Find the bug (a classic)
from sus@programming.dev to python@programming.dev on 26 Feb 2025 17:34
https://programming.dev/post/26056160 

class Node:
    def __init__(self, edges = set()):
        self.edges = edges


def main():
    foo = Node()
    bar = Node()
    quz = Node()

    foo.edges.add(bar)
    bar.edges.add(foo)

    assert(foo is not bar) # assertion succeeds
    assert(foo is not quz) # assertion succeeds
    assert(bar is not quz) # assertion succeeds
    assert(len(quz.edges) == 0) # assertion fails??


main()
spoiler

Mutable default values are shared across objects. The set in this case.

#python

threaded - newest

technohacker@programming.dev on 26 Feb 2025 17:50 next collapse

Oh I had a similar bug but with defaulted dicts. Default args are constructed once and reused. Not a problem for immutable args, but mutables like dicts (and sets I’d also assume) are all shared.

EDIT: whoops, didn’t see you spoilered the answer, my bad! If it helps, i found my bug when dealing with cross-thread stuff, so that was a fun moment to bisect

joyjoy@lemm.ee on 26 Feb 2025 18:10 collapse

You may like collections.defaultdict. Pass the constructor a factory function to be run when a key is missing.

dd = defaultdict(list)
dd['key'].append("value")
print(dd['key'])  # ["value"]
technohacker@programming.dev on 26 Feb 2025 18:46 collapse

Ah sorry I meant a default argument which was a dict, thanks for the tip tho!

m_f@discuss.online on 26 Feb 2025 18:12 next collapse

Yeah, I discovered this when a coworker wrote code like def foo(timestamp = now()) and had fun debugging why there were a bunch of duplicate timestamps.

PEP 671 would add new syntax to ease the pain, but it’s not accepted yet. It would allow for writing function definitions like one of these:

def bisect_right(a, x, lo=0, hi=>len(a), *, key=None):
def connect(timeout=>default_timeout):
def add_item(item, target=>[]):
def format_time(fmt, time_t=>time.time()):
FizzyOrange@programming.dev on 26 Feb 2025 20:34 next collapse

Yeah Pylint catches this. If you aren’t using Pylint you are writing bad Python.

milkisklim@lemm.ee on 26 Feb 2025 23:45 collapse

That’s a funny way to spell Ruff

FizzyOrange@programming.dev on 27 Feb 2025 07:45 collapse

Yeah I tried Ruff about a year ago and it only had really trivial lints so it wasn’t a good replacement for Pylint. Is it on par yet?

milkisklim@lemm.ee on 27 Feb 2025 12:03 next collapse

What do you mean by trivial? I am not necessarily the most experienced coder, but it does a great job yelling at me to keep methods short and simple.

I’d suggest taking five minutes whenever and look up the ruff ruleset to see if it would be helpful for you.

Also maybe because I don’t know how to use pylint in vs code, but the only semi useful thing it catches for me is if my venv doesn’t have a library the code imports.

Edit: For example, Ruff has caught this bug (mutable argument defaults) in my code before.

FizzyOrange@programming.dev on 27 Feb 2025 15:03 collapse

it does a great job yelling at me to keep methods short and simple

Yes style things like that are what I would consider trivial. I also think those are actively bad lints. Yes methods should be short in general, but making it a hard enforced limit means you end up getting sidetracked by refactoring when you only wanted to add one line to a method.

NostraDavid@programming.dev on 27 Feb 2025 13:07 collapse

It has implemented about half - I would check which the important ones are, and check if it’s been implemented (which is being tracked in issue 970):

github.com/astral-sh/ruff/issues/970

Astral claims Ruff has more rules total (about 800), but implemented about half of Pylint (which has 400 rules, so 200 implemented).

waz@lemmy.world on 27 Feb 2025 11:19 next collapse

Wow, I learned this bug during a job interview I had this week. I’m not much of a Python guy, but I was givin a Python coding challenge and I tried default initializing a parameter to an empty list. The guy interviewing me looked horrified and explained the problem and sent me an article about it to read later. It’s a odd coincidence coming across it twice in one week.

Michal@programming.dev on 27 Feb 2025 12:30 collapse

Static set argument?