How to check if class instance exists in a list of class instance objects?

[buran]

AFAIK, every class in Python inherits from object - which is pretty standard. I presume that one can define a class that is not hashable - class, not instance - but i will take some hacking. So I think that your warning is of little merit.

class Person:
    def __init__(self, name):
        self.name =  name

person1 = Person('John')
person2 = Person('John')

persons = [person1, person2]
print(set(persons))

In python3 and python2 the output will be

Output:
{<__main__.Person object at 0x7ff9a43b6828>, <__main__.Person object at 0x7ff9a43b6860>}

so virtually, OP cannot do what they want with set

if there is __eq__() implemented

class Person:
    def __init__(self, name):
        self.name =  name

    def __eq__(self, other):
        return self.name == other.name

person1 = Person('John')
person2 = Person('John')

persons = [person1, person2]
print(set(persons))

in python2

Error:
Traceback (most recent call last):
  File "/home/boyan/Desktop/f.py", line 12, in <module>
    print(set(persons))
TypeError: unhashable instance

in python3:

Error:
  File "/home/boyan/Desktop/f.py", line 12, in <module>
    print(set(persons))
TypeError: unhashable type: 'Person'

again, not possible to use set

---

Also note that even some 'standard' classes like list are not hashable

list1 = [1,2,3]
list2 = [5,6,7]
print(set([list1,list2]))

Error:
Traceback (most recent call last):
  File "/home/boyan/Desktop/f.py", line 3, in <module>
    print(set([list1,list2]))
TypeError: unhashable type: 'list'