Python metaclasses

#1

Post by aldanor » Sat, Aug 08, 2015, 1:01 PM UTC

Re: Python metaclasses

#2

Post by crocal » Sat, Aug 08, 2015, 4:04 PM UTC

Great tool, but not to be put in the wrong hands. The folks at Django got it right. We used it for unifying, simplifying and ensuring consistency of deeply nested configuration data entered using different means. However, in environment with less experience, I have seen "Akira-final-scene-tetsuo" monsters come to life with this.

Another loosely related suggestion: if we could stay away from singleton as examples, I think we would save ourselves a lot of future troubles. ^^

Re: Python metaclasses

#3

Post by mrsuprawsm » Sat, Aug 08, 2015, 4:25 PM UTC

This is an interesting and fairly well explained post, but a little light on real world examples.

Can anyone maybe suggest a few examples of where this could be used (or is used already?)

Re: Python metaclasses

#4

Post by pandler » Sat, Aug 08, 2015, 4:26 PM UTC

crocal wrote: post #2

Great tool, but not to be put in the wrong hands. The folks at Django got it right. We used it for unifying, simplifying and ensuring consistency of deeply nested configuration data entered using different means. However, in environment with less experience, I have seen "Akira-final-scene-tetsuo" monsters come to life with this. Another loosely related suggestion: if we could stay away from singleton as examples, I t…

I enjoyed reading through the Django ORM code to understand how it worked. Metaclasses in python are fun, but I can rarely justify actually using them for anything other than messing around. Oh but how I wish I needed to use them more!

Re: Python metaclasses

#5

Post by pandler » Sat, Aug 08, 2015, 4:30 PM UTC

mrsuprawsm wrote: post #3

This is an interesting and fairly well explained post, but a little light on real world examples. Can anyone maybe suggest a few examples of where this could be used (or is used already?)

The Django ORM or sqlalchemy good starting points. Metaclasses are what make them tick.

Re: Python metaclasses

#6

Post by akerl_ » Sat, Aug 08, 2015, 4:33 PM UTC

mrsuprawsm wrote: post #3

This is an interesting and fairly well explained post, but a little light on real world examples. Can anyone maybe suggest a few examples of where this could be used (or is used already?)

The registry really does seem to be the most common use case:

http://ivansmirnov.io/python-metaclasses/#metaclass-as-a-reg...

This enables things like plugin systems, where you want the act of importing a plugin which defines some objects to load those objects into the available plugin list.

Re: Python metaclasses

#7

Post by bkeroack » Sat, Aug 08, 2015, 5:01 PM UTC

mrsuprawsm wrote: post #3

This is an interesting and fairly well explained post, but a little light on real world examples. Can anyone maybe suggest a few examples of where this could be used (or is used already?)

I've used metaclasses to implement debug logging, where every class method logs its parameters and return values:

https://github.com/bkeroack/elita/blob/27129ea64382275effa99...

Re: Python metaclasses

#8

Post by crdoconnor » Sat, Aug 08, 2015, 5:18 PM UTC

mrsuprawsm wrote: post #3

This is an interesting and fairly well explained post, but a little light on real world examples. Can anyone maybe suggest a few examples of where this could be used (or is used already?)

I've written a few code spikes using metaclasses and to be honest, it never worked out well for me. I always found a simpler, clearer way of achieving the same outcome without them and the code that I did write felt dirty.

It's a very powerful language feature. Also very dangerous. It's a magnet for technical debt and obscure bugs.

Re: Python metaclasses

#9

Post by stuaxo » Sat, Aug 08, 2015, 5:45 PM UTC

mrsuprawsm wrote: post #3

This is an interesting and fairly well explained post, but a little light on real world examples. Can anyone maybe suggest a few examples of where this could be used (or is used already?)

I've had fun using them to make quasi-orms for CSV and Excel spreadsheet data before.

Re: Python metaclasses

#10

Post by Retra » Sat, Aug 08, 2015, 5:46 PM UTC

mrsuprawsm wrote: post #3

This is an interesting and fairly well explained post, but a little light on real world examples. Can anyone maybe suggest a few examples of where this could be used (or is used already?)

I've used it to simulate new language features. It sort of allows you to program the type system.