Wednesday, October 7, 2015

[Python] How can I patch a Python decorator before it wraps a function?

As title, Python decorator could be a problem if you want to write some unit cases that test these functions which contains decorators because you cannot use Mock to patch your decorators. Why? Please refer to the following link to check out:

Decorators are applied at function definition time. For most functions, this is when the module is loaded. (Functions that are defined in other functions have the decorator applied each time the enclosing function is called.)
So if you want to monkey-patch a decorator, what you need to do is:
  1. Import the module that contains it
  2. Define the mock decorator function
  3. Set e.g. module.decorator = mymockdecorator
  4. Import the module(s) that use the decorator, or use it in your own module

Let me give an exmple:
Here is a decorator function(user_passes_test) in the following file path

If you have a function using this decorator and need to write the test case for it as follows:

def my_dashboard_view(request):

Then, you can put your patch code in the beginning of the tests python file like this:

def mock_function(test_func, login_url=None, redirect_field_name=None):
    def wrapped_f(view_func):
        @wraps(view_func, assigned=available_attrs(view_func))
        def decorator_view(request, *args, **kwargs):
            return view_func(request, *args, **kwargs)
        return decorator_function
    return wrapped_f

from django.contrib.auth.decorators import user_passes_test
user_passes_test = mock_function

After doing so, the decorator:  user_passes_test() won't infect your test case any more~

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