嵌套函数作为Python中的属性

Question

Not sure if this is a dupe or not. Here it goes.

I need to write some Python code that looks like:

class TestClass:
    def test_case(self):
     def get_categories(self):
      return [“abc”,”bcd”]
     # do the test here

and then have a test engine class that scans all these test classes, loads all the test_case functions and for each invokes get_categories to find out if the test belongs t the group of interest for the specific run.

The problem is that get_categories is not seen as an attribute of test_case, and even if I manually assign it

class TestClass:
    def test_case(self):
     def get_categories(self):
      return [“abc”,”bcd”]
     # do the test here
     test_case.get_categories = get_categories

this is only going to happen when test_case first runs, too late for me.

The reason why this function can't go on the class (or at least why I want it to be also available at the per-function level) is that a TestClass can have multiple test cases. Since this is an already existing testing infrastructure, and the categories mechanism works (other than the categories-on-function scenario, which is of lesser importance), a rewrite is not in the plans.

Language tricks dearly appreciated.

Answer 1

Nested functions don't become attributes any more than any other assignment.

I suspect your test infrastructure is doing some severely weird things if this isn't supported (and uses old-style classes!), but you could just do this:

class TestClass:
    def test_case(self):
        # ...

    def _get_categories(self):
        return [...]
    test_case.get_categories = _get_categories
    del _get_categories

Class bodies are executable code like any other block.

Answer 2

What you need is nested classes. Functions aren't made to do what you are trying to do, so you have to move up a notch. Function attributes are mainly used as markup, whereas classes can have anything you want.

class TestClass(object):
    class TestCase(object):
         @classmethod
         def get_categories(cls):
             return ['abc', 'efg']

Note that I used @classmethod so that you could use it without instantiating TestCase() ; modify if you want to do test_case = TestCase() .