How to copy all python instances to a new class?

Question

I would like to make a deep-copy all the instances of a class, but without copying the definitions and rules of given class.

The intended use is, given I have the class "OriginalClass", its instances are the variables I am storing. While the program goes on, I want to be able to use previous "snapshots" of OriginalClass, from which I will only be using the values (I wont store new ones there, so no rules are needed to be inherited).

The above simple example works, and is very similar to what I want to do:

import copy

class VM:
    #nothing in the class
    pass

VM1=VM()
VM1.test=dict()
VM1.test['one']="hello"

def printmytext(VMPast=copy.deepcopy(VM1)):

    g.es(VMPast.test['one'])

VM1.test="Bye"

printmytext() #Returns "hello"

But the idea would be to substitute:

VMPast=copy.deepcopy(VM1)

For the working code I dont know how to write, which will copy the instances and dictionaries but not the definitions.

The reason is, when using the deepcopy version in my program (this way:)

VMPast=copy.deepcopy(VM1)
VM1.MyMethod(VMPast.MyButton[-1])

it throws the following error:

AttributeError: AttributeSetter instance has no __call__ method

Because the value of:

VMPast.MyButton[-1]

Is been sent as:

'__deepcopy__'

Instead of the actual substituted value I would intend... (the button), so if VMPast was already a working copy of the instances inside VM, this would work inestad of returning deepcopy! Another thing is, the instances are dynamic, I dont know in advance the names or values that the original class will have at that moment. Thank you very much!

Your `printmytext()` function creates **one** deep copy, which it stores as a default value with the function definition. It will not create a deep copy each time it is called. — Martijn Pieters, Jul 19 '13 at 11:15
A deep copy **never** copies the class definition, only the instance attributes. — Martijn Pieters, Jul 19 '13 at 11:16
Then how can I access the copied values without them returning __deepcopy__(memo) instead in the more complex script? I cant call "VMPast.MyButton[-1]" because it will return a __deepcopy__(memo) to which my methods in the VM class are not ready to use... — I want badges, Jul 19 '13 at 11:19
You need to share your original code and full traceback; I don't yet know what is going on for your code there, but you are very much misunderstanding what a deepcopy does or how Python instances and classes relate. — Martijn Pieters, Jul 19 '13 at 11:20
It looks as if the original class materializes a `__deepcopy__` attribute incorrectly when the `copy` module looks for a special copy-customisation hook. Without knowing what code we are talking about here that is impossible to be certain about. — Martijn Pieters, Jul 19 '13 at 11:22
Martin, after researching on your answer, I have realized the code fails just then calling copy.deepcopy(VM1). The code is here: http://snipt.org/AKhc7 Just executing that will throw the exception Im having,its just that copy module is not working with my class :\ — I want badges, Jul 19 '13 at 11:34

Martijn Pieters · Accepted Answer · 2013-07-19T11:43:32.947

A deep copy never copies the class definition, only the instance attributes.

It instead creates a new instance. Python instances only hold a reference to the class definition. Just call MyMethod on that instance:

VMPast.MyMethod()

provided the class has such a method.

It is your specific class that prevents a successful deep-copy:

class VariableManagerClass:
    def __getattr__(self, attr):
        return AttributeSetter(self, attr)

The copy.deepcopy() function looks for a .__deepcopy__() method on the instance, and your __getattr__ hook instead returns a AttributeSetter class.

You can prevent this by testing your attr a little first:

class VariableManagerClass:
    def __getattr__(self, attr):
        if attr.startswith('_'):
            raise AttributeError(attr)
        return AttributeSetter(self, attr)

This signals that your class does not handle private attributes or special method names.

Awesome Martin, Im sad I dont deeply understand why, but it works, Im really grateful for this :) — I want badges, Jul 19 '13 at 12:02

score 0 · Answer 2 · edited May 23 '17 at 12:29

If you only care about the values (attributes) of the class I think you could just call the __dict__ method to get those.

In [1]: class Foo():
   ...:     pass
   ...: 

In [2]: foo = Foo()

In [3]: foo.bar1 = 'derp'

In [4]: foo.bar2 = 'derp derp'

In [5]: foo.__dict__
Out[5]: {'bar1': 'derp', 'bar2': 'derp derp'}

If this is what asking, here is a related question: List attributes of an object

How to copy all python instances to a new class?

2 Answers2

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