Circular dependency in Python

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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?

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.

Python Circular Imports, A circular dependency occurs when two or more modules depend on each other. This is due to the fact that each module is defined in terms of the other (See Figure 1). The code above depicts a fairly obvious circular dependency. functionA() calls functionB() , thus depending on it, and functionB() calls functionA() . Circular dependency in Python. Related. 4293. How do I merge two dictionaries in a single expression? 4508. Calling an external command from Python. 5381.

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()
    ...

Yet another solution to dig you out of a circular import hole in Python , Python allows it. Depending on how both modules interact, you might not even notice there is cycle in the dependency chain. However, when� Instantly we have a problem: a circular dependency. Thing needs ThingList in order to construct ThingLists. ThingList needs Thing in order to verify that it is made of and only of Things. But Python won't let me import Thing into thinglist.py. To get around this, I could put both classes into the same file.

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)

How to avoid bidirectional class and module dependencies , There are two main ways to deal with circular dependencies: strictly tied to Python, you can use, as a general rule, the Dependency Inversion� 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)

You may not need to import Path in node.py in order for Path and Node to make use of one another.

# in __init__.py  (The order of imports should not matter.)
from .node import Node
from .path import Path

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

  def return_something_pathy(self): 
    ...

# in node.py
class Node
  def __init__(self, path): 
    self.path = path
    ...

  def a_node_method():
    print(self.path.return_something_pathy())

To make it clear that Node is making use of Path, add type hinting. There is a feature available starting with Python 3.7 to support forward references in type annotations, described in PEP 563.

# in node.py  (Now with type hinting.)
from __future__ import annotations

class Node
  def __init__(self, path: Path): 
    self.path = path
    ...

  def a_node_method():
    print(self.path.return_something_pathy())

I came across a Yet another solution to dig you out of a circular import hole in Python is a great blog post which taught me this.

How do we handle circular dependency between Python classes?, We have the following code which shows circular dependency between the python classes.class P(): q = Q().do_something(); def� I'm working on a simple bake script - if the materials are just colors the script works - but if any of the materials include a texture - then I get a "Circular dependency for image.." when I manually replicate process of this script - baking works fine. The script works like this: for each object within a group named 'bake':

Why cyclic dependency errors occur, If you've been using Python for more than just simple proof-of-concept apps, you have probably encountered issues due to circular dependencies� Circular dependency in pandas dataframe Tag: python,python-2.7,pandas The following code places bets on coin flip results. You start with £100 and risk 5% on each flip, but because my code calculates bet size based on your starting balance, the bet is always £5.

Circular Dependencies in Python – C0DE MAN1AC, The circular dependency problem: The issue: Suppose you have two modules, module A and module B. Module A uses some classes from� The circular dependency problem: Suppose you have two modules, module A and module B. Module A uses some classes from module B and module B uses some classes from module A. This will result in errors when you try to use any of the modules, since none of the modules can be loaded without loading the other module.

Structuring Your Project — The Hitchhiker's Guide to Python, Multiple and messy circular dependencies: if your classes Table and Chair in furn .py need to import Carpenter from workers.py to answer a question such as� The situation we got into was a circular dependency in our Guice module, our DI configuration. A circular dependency isn’t hard to understand, it basically just means we told Guice how to construct

Comments
  • duplicate? stackoverflow.com/questions/744373/python-cyclic-imports
  • Are you trying to have one class per file? This is why that rarely works out well.
  • Agree with S.Lott. Python is not Java. You don't need one class per file.
  • A couple of people have said "you don't need one class per file" and words to the effect "don't try to be Java". OK - but it's off the point. Class definitions can get very large and bundling them into the same file can make for a very large, unreadable file. In a program I am working on with 8 mutually dependent classes, each of which is several hundred lines in length, I see no benefit in keeping them in the same file and a considerable benefit in keeping them separate.
  • also just as a reference, it seems circular imports are allowed on python 3.5 (and probably beyond) but not 3.4 (and probably bellow).
  • Okay, but the thing is, I have two other modules tree.py and block.py in that package that require node.py and are required by path.py. So am I supposed to put them all in one file? I liked having one module per class.
  • Have you tried my suggestion? It'll probably work. Just move the import to the end of the file. I advice you to read the article to understand why this is happening.
  • By the way, is there some reason other than you "liking" it that you want one class per module? The few times I've seen this preference, it was because it's Java-like.