Python @programming.dev sus @programming.dev 4mo ago

Find the bug (a classic)

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.

12 comments

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
- 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"]
  
  Ah sorry I meant a default argument which was a dict, thanks for the tip tho!
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()):
Yeah Pylint catches this. If you aren't using Pylint you are writing bad Python.
- That's a funny way to spell Ruff
  
  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?
Static set argument?
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.

12 comments