How to compare two instances in Python?

Question

Is there possibility to compare two 'instances'? I have one variable and one list. Variable have type 'instance', items in list have also the same type. When I compare variable with the same item in list:

cities = [USA, Poland, England, GB, Italy]
variable = Italy

variable == cities[-1]

I received 'False' as output. I'm 100% sure that both elements are the same.

Thanks in advance!

Answer 1

There are two types of objects in Python. Mutable, and immutable.

Immutable

• State cannot be changed.
• Usually thought of as "primitive" types.
• int, float, string, tuple, etc.

Mutable

• State can be updated and changed.
• list, dict, set, bytearray, any object that is created via the class token.

---

Depending on the type that you are discussing when you say variable this will affect the operator ==. Immutable types will always be checked against the actual value (e.g. 1 == 1 is True), where mutable types are checked against the object's __eq__ method (which overloads the == sign).

All of the mutable types listed - except new objects initialized with class - have a built-in __eq__ methods that are used when the == sign is present. Assuming you are using your own object, take the following for example:

class Obj:
    def __init__(self, integer):
        self.integer = integer

print(Obj(1) == Obj(1)) # False

Notice that despite integer being equal for each Obj, due to the fact Obj is a mutable type without the __eq__ method Python will check if the objects are equal to each other based on their space in memory- in other words, for it to be True, the object must be the exact same one you initialized.

class Obj:
    def __init__(self, integer):
        self.integer = integer

obj = Obj(1)

print(obj == obj)  # True

To manually overload the == sign, you must use the __eq__ method:

class Obj:
    def __init__(self, integer):
        self.integer = integer

    def __eq__(self, other):
        # Comparison of two integers.
        return self.integer == other.integer

print(Obj(1) == Obj(1))  # True