Andrea Crotti wrote: > I've wasted way too much time for this, which is surely not a Python bug, > not something that surprised me a lot. > > I stupidly gave for granted that adding an object to a set would first > check if there are equal elements inside, and then add it. > > As shown below this is not clearly the case.. > Is it possible to get that behaviour implementing another magic method > in my C class or I just have use another function to check (as I'm doing > now). > > class C(object): > > def __init__(self, x): > self.x = x > > def __eq__(self, other): > return self.x == other.x > > > if __name__ == '__main__': > s = set() > c1 = C(1) > c2 = C(1) > assert c1 == c2 > s.add(c1) > s.add(c2) > > print len(s)
Python's sets are hash-based; you have to implement a __hash__() method for the elements that ensures that c1 == c2 implies hash(c1) == hash(c2). >>> class C(object): ... def __init__(self, x): self.x = x ... def __eq__(self, other): return self.x == other.x ... def __hash__(self): return hash(self.x) ... >>> c1 = C(1) >>> c2 = C(1) >>> c1 == c2 True >>> s = set() >>> s.add(c1) >>> s.add(c2) >>> len(s) 1 -- http://mail.python.org/mailman/listinfo/python-list
