enhancement: introduce an utility function find_by_type_or_id

add an utility function .processors.find_by_type_or_id to find
appropriate processor object by data type *or* Processor's ID, and
some test code.

anyconfig/processors.py

@@ -113,6 +113,37 @@ def pred(pcls):
     return processor()
 
 
+def find_by_type_or_id(type_or_id, prs,
+                       cls=anyconfig.models.processor.Processor):
+    """
+    :param type_or_id:
+        Type of the data to process or ID of the processor class or
+        :class:`anyconfig.models.processor.Processor` class object or its
+        instance
+    :param prs: A list of :class:`anyconfig.models.processor.Processor` classes
+    :param cls: A class object to compare with `type_or_id`
+    :return:
+        Most appropriate processor instance to process files of given data type
+        or processor `type_or_id` found by its ID or None
+    :raises: UnknownProcessorTypeError
+    """
+    if isinstance(type_or_id, cls):
+        return type_or_id
+
+    if type(type_or_id) == type(cls) and issubclass(type_or_id, cls):
+        return type_or_id()
+
+    def pred(pcls):
+        """Predicate"""
+        return pcls.cid() == type_or_id or pcls.type() == type_or_id
+
+    processor = find_with_pred(pred, prs)
+    if processor is None:
+        raise UnknownProcessorTypeError(type_or_id)
+
+    return processor()
+
+
 def find_by_fileext(fileext, prs):
     """
     :param fileext: File extension

tests/processors.py

@@ -32,7 +32,13 @@ class B(anyconfig.models.processor.Processor):
     _extensions = ['yaml', 'yml']
 
 
-PRS = [A, A2, A3, B]
+class C(anyconfig.models.processor.Processor):
+    _cid = "dummy"
+    _type = "yaml"
+    _extensions = ['yaml', 'yml']
+
+
+PRS = [A, A2, A3, B, C]
 
 
 class Test_10_Processor(unittest.TestCase):
@@ -75,6 +81,11 @@ def test_20_find_by_type(self):
         self.assertRaises(UnknownProcessorTypeError, TT.find_by_type,
                           "X", PRS)
 
+    def test_24_find_by_type_or_id(self):
+        self.assertTrue(isinstance(TT.find_by_type_or_id("json", PRS), A3))
+        self.assertTrue(isinstance(TT.find_by_type_or_id("yaml", PRS), B))
+        self.assertTrue(isinstance(TT.find_by_type_or_id("dummy", PRS), C))
+
     def test_30_find_by_fileext(self):
         self.assertEqual(TT.find_by_fileext("jsn", PRS), A3)
         self.assertEqual(TT.find_by_fileext("yml", PRS), B)