Is it possible to patch a class instance with an existing Singleton instance?

Question

I have a class representing our DB layer which is being instantiated internally in some classes (I cannot pass it as an outside parameter)
For example:

class MyClass(object):
    def __init__(self):
        self.dbapi = DatabaseAPI()
        self.timeout = 120

    def some_methods(self):
        pass

We're writing some Unit tests, and we'd like to mock the self.dbapi with an existing instance which we'll be creating before the test runs.

for example:

my_dbapi = DatabaseAPIMock()
...
...
@patch('MyModule.DatabaseAPI', my_dbapi)
def my_test(self):
    my_class = MyClass() #<---This is where I'm not able to mock the DatabaseAPI

This is what I tried to achieve so far, but from debugging the code I see that the self.dbapi is instantiated with the real object and not with the pre-made mock.

What am I missing?
BTW, we're running python 2.7

Thanks in advance!

Can you explain how you import DatabaseAPI in MyModule?

Michele d'Amico
– Michele d'Amico

2015-03-25 13:17:21 +00:00
Commented Mar 25, 2015 at 13:17 — Michele d'Amico
– Michele d'Amico, Commented Mar 25, 2015 at 13:17
I've completely changed my answer.

Michele d'Amico
– Michele d'Amico

2015-03-25 13:45:10 +00:00
Commented Mar 25, 2015 at 13:45 — Michele d'Amico
– Michele d'Amico, Commented Mar 25, 2015 at 13:45

Daniel Roseman · Accepted Answer · 2015-03-25 12:56:38Z

2

You're patching the wrong thing. You need to patch in the module that is assigning the attribute, ie the one that contains the target class.

It would be easier if you defined a method in your target class that gets your DatabaseAPI object. That way you can much more easily patch it.

class MyClass(object):
    def __init__(self):
        self.dbapi = self.get_db_api()
        self.timeout = 120

    def get_db_api():
        return DatabaseAPI()

and the test becomes:

@patch('my_module.MyClass.get_db_api')
def my_test(self, my_method):
    my_method.return_value = my_dbapi

answered Mar 25, 2015 at 12:56

Daniel Roseman

602k68 gold badges910 silver badges923 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Meny Issakov Over a year ago

HI Daniel, Thanks for the quick response, It's my fault that I 'skipped' that part, but in my code I actually patch the full class path, including modules. I'll update the question

Meny Issakov Over a year ago

Regarding your second recommendation, We have few hundred of classes written in a similar way to MyClass. I'm trying to create a single patch decorator that will suit all of them without needing to create a different patch for each test (as the class name changes). Also, I'd rather not to change the code of those classes unless there won't be any other way.

WestCoastProjects Over a year ago

I have a similar scenario. I can not change the target class and it internally instantiates a utility class instance, which although can be mocked in the testing class it does not affect what is used inside the target class. So the above approach does not work

Michele d'Amico · Accepted Answer · 2015-03-28 14:38:33Z

0

Short answer: use

@patch('MyModule.DatabaseAPI', return_value=my_dbapi)

instead.

In MyModule DatabaseAPI is something like a factory but my_dbapi is a instance. So by set the instance as return value of your factory you mock self.dbapi reference.

If in production code you can change the design a little bit consider to move as @DanielRoseman's answer propose: it is a better design.

edited Mar 28, 2015 at 14:38

answered Mar 25, 2015 at 12:56

Michele d'Amico

24k10 gold badges78 silver badges78 bronze badges

Collectives™ on Stack Overflow

Is it possible to patch a class instance with an existing Singleton instance?

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related