andy Robinson, Mark Hammond)
- pywin32 docs
- "Early Vs Late Binding In Win32Com" Youtube video (by "Sigma Coding")
API references:
- Office Suite VBA Documentation
Use COM Constants
Tutorials:
- Python - Win32COM - Core Concepts
- Python Outlook Automation | pywin32

Basic operations

Luca Salvarani — Sun, 30 Oct 2022 14:41:39 +0000

Let's dive into the basic operations...

Dispatch methods

A COM object can be created in various ways:

win32com.client.Dispatch
win32com.client.DispatchEx
win32com.client.DispatchWithEvents
win32com.client.dynamic.Dispatch
win32com.client.gencache.EnsureDispatch

1. `win32com.client.Dispatch`

This is the most common way to start an instance of a COM object.
It tries to automatically detect and use the best dispatching method (early or late binding).
In fact, behind the scenes, this method uses dynamic.Dispatch or gencache.EnsureDispatch. The order in which the dispatch types are tested is the following (the first valid dispatch is returned):

Early binding (also called "Static Dispatch") if possible
Late binding (also called "Dynamic Dispatch") otherwise

from win32com import client

excel = client.Dispatch('Excel.Application')

2. `win32com.client.DispatchEx`

Same as win32com.client.Dispatch, but allows to create a DCOM object on a remote machine.

For example, in the following code snippet, we will open Excel on the server with IP 192.168.1.17:

from win32com import client

server_name = "192.168.1.17"
excel = client.DispatchEx('Excel.Application', server_name)

3. `win32com.client.DispatchWithEvents`

Same as win32com.client.Dispatch, but allows to respond to events triggered by the application.

For example, in the following code a message will be printed on screen every time a user performs a double click on a cell:

from win32com import client

class ExcelEvents:
    def BeforeDoubleClick (self, Target, Cancel):
        print("A double click has occurred")

excel = client.DispatchWithEvents('Excel.Application', ExcelEvents)

4. `win32com.client.dynamic.Dispatch`

This method completely bypasses the COM cache, directly querying the object instead. It can solve some problems with the cache, but it is slower and is subject to application rules (for example in Excel everything is case insensitive, so excel.VISIBLE is equal to excel.Visible) but it can

from win32com import client

excel = client.dynamic.Dispatch('Excel.Application')

PRO:

Case insensitive lookup

CONS:

Slower than gencache.EnsureDispatch

5. `win32com.client.gencache.EnsureDispatch`

This method forces the use/creation of the cache.

The cache can be found inside the folder: C:\Users\{USERNAME}\AppData\Local\Temp\gen_py

from win32com import client

excel = client.gencache.EnsureDispatch('Excel.Application')

Known bugs

Using this method you might encounter some bugs involving the cache, like the infamous AttributeError: 'module' object has no attribute 'CLSIDToPackageMap'.

This problem in particular can be solved in two ways:

deleting the cache folder (%TEMP%/gen_py) so that the next call to EnsureDispatch will trigger a cache refresh;
forcing a cache refresh yourself;

Then all you need to do is running again the client.gencache.EnsureDispatch function and you'll be good to go!

PRO:

Faster than dynamic.Dispatch thanks to the cache

CONS:

More subject to errors (most notably the AttributeError cited before) caused by the cache

DEV Community: Luca Salvarani

Introduction to COM

Useful sources

Basic operations

Dispatch methods

1. win32com.client.Dispatch

2. win32com.client.DispatchEx

3. win32com.client.DispatchWithEvents