Python: deepcopy does not work on user-defined classes?

Question

In the following example I would expect deepcopy to create a copy of field and not just copy the reference. What happens here and is there an easy way around it?

from copy import deepcopy

class Test:
    field = [(1,2)]

t1 = Test()
t2 = deepcopy(t1)

t2.field[0]=(5,10)

print t1.field # [(1,2)] expected but [(5,10)] obtained
print t2.field # [(5,10)] expected

Output:

[(5, 10)]
[(5, 10)]

Williams

possible duplicate of How to copy a python class? – Norvin

Answer 1

Deep copying (by default) only applies to instance level attributes - not class level - It doesn't make much sense that there's more than one unique class.attribute...

Change your code to:

class Test:
    def __init__(self):
        self.field = [(1,2)]

Pyrogen

Unless he overrides __copy__/__deepcopy__. Still though, that would be pretty odd. – Takara

This is actually not an answer to the question, but a workaround to the problem. If you dont want to/cant instantiate the class in every case, e.g.: you have a @classmethod, you cannot apply this workaround... – Norvin

Answer 2

Another approach with python 3.8 (I am not sure about previous versions) could be creating a __init__ method that deep-copies every class level attribute, like:

from copy import deepcopy

class Test:
    field = [(1,2)]
    def __init__(self):
        for attr in dir(self):
            if not attr.startswith('_'):
                setattr(self, attr, deepcopy(getattr(self, attr)))

t1 = Test()
t2 = deepcopy(t1)

t2.field[0]=(5,10)

print(t1.field) # [(1,2)]
print(t2.field) # [(5,10)]

of course you can also replace the dir with inspect.getmembers.

Lorrettalorri

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