Python Mutable Defaults Are The Source of All Evil - Florimond Manca

[utterances-bot]

Python Mutable Defaults Are The Source of All Evil - Florimond Manca

How to prevent a common Python mistake that can lead to horrible bugs and waste everyone's time.

https://florimond.dev/en/posts/2018/08/python-mutable-defaults-are-the-source-of-all-evil/

[JC-swEng]

Thank you very much for this lesson!!

[rfichi]

Hey man thanks for this post, I always set None for default arguments because is a good practice, but never actually understood why? Now I know what can happen if a use mutable arguments as default!

[fernandomff123]

Python default arguments are evaluated only once, when the function is defined, not when its called. So if the default argument is a mutable and is mutated inside the function, it wil stay mutated when you call the function again.

[japita-se]

You saved my day!

[pinqwing]

I have programmed in python for what, 15 years, and NEVER noticed this until yesterday when hell broke loose and I failed to understand why. I must remind here that programming languages are made for humans, exclusively. So they must be designed in such a way that it is easy for humans to do he right thing and harder to write defects. I don't know how many defects I have introduced all hese years because of this. That is a bad symptom in itself. Any design that allows developers to make mistakes w/o noticing is a bad design, whatever the reasoning behind it. Methinks the python team needs more psychologists rather than mathematicians.

And the workaround (if x is None: x = []) is just plain ugly too.

I have invested too much in python to change now, but had I known this upfront, I quite probably would not have chosen it.

[k0pernikus]

And people like to make fun of PHP. I'm just glad that I stumbled over this madness in my first week of learning python. This is insanity.