[chadykamar]

“Metaclasses are deeper magic than 99% of users should ever worry about. If you wonder whether you need them, you don’t (the people who actually need them know with certainty that they need them, and don’t need an explanation about why).”

— Tim Peters

[tybug]

I realize this quote isn't meant to be taken literally, and it's a nice one-liner encapsulation of metaclasses, but it always bothered me. The people who "actually need metaclasses" were, at some point, learning about metaclasses for the first time and were "wondering whether they need them".

This quote ignores the middle ground of users who have a valid use case, but don't yet understand metaclasses. Not great advice for people who are trying to learn metaclasses.

[jcranmer]

I do sympathize with this sentiment, but there are times when "if you're wondering if you need it, you don't need it" is a very true statement, and Python metaclasses are in that boat. To see why, let me explain in more detail:

There's a "basic" [1] understanding of Python's object model, one which understands that most of Python is actually syntactic sugar for calling certain special methods. For example, the expression a[b] is "really" just calling a.__getitem__(b). Except that's not actually true; the "real" object model involves yet more dispatching to get things to work. Metaclasses allow you to muck with that "yet more dispatching" step.

So when do you need metaclasses? When you need to do that low-level mucking--the kind of mucking most won't know about until actually needed. If all you know about metaclasses is kind of what they are, then you very likely haven't learned enough about them to use them to actually need them. Conversely, if you've learned those details well enough to need to muck with them, then you've also learned enough to the point that you can answer the question as to whether or not you need metaclasses.

[1] I suspect most Python users don't even have this level of understanding, which is why I put it in scare quotes.

[nas]

I suspect Tim's meaning was that ordinary users don't need to know how metaclasses work in detail. It is enough that people know they exist (i.e. do something at time class is defined). If you need them, you are doing deep enough magic that you can take the time to learn how they work.

Maybe he was also suggesting that people should generally not be using them. In my experience, it is exceedingly rare to need them. In 29 years of writing Python code, I think I've used them once or twice.