Python: circular imports needed for type checking

Question

First of all: I do know that there are already many questions and answers to the topic of the circular imports.

The answer is more or less: \"Design your Module/Class s

Answer 1

Possible duplicate: Python type hinting without cyclic imports

You should use Forward Reference (PEP 484 - Type Hints):

When a type hint contains names that have not been defined yet, that definition may be expressed as a string literal, to be resolved later.

So instead of:

class Tree:
def __init__(self, left: Tree, right: Tree):
    self.left = left
    self.right = right

do:

class Tree:
def __init__(self, left: 'Tree', right: 'Tree'):
    self.left = left
    self.right = right

Answer 2

The best solution is to not check types.

The other solution is to not create an instance of, and not reference at all, Foo or Bar until both classes are loaded. If the first module is loaded first, don't create a Bar or refer to Bar until after the class Foo statement is executed. Similarly, if the second module is loaded first, don't create a Foo or reference Foo until after the class Bar statement is executed.

This is basically the source of the ImportError, which could be avoided if you did "import foo" and "import bar" instead, and used foo.Foo where you now use Foo, and bar.Bar where you now use Bar. In doing this, you no longer refer to either of them until a Foo or Bar is created, which hopefully won't happen until after both are created (or else you'll get an AttributeError).

Answer 3

You could just defer the import in bar.py like this:

class Bar(object):

    def __init__(self, arg_instance_of_foo):
        from foo import Foo
        if not isinstance(arg_instance_of_foo, Foo):
            raise TypeError()

Answer 4

You can program against interface (ABC - abstract base class in python), and not specific type Bar. This is classical way to resolve package/module inter-dependencies in many languages. Conceptually it should also result in better object model design.

In your case you would define interface IBar in some other module (or even in module that contains Foo class - depends on the usage of that abc). You code then looks like this:

foo.py:

from bar import Bar, IFoo

class Foo(IFoo):
    def __init__(self):
        self.__bar = Bar(self)

# todo: remove this, just sample code
f = Foo()
b = Bar(f)
print f
print b
x = Bar('do not fail me please') # this fails

bar.py:

from abc import ABCMeta
class IFoo:
    __metaclass__ = ABCMeta

class Bar(object):
    def __init__(self, arg_instance_of_foo):
        if not isinstance(arg_instance_of_foo, IFoo):
            raise TypeError()