Hi,
I'm trying to understand Python 3's comparisons.
class Eq:
def __init__(self, x=""):
self.x = x
def __str__(self):
return self.x
def __eq__(self, other):
return str(self) == str(other)
class Lt:
def __init__(self, x=""):
self.x = x
def __str__(self):
return self.x
def __lt__(self, other):
return str(self) < str(other)
class LtEq:
def __init__(self, x=""):
self.x = x
def __str__(self):
return self.x
def __eq__(self, other):
return str(self) == str(other)
def __lt__(self, other):
return str(self) < str(other)
pairs = ((Eq("a"), Eq("b")), (Lt("a"), Lt("b")), (LtEq("a"), LtEq("b")))
for a, b in pairs:
print("comparing", type(a))
try:
print("a < b", a < b)
except TypeError as err: # TypeError, err in Python 2
print(err)
# etc, for all the other comparisons
For Python 2 I get this output:
comparing <type 'instance'> # Eq
a < b True
a <= b True
a == b False
a != b True
a > b False
a >= b False
comparing <type 'instance'> # Lt
a < b True
a <= b True
a == b False
a != b True
a > b False
a >= b False
comparing <type 'instance'> #LtEq
a < b True
a <= b True
a == b False
a != b True
a > b False
a >= b False
Clearly this is bad since class Eq has no ordering and class Lt has no
notion of equality.
For Python 3 I get this output:
comparing <class '__main__.Eq'>
unorderable types: Eq() < Eq()
unorderable types: Eq() <= Eq()
a == b False
a != b True
unorderable types: Eq() > Eq()
unorderable types: Eq() >= Eq()
comparing <class '__main__.Lt'>
a < b True
unorderable types: Lt() <= Lt()
a == b False
a != b True
a > b False
unorderable types: Lt() >= Lt()
comparing <class '__main__.LtEq'>
a < b True
unorderable types: LtEq() <= LtEq()
a == b False
a != b True
a > b False
unorderable types: LtEq() >= LtEq()
This is much better in the case of classes Eq and Lt. But I don't
understand why class LtEq does not handle <= or =>. Bear in mind that
for class Eq I only defined ==, Python 3 created != for me; similarly
for class Lt I defined < and Python created >. Or is my code for LtEq
wrong?
I know it isn't a problem creating a class decorator or metaclass to
"complete" LtEq; I'm just trying to understand how Python 3 comparisons
work.
Thanks!
--
Mark Summerfield, Qtrac Ltd., www.qtrac.eu
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