Python mock global function that is used in class

mockingpythonunit testing

I can't seem to get my head around mocking in Python. I have a global function:

a.py:

def has_permission(args):
    ret_val = ...get-true-or-false...
    return ret_val

b.py:

class MySerializer(HyperlinkedModelSerializer):

     def get_fields():
         fields = super().get_fields()
         for f in :
             if has_permission(...):
                 ret_val[f.name] = fields[f]
         return ret_val

c.py:

class CountrySerializer(MySerializer):
    class Meta:
        model = Country

Question: Now i want to test c.py, but i want to mock the has_permission function that is defined in a.py, but is called in the get_fields-method of the class MySerializer that is defined in b.py … How do i do that?

I've tried things like:

@patch('b.MySerializer.has_permission')

and

@patch('b.MySerializer.get_fields.has_permission')

and

@patch('a.has_permission')

But everything i try either just doesn't work and has_permission is still executed, or python complains about that it can't find the attribute 'has_permission'

with the patching done in:

test.py

class TestSerializerFields(TestCase):
    @patch(... the above examples....)
    def test_my_country_serializer():
        s = CountrySerializer()
        self..assertTrue(issubclass(my_serializer_fields.MyCharField, type(s.get_fields()['field1'])))

Best Answer

You need to patch the global in the b module:

@patch('b.has_permission')

because that's where your code looks for it.

Also see the Where to patch section of the mock documentation.

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Python – Are static class variables possible in Python

Variables declared inside the class definition, but not inside a method are class or static variables:

>>> class MyClass:
...     i = 3
...
>>> MyClass.i
3

As @millerdev points out, this creates a class-level i variable, but this is distinct from any instance-level i variable, so you could have

>>> m = MyClass()
>>> m.i = 4
>>> MyClass.i, m.i
>>> (3, 4)

This is different from C++ and Java, but not so different from C#, where a static member can't be accessed using a reference to an instance.

See what the Python tutorial has to say on the subject of classes and class objects.

@Steve Johnson has already answered regarding static methods, also documented under "Built-in Functions" in the Python Library Reference.

class C:
    @staticmethod
    def f(arg1, arg2, ...): ...

@beidy recommends classmethods over staticmethod, as the method then receives the class type as the first argument, but I'm still a little fuzzy on the advantages of this approach over staticmethod. If you are too, then it probably doesn't matter.

Best Answer

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Python – Are static class variables possible in Python

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