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

Question

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]):
                self._data[name] = value

                # more stuff goes here, specially indexing dependencies, so we can 
                # do Index(some_class, name_of_property, some.object) to find all   
                # objects of some_class or its children where
                # given property == some.object

            else:
                raise Exception('Some misleading message')
        else:
            self.__dict__[name] = value    

    def __gettattr__(self, name):
        return self._data[name]

# USERS 

class User(Entity):
    _defs  = {'team':Team}

class DPLUser(User):
    _defs  = {'team':DPLTeam}

class PythonUser(DPLUser)
    pass

class PerlUser(DPLUser)
    pass

class FunctionalUser(User):
    _defs  = {'team':FunctionalTeam}

class HaskellUser(FunctionalUser)
    pass

class ErlangUser(FunctionalUser)
    pass

# TEAMS

class Team(Entity):
    _defs  = {'leader':User}

class DPLTeam(Team):
    _defs  = {'leader':DPLUser}

class FunctionalTeam(Team):
    _defs  = {'leader':FunctionalUser}

and now some usage:

t1 = FunctionalTeam()
t2 = DLPTeam()
t3 = Team()

u1 = HaskellUser()
u2 = PythonUser()

t1.leader = u1 # ok
t2.leader = u2 # ok
t1.leader = u2 # not ok, exception
t3.leader = u2 # ok

# now , index

print(Index(FunctionalTeam, 'leader', u2)) # -> [t2]
print(Index(Team, 'leader', u2)) # -> [t2,t3]

So, it works great (implementation details ommitted, but there is nothing complicated) besides of this unholy circular import thing.

Answer 1

Circular imports are not inherently a bad thing. It's natural for the team code to rely on user whilst the user does something with team.

The worse practice here is from module import member. The team module is trying to get the user class at import-time, and the user module is trying to get the team class. But the team class doesn't exist yet because you're still at the first line of team.py when user.py is run.

Instead, import only modules. This results in clearer namespacing, makes later monkey-patching possible, and solves the import problem. Because you're only importing the module at import-time, you don't care than the class inside it isn't defined yet. By the time you get around to using the class, it will be.

So, test/users.py:

import test.teams

class User:
    def setTeam(self, t):
        if isinstance(t, test.teams.Team):
            self.team = t

test/teams.py:

import test.users

class Team:
    def setLeader(self, u):
        if isinstance(u, test.users.User):
            self.leader = u

from test import teams and then teams.Team is also OK, if you want to write test less. That's still importing a module, not a module member.

Also, if Team and User are relatively simple, put them in the same module. You don't need to follow the Java one-class-per-file idiom. The isinstance testing and set methods also scream unpythonic-Java-wart to me; depending on what you're doing you may very well be better off using a plain, non-type-checked @property.