How to create a dynamically updating list of class instances, satisfying a particular condition? (Python)

Question

I want to create a list of class instances that automatically updates itself following a particular condition on the instance attributes.

For example, I have a list of object of my custom class Person() and I want to be able to generate a list that always contains all the married persons, ie all persons having the attribute 'MAR_STATUS' equal to 'MARRIED'.

Is this possible at all in Python? I have used a C++ precompiler for microsimulations that had a very handy built-in called "actor_set" which did exactly this. But I have no idea of how it was implemented in C++.

Thank you.

Answer 1

List comprehension:

[person for person in people if person.MAR_STATUS == 'MARRIED']

If you need to assign it to a variable and you want that variable to automatically update on every access, you can put this same code in a lambda, a normal function, or, if your variable is a class member, in a property getter.

Answer 2

It is poor form to have "action at a distance" / mutations / side-effects unless it is very carefully controlled.

That said, imperative language will let you do this, if you really want to, as follows. Here we use python's [ property getters and setters ]:

MARRIED_SET = set()
def updateMarriedSet(changedPerson):
    if hasattr(changedPerson,'married') and changedPerson.married==Person.MARRIED:
        MARRIED_SET.add(changedPerson)
    else:
        MARRIED_SET.discard(changedPerson)

class Person(object):
    ...

    @property
    def married(self):
        """The person is married"""
        return self._married

    @married.setter
    def married(self, newStatus):
        self._married = newStatus
        updateMarriedSet(self)

    @married.deleter
    def married(self):
        del self._married
        updateMarriedSet(self)

I can imagine this might, possibly, be useful to ensure accesses to getMarriedPeople() runs in O(1) time rather than amortized O(1) time.

Answer 3

The simple way is to generate the list on the fly eg, as shown in @sr2222's answer.

As an alternative you could call an arbitrary callback each time MAR_STATUS changes. Use __new__ if Person instances are immutable or make MAR_STATUS a property and call registered callbacks in the setter method (see notifications in traits library for a more complex implementation).