Overwriting object instance does not release memory?

Question

I have a python object which is essentially a collection of other object instances. You can append other objects to it (which it just stores in a list). It is created when reading a file, eg:

def file_reader(file):
    obj = MyCollection()

    for line in file:
        other_obj = line_reader(line)
        obj.append(other_obj)
    return obj

If I then try to overwrite the object later (by reading a different file), the original data is not deleted, the object is just extended. Strangely, this seems to happen if I use different references:

obj1 = file_reader(file)
obj2 = file_reader(file1)

I suspect I have some kind of problem with circular referencing, but I can't quite grasp the logic. Anyone have an idea?

Edit: The essential part of MyCollection looks like:

class MyCollection(object):

    def __init__(self, objs = []):
        self.objs = objs

   def append(self, obj):
        self.objs.append(obj)

Could you show the code for MyCollection()? I'm betting the list is a class attribute, not an instance attribute. — jonrsharpe
– jonrsharpe, Commented Jan 27, 2014 at 13:56
Could you provide us, please, with the code of MyCollection object? It could be helpful. — Marek
– Marek, Commented Jan 27, 2014 at 13:58

jonrsharpe · Accepted Answer · 2018-08-07 14:07:55Z

8

I'll take a not-so-wild guess that you currently have something like this:

class MyCollection():
    listOfThings = []
    def __init__(self):

In this case, listOfThings is a class attribute, it is shared amongst all instances of the MyCollection class. Instead, you want an instance attribute, separate for each instance:

class MyCollection():
    def __init__(self):
        self.listOfThings = []

Edit: so close!

The issue you have is in:

def __init__(self, objs = []):

Here, objs is known as a "mutable default parameter". The list in the default parameter values is shared for all instances of that class that don't pass anything to __init__ (defining a = MyCollection([]), for instance, would create a new instance with a separate objs list).

You should use:

def __init__(self, objs=None):
    if objs is None:
        objs = []

Canonical question covering this: "Least Astonishment" and the Mutable Default Argument

edited Aug 7, 2018 at 14:07

answered Jan 27, 2014 at 14:01

jonrsharpe

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3 Comments

jramm Over a year ago

Thanks a lot! I had to look up 'mutable default parameter', and presto, my exact example is in the guide: docs.python-guide.org/en/latest/writing/gotchas

jramm Over a year ago

BTW, list is not used as the actual class is a fair bit bigger with other methods. You hit the problem though :)

jonrsharpe Over a year ago

Yes, this is a classic example of something that is incredibly easy to find information on, but only if you already know what it's called!

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