What Are The Measures To Call A Python Code A Policy-based Design?

November 17, 2024 Post a Comment

Description I wonder if the code I am showing can be considered as an example of policy-based design in Python. Also, I would like to know if have you seen python modules using som

Solution 1:

The answer is the product of what I learn based on my research and some comments from StackOverflow.

I wrote a detail blog about these approaches called Mixin and policy-based design in python. In that post, I also discuss several extensions like implementations using decorators, adding default classes with mixins, and more ...

Design requirements and implementations

Requirement

Design a Python module such users can create classes with new behavior and interfaces by inheriting from a predefined set of classes. This approach provides a large number of classes available to the users based on all possible ways they can be combined into new ones.

This requirement can be implemented using mixin and policy-based implementations.

Mixin implementation

Mixin implementation is defined as follow:

Module classes are divided between mixin and base classes.
Mixin classes implement modifications to the behavior or interface of a base class.
Users create a new class by combining one or more mixin(s) with base classes by inheritance.

classMixin1Class:
    """Implementation of some Mixin1 class."""classMixin2Class:
    """Implementation of some Mixin2 class."""
...
classBasedClass:
    """Implementation of based class."""# End user creating new classclassNewClass(Mixin1Class, [Mixin2Class, ...], BasedClass):
    pass

References:

Policy-based implementation

Policy-based implementation is defined as follow:

Module classes are divided between policy and host classes.
Policy classes implement modifications to the behavior or interface of the host.
Host classes are defined withing class factories i.e., a function that returns type objects.
Users invoke the creation of new class using class factories.
Policy classes are passed as arguments to the class factory.
Withing the class factory, the host class is created with all the policy classes as its parents.

classPolicy1Class:
    """Implementation of some Policy1 class."""classPolicy2Class:
    """Implementation of some Policy2 class."""
...
defHostClassFactory(Policy1, Policy2=Policy2Class, ...):
    """Create a HostClass and return it."""classHostClass(Policy1, Policy2, ...):
        """Concrete implementation of the host class."""return HostClass

# End user invoking new class
NewClass = HostClassFactory(Policy1Class, [Policy2Class,...])

References:

Comparison between implementations

Here a list of the differences between approaches:

The relationship between classes is not the same affecting the Python's Method Resolution Order or MRO, see figure below.
The mixin's base classes are defined or instantiated when the user creates the new class.
However, policy-based host class definition is delayed until the user calls the factory function.
It is possible to provide default policy classes, but you cannot have mixin default classes.

theprettymind1987