candide wrote:
In which cases should we use the is() function ? The is() function
compares identity of objects rather than values so I was wondering in
which circumstances comparing identities of objects is really vital.
Examining well reputated Python source code, I realize that is()
function is mainly used in the following set form :
spam is None
But how much "spam is None" is different from "spam == None" ?
is() function makes comparaison of (abstract representation of)
adresses of objects in memory. Comparing addresses of objects is a low
level feature performed by low level langages such as C but seldom
needed in high level languages like Python, isn'it ?
I remember meeting a use case where testing identity is required, when
you are searching for an instance containing a specific object:
class Someone:
def __init__(self, name, car):
self.name = name
self.car = car
class Car:
def __init__(self, brand):
self.brand = brand
def __eq__(self, other):
return self.brand == other.brand
people = { 'bob':Someone('bob', Car('chrys')), 'cindy': Someone('cindy',
Car('Volk')), 'carlos':Someone('carlos', Car('Volk'))}
aCar = people['carlos'].car
print "people owning a Volk car", [ people[ppl].name for ppl in people
if people[ppl].car == Car('Volk')]
print "people owning Carlos's car", [ people[ppl].name for ppl in people
if people[ppl].car is aCar]
people owning a Volk car ['carlos', 'cindy']
people owning Carlos's car ['carlos']
JM
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