Swizzling in Python

Story Time

Ever come across a Python library with some kind of shortcoming that you know you can fix, but don’t want to fork, fix, pull-request, wait five weeks, then update in order to get moving? Are you even too lazy to do just the first two steps and point your dependency manager at your own fork? Don’t worry, you can probably fix it in code.

Right now I’m writing a Slack bot library that will support, in addition to the commonplace single command/response paradigm, a more conversational paradigm for multi-step commands and workflows. So, like a good programmer, I set up my module structure, my setup.py, and got moving. This was after a little bit of proof-of-concept code sketching, in which I came up with the general flow of connecting with the official Slack client for python, aptly named python-slackclient. So, I ported the flow into my library’s code, with some nicer structure, fired up a virtualenv, ran python setup.py develop, and then wrote a little wrapper script to invoke the client.

Boom, error. Weird, right? My proof-of-concept had worked fine…

After looking at Slack’s code, I found that it was cleansing an exception from the websocket-client library, where it was having problems locating my system’s Root Certificate Authority file. That’s a problem I don’t really want to deal with right now. I have a feeling it has to do with the path magic that python setup.py develop does, so I’m willing to let it go. Nonetheless, I’m still stuck with it not working, and no way to tell the Slack client to get its head out of its ass and find the Root CA file for the websocket client.

Or is there?

Investigation

The websocket-client method, create_connection takes a URL by default but also takes some kwargs, one of which is sslopt, a dictionary of values describing SSL options, including the location of the Root CA file. Additionally, the ssl standard library includes methods for retrieving the location of said file. But, again, the calling of create_connection is within the Slack client’s code! What do I do?

Swizzle that shit!

Swizzling, for the uninitiated, is the same thing as monkey-patching, which is to say, replacing at runtime the implementation of a method on a class. Alas, it’s a little more complex in this case, since the method I want to swizzle is on the class of an object, an instance of which is held by the Slack client. So, I ended up using a subclass of the client and overloading the initializer to dig into the client’s state and swizzle the method in question.

Check it out!

Note that you have to bind the newly-defined method to the class, meaning that if you only have a reference to an instance, you need to use the __class__ attribute to get the class itself. Don’t forget self as the first argument; this is important for the method to bind correctly to the class/instance.

Oh, so, by the way, that Slack bot library I’m working on can be found here: SlackBorg.



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