Python - updating class attribute (monkey patching?)?

Question

So there is a class called Field and it has class attribute _slots, this attribute is dictionary.

So it looks like this (that class also use custom __metaclass__):

class Field(object):

  _slots = {
    'key1': False,
    'key2': None,
    ...
}

Class content is too big to be pasted here, so I'm providing a link to it, src

Now I need that when you instantiate this class, it will have my custom key/value pair inside that attribute.

So I tried this (in my own module):

from openerp import fields

fields.Field._slots['custom_key'] = None

When I print after this assignment, then it shows me that I have set this key/value pair, but actually when that class is loaded, it never gets my custom_key. Though if I monkey patch some method, it works, but for class attribute it does not. Is it even possible to do that (or am I doing something wrong here?)?

Though if I write simple test scenario with my own simple class, like:

(f.py file)

class F(object):
    _t = {'a': 1}

(other python file)

import f

f.F._t['b'] = 10

f_new = f.F()
print f_new._t

Then this one works.

Update

I also tried this (modifying code directly) in MetaField class that is used as __metaclass for Field (still that is ignored when Field class uses _slots:

def __new__(meta, name, bases, attrs):
    """ Combine the ``_slots`` dict from parent classes, and determine
    ``__slots__`` for them on the new class.
    """
    base_slots = {}
    for base in reversed(bases):
        base_slots.update(getattr(base, '_slots', ()))

    slots = dict(base_slots)
    slots.update(attrs.get('_slots', ()))

    attrs['__slots__'] = set(slots) - set(base_slots)
    attrs['_slots'] = slots
    attrs['_slots']['custom_key'] = None  # <= Modification here
    return type.__new__(meta, name, bases, attrs)