Hashability questions

[Chris Angelico]

On Tue, May 15, 2012 at 3:27 PM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> Why?  I can't see any purpose in implementing __eq__ this way, but I
> don't see how it's "broken" (assuming that __hash__ is actually
> implemented somehow and doesn't just raise TypeError).  The
> requirement is that if two objects compare equal, then they must have
> the same hash, and that clearly holds true here.
>
> Can you give a concrete example that demonstrates how this __eq__
> method is dangerous and broken?

Its brokenness is that hash collisions result in potentially-spurious
equalities. But I can still invent a (somewhat contrived) use for such
a setup:


class Modulo:
	base = 256
	def __init__(self,n):
		self.val=int(n)
	def __str__(self):
		return str(self.val)
	__repr__=__str__
	def __hash__(self):
		return self.val%self.base
	def __eq__(self,other):
		return hash(self)==hash(other)
	def __iadd__(self,other):
		try:
			self.val+=other.val
		except:
			try:
				self.val+=int(other)
			except:
				pass
		return self

Two of these numbers will hash and compare equal if they are equal
modulo 'base'. Useful? Probably not. But it's plausibly defined.

ChrisA