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Here at Dropbox, we often use Python in uncommon ways. This module allows you to write COM-compatible code using only Python.
For example, the Dropbox desktop client feature that allows you to upload photos from a camera uses comtypes to access Windows Autoplay. But before we talk about comtypes, we have to talk about COM. The Component Object Model is a standard introduced by Microsoft back in It allows two software components to interact without either one having knowledge of how the other is implemented, even if the components are written in different languages, running in different processes, or running on different machines and different platforms.
And it does all this using a standard that was created almost 20 years ago. On Windows, COM is both a standard and a service. It provides all the systems and utilities necessary to make inter-component compatibility possible.
The standard requires interfaces to be compiled into a binary format that is language agnostic. For this purpose, it includes a specification for its own interface language—the Microsoft Interface Definition Language, aka MIDL—which is compiled into a binary called a type library that is then included inside a runnable such as a. Well, parlez-vous version 1? COM handles these things for you, but this functionality comes at a cost.
It involves a fair amount of complexity, which is unfortunately necessary, and a lot of syntactic convolution, which is just plain unfortunate.
COM requires incantations such as. To ensure unambiguity in class and interface identification, everything is referenced by GUIDs, which are undescriptive and unwieldy at best. And when creating a COM server, the sheer number of configuration options at every step of the way can be paralyzing. I came to Dropbox with enough knowledge of COM to squeak by, and still consider myself no more than an advanced novice. If one were to attempt to write pure Python code that used or, heaven forbid, implemented a COM object, one would need to generate and parse the binary type library files that specify COM interfaces, perform all the complex Windows registry rituals, track all the reference counts to COM objects, as well as correctly write the endless syntactical mumbo jumbo.
Fortunately for us, the comtypes module exists to abstract almost all of this horribleness away from us. If COM is black magic, then comtypes is the mysterious witch doctor service that you contract to perform the black magic for you. For simple tasks, everything likely works fine. Unfortunately, if you need to do anything very complex, you run the risk of being left in the dark as to what sort of invocations were performed, only to find the demon knocking on your door.
Still, comtypes makes life much easier. This slightly simplified sample code allows access to the contents of a camera attached to the computer. The deviceobj is a Python wrapper around a COM object that is actually implemented elsewhere on the system, one that represents a camera we can interface with.
Underneath, comtypes will be busy CoCreateInstancing, QueryInterfacing, and wrapping ctypes objects with Python objects for your ease of use. The magic begins in the comtypes.
The handy helper function GetModule will take a binary file like a. It is declared as two mystery types followed by three 32 bit ints.
Further digging will reveal that one of the unknown types is an enum, but the other is another struct involving more unknown types. For it to be usable in Python, you have to break things down into known types without the benefit of importing hundreds of Windows headers.
GetModule will do all of these things for you, but the generated code hides the interesting part, the wrapper classes that actually proxy to the real COM objects underneath. In COM, IUnknown is the grandfather of all interfaces, the interface which all other interfaces inherit from. It contains only three functions:. In comtypes land, IUnknown is actually a base class. The class IUnknown itself is actually pretty simple. For the uninitiated, all Python classes are actually objects, and metaclasses are things that make classes.
Open , which takes in two parameters and returns nothing; and IPortableDevice. Content , which takes no parameters and returns one. The wrappers then proxy the method calls into an actual COM object that was instantiated under the covers, wrap the return values in Python types, and return them to you, transforming returned error codes into Python exceptions along the way. The most painful such incident brought development to a dead halt for two days.
The only symptom was that the program would occasionally crash after reading a bunch of images from a camera. The bug was non-deterministic and no exception was generated. After endless hours of printing and prodding, I finally found the root cause of the problem. Additional calls to AddRef and Release are only necessary if the user makes copies of the reference to the object.
This in itself is only passingly strange, because it usually works. However, the clever wrapping of COM objects sometimes results in comtypes objects being created unexpectedly, and then also deleted unexpectedly.
For example, in the following code. In the process, it unexpectedly creates an instance of IDeviceItem. More importantly, it unexpectedly destroys an instance of IDeviceItem.
So, the following code. After the first Release call, the COM object is considered deleted. Whenever the garbage collector for that COM object is triggered, everything explodes. Save your reference into a Python object and keep it around. This results in exactly one call to Release , which occurs when the Python object deviceitem is destroyed. All of this happened within my first few months at Dropbox, and I barely spoke Python at the time.
I learned what a metaclass was before I had fully mastered list slicing syntax. Meanwhile, my counterpart on the Mac camera uploads side was not having the easiest time either. It made me feel a little bit better about dealing with incorrect vtable pointers and bad reference counting.
Discovering comtypes was an integral part of the development of the photo feature, and it certainly presented enough excitement to be considered an adventure. Reference counting bugs are the price you pay when you work with low-level code, but it certainly would have been much more work to write our own Python wrappers around COM. COM may be difficult to use, and comtypes occasionally frustrating, but with a working knowledge of how to use the first and how to work around unexpected pitfalls in the second, we can plow ahead with future Windows features.
In fact, not long after we released the camera feature, we found ourselves again needing to interact with a COM component. With all this knowledge under my hat, I made myself a COM client in no time. It was a glorious victory. Hi — great article! This article just reminds of the nightmares that used to be. I still think in COM sometimes, I loved working with it.
COM is possibly one of the worst psuedo technologies that ever existed. And the reason why it existed was to obfuscate programming windows. As a technology COM itself is a spectacular failure. SO spectacular that MS has done nothing but try and paper over it again and again as if we wont notice its there. COM is designed to solve binary compatibility between modules implemented on different programming languages.
This you cannot get directly with plain C. The concepts it has are SAME as you would find in any similar technology and are not bound to any particular programming language. COM is there to stay. It is all over Windows and it does its job well. If you wanted to solve same problem that COM solves, your solution would not be much different.
Just when you think you understand COM this book will whack you in the head and show you how little you really know. In COM, objects are created and destroyed and objects implement interfaces, e. Actually it says a bit earlier in the article that on the Python side, IDeviceItem is a class.
Skip to content Here at Dropbox, we often use Python in uncommon ways. Comtypes If COM is black magic, then comtypes is the mysterious witch doctor service that you contract to perform the black magic for you. Content for item in contents: GetModule The magic begins in the comtypes.
It contains only three functions: So, the following code for i in range Microsoft rules specify that the beard requirement can be waived if you wear a tie. Takes me back… Now to try and forget before the nightmares set in Like Like. Ive been MS dev tool free for 10 years now, and Ill never go back.