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 wrapped with the 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")

Note

Since the singleton pattern allows to create only one instance from the class, an __init__ method would be called once with args and kwargs passed at the first call. Arguments of all future calls would be completely ignored and would not impact the existing instance at all.

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.