singleton-decorator

A testable singleton decorator allows easily create a singleton objects just adding a decorator to class definition but also allows easily write unit tests for those classes.

A problem

If you use a simple singleton pattern based on a decorator function that wraps a class with inner wrapper function like this:

def singleton(cls):
    instances = {}
    def wrapper(*args, **kwargs):
        if cls not in instances:
          instances[cls] = cls(*args, **kwargs)
        return instances[cls]
    return wrapper

it works fine with your classes, but it makes impossible a direct access to the class object without decorator. So you cannot call methods using a class name in unit tests:

@singleton
class YourClass:
    def method(self):
        ...
YourClass.method(...)

this code would not work because YouClass actually contains a wrapper function but not your class object. Also this approach causes another problem if your tests require separate instances of the objects, so a singleton pattern could break an isolation of different tests.

Solution

The singleton-decorator offers a simple solution to avoid both of these problems. It uses a separate wrapper object for each decorated class and holds a class within __wrapped__ attribute so you can access the decorated class directly in your unit tests.

Installation

To install the singleton-decorator just type in the command line:

$ pip install singleton-decorator

Usage

At first import the singleton decorator:

from singleton_decorator import singleton

Then decorate you classes with this decorator:

@singleton
class YourClass:
    ...

That’s all. Now you could create or get existing instance of your class by calling it as a simple class object:

obj = YourClass()  # creates a new instance
obj2 = YourClass()  # returns the same instance
obj3 = YourClass()  # returns the same instance
...

You also could pass args and kwargs into constructor of your class:

obj = YourClass(1, "foo", bar="baz")

Unit testing

In your unit tests to run the methods of decorated classes in isolation without instantiation the object (to avoid running a constructor code), use the __wrapped__ attribute of the wrapper object:

# your_module.py
@singleton
class YourClass:
    def your_method(self):
        ...

# tests.py
class TestYourClass(TestCase):
    def test_your_method(self):
        obj = mock.MagicMock()
        YourClass.__wrapped__.your_method(obj)
        ...

This test runs a code of the your_method only using a mock object as the self argument, so the test would be run in complete isolation and would not depend on another pieces of your code including a constructor method.