After my initial scoping, I’d reached the point where there were some self contained tasks. We set up a mini team, with Dan and Anjani keen to be involved as well! Dan took a look at the Python part of the task, which was fairly well defined. I continued to look at how the C code worked, whilst Anjani was finishing off an unrelated issue during the morning.

Finding the Hidden Code

I was finally able to find a place where the C code needs to be changed! This took me a lot longer than expected. In general I think the documentation is quite lacking and the only way to figure out how things worked was to read the code, which proved quite slow going. The functionality I was interested in required following a long code path.

The key breakthrough came when comparing the setattro functions of PyCStructType and UnionType, both of which call PyCStructUnionType_update_stgdict. This is a key function that contains a lot of the behaviour we’ll need to modify!

It’s a large function (several hundred lines) and begins with a comment “Hack Alert” which is a little worrying. But reading through it revealed some changes we’ll need to make. One of the key areas is pep-3188, in particular ensuring that Unions also specify the structured format data string.

By this time Anjani was ready to help out, so I asked her to take a look at this function in more detail and make the necessary changes.

Understanding the rest of the C code

Even though I’ve found where some of the key code lives I’m still not sure how the rest of the code fits together - there is still a fair amount of investigation work to do.

I’m currently looking into why UnionType_setattro uses PyObject_GenericSetAttr but PyCStructType_setattro uses type_setattro. The generic function is just a wrapper around PyObject_GenericSetAttrWithDict, whereas type_setattro does some additional Unicode handling and checking of the ‘name’ before calling into PyObject_GenericSetAttrWithDict. Again I can’t find any documentation for this, so it isn’t clear to me yet what the additional handling is needed for, or whether it relates to Endianness at all.

Next Time

My plan for next time is to continue to look into the setattro behaviour and hopefully get to the bottom of what the unicode handling is about. There are a few other bits and pieces of investigation to do, which will likely keep me busy for a while yet!