How to unit test a method called within __init__() function?

Question

I am trying to test a class method which is called within an__init__ function.

class abc:

    def __init__(path):
         list = []
         foo(path) 
         bar('hello') # test function bar

    def foo(self, path):
          # does a bunch of stuff and creates internal list 
          list =

    def bar(self):
         # does a bunch of stuff and uses list

I would like to write a test for method bar here which I guess must be called through an instance of class abc. I can mock list array for this test, but cannot understand how to avoid the call to foo().

Answer 1

Just mock foo method for the time of testing bar. You can use patch.object.

A full example below:

import unittest
from unittest.mock import patch


class MyClass:

    def __init__(self, path):
        self.list = []
        self.foo(path)
        self.bar('/init')

    def foo(self, path):
        self.list.append(path)

    def bar(self, path):
        self.list.insert(0, path)


class MyTestClass(unittest.TestCase):

    @patch.object(MyClass, 'foo')
    def test_bar_decorated(self, mock):
        a = MyClass('/foo')
        a.bar('/bar')
        self.assertEqual(a.list, ['/bar', '/init'])  # .foo() wasn't invoked


if __name__ == '__main__':
    unittest.main()

Notice that, a mock is created for you and passed in as an extra argument to the decorated function (we don't use it in this test). To avoid that you can use context manager version of patch.object:

    def test_bar_context_manager(self):
        with patch.object(MyClass, 'foo'):
            a = MyClass('/foo')
            a.bar('/bar')
            self.assertEqual(a.list, ['/bar', '/init'])  # same behaviour