Question

There are several threads on Python garbage collection in SO, and after reading about five, plus some doc on line, i am still not sure as to how garbage collection works and how i should manage objects which i am not using. In fact somewhere i read one should not do anything about collecting garbage, others tell one should del objects, while others again explain de-referencing an object is enough for Python to collect it as garbage.

So, at the risk of creating a duplicate, i will ask the question again, but differently, hoping to get more comprehensive and clearer information.

In my case i want to make a small simulation with objects representing people. Several instances of the Person() class will be created. It should exist for some time until it virtually “dies” while other instances will be created.

Now how do i make this Person() instance “die” (assuming many many of these instances will be created and i don’t want these instances to hang out like ghosts)?

There are several ways i can reference an object:

john = Person('john')

or

people = []
people.append(Person('john'))

or

people = {}
people['john'] = Person('john')

What is the best way to keep my program clean, freeing resources optimally? And what is the best way then to reference my object so i can control the deletion of the object?

Answer 1

I find that most programs create and dispose of objects quite naturally, so I never normally worry about it.

Some examples:

person = Person('john')
person = Person('james')
# Whoops! 'john' has died!

people = []
people.append(Person('john'))
# ...
# All 'Persons' live in people
people = []
# Now all 'Persons' are dead (including the list that referenced them)

class House():
    def setOwner(self, person):
        self.owner = person

house.setOwner(people[0])
# Now a House refers to a Person
people = []
# Now all 'Persons' are dead, except the one that house.owner refers to.

What I assume you are after is this:

people = {}
people['john'] = Person('john')

def removePerson(personName):
    del people[personName]

removePerson('john')

In this case people is the master list and you can control when a Person gets added and removed from the list (its a dictionary).

You may have to think through the concept of a person being created and then dying very thoroughly: Once created how does the person first interact with the simulation. Upon death, how should you untangle the references? (Its ok for a person to refer to other stuff, its things like House in my example that would keep a person alive. You could have other objects hold on to just the name of the person).