Deprecate This

As a fact of life, in bigger projects you often cannot just delete something – be it function, method, class or module. Replacing all its usages with whatever is the new recommendation – if any! – is typically outside of your influence, capabilities or priorities. By no means it should be treated as lost cause, though; any codebase would be quickly overwhelmed by kludges if there were no way to jettison them.

To reconcile those two opposing needs – compatibility and cleanliness – the typical approach involves a transition period. During that time, the particular piece of API shall be marked as deprecated, which is a slightly theatrical term for ‘obsolete’ and ‘not intended for new code’. How effective this is depends strongly on target audience – for publicly available APIs, someone will always wake up and start screaming when the transition period ends.

For in-project interfaces, however, the blow may be effectively cushioned by using certain features of the language, IDE, source control, continuous integration, and so on. As an example, Java has the @Deprecated annotation that can be applied to functions or classes:

If the symbol is then used somewhere else, it produces a compiler warning (and visual cue in most IDEs). These can be suppressed, of course, but it’s something you need to do explicitly through a complementary language construct.

So I had this idea to try and add similar mechanism to Python. One part of it is already present in its standard library: we have the warnings module and a built-in category of DeprecationWarnings. These can be ignored, suppressed, caught or even made into errors.
They are also pretty powerful, as they allow to deprecate certain code paths and not just symbols, which can be useful when introducing new meanings for function parameters, among other things. At the same time, it means using them is irritatingly imperative and adds clutter:

And in this particular case, it also doesn’t work as intended, for reasons that will become apparent later on.
What we’d like instead is something similar to annotation approach that is available in Java:

Given that the @-things in Python (decorators, that is) are significantly more powerful than the Java counterparts, it shouldn’t be a tough call to achieve this…

Surprisingly, though, it turns out to be very tricky and quite arcane. The problems lie mostly in the subtle issues of what exactly constitutes “usage” of a symbol in Python, and how to actually detect it. If you try to come up with a few solutions, you’ll soon realize how the one that may eventually require walking through the interpreter call stack turns out to be the least insane one.

But hey, we didn’t go to the Moon because it was easy, right? ;) So let’s see how at least we can get started.