Python: Circular Imports Needed For Type Checking

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 structure pro

Solution 1:

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).

Solution 2:

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

classFoo(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
classIFoo:
    __metaclass__ = ABCMeta

classBar(object):
    def__init__(self, arg_instance_of_foo):
        ifnotisinstance(arg_instance_of_foo, IFoo):
            raise TypeError()

Solution 3:

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

classBar(object):

    def__init__(self, arg_instance_of_foo):
        from foo import Foo
        ifnotisinstance(arg_instance_of_foo, Foo):
            raise TypeError()

Solution 4:

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:

classTree:def__init__(self, left: Tree, right: Tree):
        self.left = left
        self.right = right

do:

classTree:def__init__(self, left:'Tree', right:'Tree'):
        self.left = left
        self.right = right