are user defined classes mutable

class, immutability, mutable, python

Solution

User classes are considered mutable. Python doesn't have (absolutely) private attributes, so you can always change a class by reaching into the internals.

For using your class as a key in a `dict` or storing them in a `set`, you can define a `.__hash__()` method and a `.__eq__()` method, making a promise that your class is immutable. You generally design your class API to not mutate the internal state after creation in such cases.

For example, if your engines are uniquely defined by their id, you can use that as the basis of your hash:

class Engine(object):
    def __init__(self, id):
        self.id = id

    def __hash__(self):
        return hash(self.id)

    def __eq__(self, other):
        if isinstance(other, self.__class__):
            return self.id == other.id
        return NotImplemented

Now you can use instances of class Engine in sets:

>>> eng1 = Engine(1)
>>> eng2 = Engine(2)
>>> eng1 == eng2
False
>>> eng1 == eng1
True
>>> eng1 == Engine(1)
True
>>> engines = set([eng1, eng2])
>>> engines
set([<__main__.Engine object at 0x105ebef10>, <__main__.Engine object at 0x105ebef90>])
>>> engines.add(Engine(1))
>>> engines
set([<__main__.Engine object at 0x105ebef10>, <__main__.Engine object at 0x105ebef90>])

In the above sample I add another `Engine(1)` instance to the set, but it is recognized as already present and the set didn't change.

Note that as far as lists are concerned, the `.__eq__()` implementation is the important one; lists don't care if an object is mutable or not, but with the `.__eq__()` method in place you can test if a given engine is already in a list:

>>> Engine(1) in [eng1, eng2]
True

Problem

Say I want to create a class for `car`, `tractor` and `boat`. All these classes have an instance of `engine` and I want to keep track of all the engines in a single list. If I understand correctly if the motor object is mutable i can store it as an attribute of `car` and also the same instance in a list. I cant track down any solid info on whether user defined classes are mutable and if there is a choice to choose when you define them, can anybody shed some light?

Original source

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