How can I “override” deepcopy in Python?

I'd like to override __deepcopy__ for a given SQLAlchemy-mapped class such that it ignores any SQLA attributes but deepcopies everything else that's part of the class.

I'm not particularly familiar with overriding any of Python's built-in objects in particular but I've got some idea as to what I want.

Let's just make a very simple class User that's mapped using SQLA.

class User(object):
    def __init__(self, user_id=None, name=None):
        self.user_id = user_id
        self.name = name

I've used dir() to see, before and after mapping, what SQLAlchemy-specific attributes there are and I've found _sa_class_manager and _sa_instance_state.

Questions
Provided those are the only ones how would I ignore that when defining __deepcopy__?
Also, are there any attributes the SQLA injects into the mapped object?

(I asked this in a previous question (as an edit a few days after I selected an answer to the main question, though) but I think I missed the train there. Apologies for that.)

Edit - Fixed code thanks to zifot's answer

The only thing I got out of the Python docs is that you need to define deepcopy with memo as an extra argument. After a teensy bit of digging around I tried this out:

def __deepcopy__(self, memo):
    dpcpy = self.__class__()
    memo[id(self)] = dpcpy
    for attr in dir(self):
        if not attr.startswith('_'):
            value = getattr(self, attr)
            setattr(dpcpy, attr, copy.deepcopy(value, memo))
    return dpcpy

Then I created an instance of User as:

snake = User(913, 'Snake,S.')

After that, I tried a deepcopy operation as:

snake_dc = copy.deepcopy(snake)

...and snake_dc still has the SQLA attributes in it...

I'm all open to help, suggestions, etc.

回答1:

mavnn is right. For example try change your init of User to:

def __init__(self, user_id = None, name = None):
        self.user_id = user_id
        self.name = name

As for copying mapped instances, I recommend reading this thread

回答2:

When (deep-)copying you should not be calling __init__ but instead call __new__.

An example:

def __copy__(self):
    cls = self.__class__
    newobject = cls.__new__(cls)
    newobject.__dict__.update(self.__dict__)
    return newobject

回答3:

I'm not an expert on deepcopy, but from the error it looks like you need a parameterless constructor to call self.__class__() without parameters.

回答4:

To exclude sqlalchemy columns and mapped attributes you would do something along the lines of:

for attr in dir(self):
    if not self._sa_class_manager.mapper.has_property(key):
        ...