python circular imports once again (aka what's wrong with this design)

Let's consider python (3.x) scripts:

main.py:

from test.team import team
from test.user import user

if __name__ == '__main__':
    u = user()
    t = team()
    u.setTeam(t)
    t.setLeader(u)

test/user.py:

from test.team import team

class user:
    def setTeam(self, t):
        if issubclass(t, team.__class__):
            self.team = t

test/team.py:

from test.user import user

class team:
    def setLeader(self, u):
        if issubclass(u, user.__class__):
            self.leader = u

Now, of course, i've got circular import and splendid ImportError.

So, not being pythonista, I have three questions. First of all:

i. How can I make this thing work ?

And, knowing that someone will inevitably say "Circular imports always indicate a design problem", the second question comes:

ii. Why is this design bad?

And the finally, third one:

iii. What would be better alternative?

To be precise, type checking as above is only an example, there is also a index layer based on class, which permits ie. find all users being members of one team (user class has many subclasses, so index is doubled, for users in general and for each specific subclass) or all teams having given user as a member

Edit:

I hope that more detailed example will clarify what i try to achieve. Files omitted for readibility (but having one 300kb source file scares me somehow, so please assume that every class is in different file)

# ENTITY

class Entity:
    _id    = None
    _defs  = {}
    _data  = None

    def __init__(self, **kwargs):
        self._id   = uuid.uuid4() # for example. or randint(). or x+1.
        self._data = {}.update(kwargs)

    def __settattr__(self, name, value):
        if name in self._defs:
            if issubclass(value.__class__, self._defs[name]):
                s