this post was submitted on 26 Feb 2025

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24

Find the bug (a classic) (self.python)

submitted 5 days ago by sus to c/python

12 comments fedilink hide all child comments

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.

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[–] [email protected] 3 points 5 days ago* (last edited 5 days ago)

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()):