How to Handle Circular Imports in Python with Immutable and Mutable Classes?

Avoiding Circular Imports in Python with Immutable and Mutable Classes

Circular imports are typically considered a symptom of poor code design. However, in certain scenarios, circular dependencies between classes may be necessary. To avoid import errors in such cases, one must carefully manage the import statements and the placement of circular references.

Example Scenario

Consider two classes, A and B, where A has a mutable attribute (a list) and B an immutable attribute (a tuple). Each class needs a method that takes an instance of the other class and returns an instance of itself, transforming the attributes accordingly.

Solutions

There are several approaches to avoid circular imports in this scenario:

Absolute Imports:

In a.py and b.py, use absolute imports to ensure the modules can import each other directly:

# a.py
from package import b

# b.py
from package import a

Deferring Import:

Delay importing the other class until it is actually needed within a function:

# a.py
def convert_to_list(b_obj):
    from package import b
    return a.A(list(b_obj.immutable_attr))

# b.py
def convert_to_tuple(a_obj):
    from package import a
    return b.B(tuple(a_obj.mutable_attr))

Intermediate Module:

Create an intermediate module that imports both a and b. This module can then be imported by both a and b to avoid circular imports:

# package/common.py
import a
import b

In a.py and b.py, import the intermediate module:

# a.py
from package import common

# b.py
from package import common

Avoiding Circular References in Class Definitions:

Circular references in class definitions should be avoided. Instead, consider creating a factory function that takes the class's attributes as arguments and returns an instance of the appropriate class:

# a.py
def create_a(list_attr):
    return A(list(list_attr))

# b.py
def create_b(tuple_attr):
    return B(tuple(tuple_attr))

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