I stumbled across this unexpected behaviour with Python 2.7 and 3.3. When
you look up a key in a dict, the key is sometimes compared against other
keys twice instead of just once.
First, a class that reports when it is being tested for equality, with a
fixed hash so that we get collisions inside the dict:
class Spam:
def __init__(self, label):
self.label = label
def __str__(self):
return self.label
def __hash__(self):
return 100
def __eq__(self, other):
print("comparing %s with %s" % (self, other))
return self is other
Create a dict and prepare for collisions:
x = Spam("x")
y = Spam("y")
d = {100: 1} # hash(100) == 100
When we add x to the dict, it collides with the existing key 100, so I
expect one call to Spam.__eq__, and that's exactly what I get:
py> d[x] = 200
comparing x with 100
But when I try adding y to the dict, it collides with 100, then it
collides with x, then it apparently collides with x a second time:
py> d[y] = 300
comparing y with 100
comparing x with y
comparing x with y
I expected only two calls to __eq__, not three: first comparing with 100,
then comparing with x. Checking for keys gives the same result:
py> x in d
comparing x with 100
True
py> y in d
comparing y with 100
comparing x with y
comparing x with y
True
What's more, checking for a key which is not present also compares three
times instead of twice:
py> Spam("z") in d
comparing z with 100
comparing x with z
comparing x with z
comparing y with z
False
I expected it to compare z with 100, then x *once*, then y, then return
False.
Why is the dict lookup comparing against x twice? It doesn't seem to be
the fault of x. If I create a slightly different dict, with the
collisions in a different order:
py> e = {x: 100}
py> e[100] = 200
comparing x with 100
py> e[y] = 300
comparing x with y
comparing y with 100
comparing y with 100
--
Steven
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