Circular dependency in Python

Question

I have two files, node.py and path.py, which define two classes, Node and Path, respectively.

Up to today, the definition for Path referenced the Node object, and therefore I had done

from node.py import *

in the path.py file.

However, as of today I created a new method for Node that references the Path object.

I had problems when trying to import path.py: I tried it, and when the program ran and called the Path method that uses Node, an exception rose about Node not being defined.

What do I do?

Answer 1

Importing Python Modules is a great article that explains circular imports in Python.

The easiest way to fix this is to move the path import to the end of the node module.

Answer 2

One other approach is importing one of the two modules only in the function where you need it in the other. Sure, this works best if you only need it in one or a small number of functions:

# in node.py 
from path import Path
class Node 
    ...

# in path.py
class Path
  def method_needs_node(): 
    from node import Node
    n = Node()
    ...

Answer 3

I prefer to break a circular dependency by declaring one of the dependencies in the constructor of the other dependent class. In my view this keeps the code neater, and gives easy access to all methods who require the dependency.

So in my case I have a CustomerService and a UserService who depend on each other. I break the circular dependency as follows:

class UserService:

    def __init__(self):
        # Declared in constructor to avoid circular dependency
        from server.portal.services.admin.customer_service import CustomerService
        self.customer_service = CustomerService()

    def create_user(self, customer_id: int) -> User:
        # Now easy to access the dependency from any method
        customer = self.customer_service.get_by_id(customer_id)