patch method that uses external library method calls

Question

The foo class links the external library to an atribute so I can use self.external_class.externalClassMethod. But to do testing, I need to patch out this method call so I can continue testing the rest of the method. I have tried everything in the @patch decorator, nothing worked :(

import os
from unittest import TestCase
from unittest.mock import patch

class Foo(object):
    def __init__(self):
        self.os_handle = os
        self.string = None
    def path(self):
        try:
            print(self.os_handle.getNothing())
        except Exception:
            raise Exception
        self.string = "circumvented the faulty os_handle method call"
    
class TestFoo(TestCase):
    def testClass(self):
        self.assertEqual(1,1)
    def testLibraryCall(self):
        with self.assertRaises(Exception) as cm:
            foo = Foo()
            foo.path()
        self.assertEqual(cm.exception.__class__, Exception)
    # @patch('os', 'getNothing', ...)  # WHAT DO I PATCH?????
    def testLibraryCallNoException(self):
        foo = Foo()
        foo.path()
        self.assertEqual(foo.string, "circumvented the faulty os_handle method call")

save above code in my_class.py and run above code with $ python -m unittest my_class

Answer 1

Try with the following modification:

import os
from unittest import TestCase
from unittest.mock import patch

class Foo(object):
    def __init__(self):
        self.os_handle = os
        self.string = None
    def path(self):
        try:
            print(self.os_handle.getNothing())
        except Exception:
            raise Exception
        self.string = "circumvented the faulty os_handle method call"
    
class TestFoo(TestCase):
    def testClass(self):
        self.assertEqual(1,1)
    def testLibraryCall(self):
        with self.assertRaises(Exception) as cm:
            foo = Foo()
            foo.path()
        self.assertEqual(cm.exception.__class__, Exception)
    
    def testLibraryCallNoException(self):
        foo = Foo()
        with patch.object(foo, "os_handle") as mock_os_handle: 
            foo.path()
            self.assertEqual(foo.string, "circumvented the faulty os_handle method call")

I have modified only the method testLibraryCallNoException():

• I have used patch.object() instead patch(): in this way the attribute self.os_handle is substitute by the mock object mock_os_handle. The effect of this substitution is that in your production code the instruction self.os_handle.getNothing() does nothing as you want (I have omitted some details in the explanation!)
• I have added self. to assertEqual(foo.string, "circumvented the faulty os_handle method call") to solve an other error in the test code.