featuretools_primitives entry point expects list of primitive classes (…

…#529)

* the featuretools_primitives entry point expects a list of primitive classes

* add example use of entry point

* only import trans or agg primitives from entrypoint

featuretools/primitives/__init__.py

@@ -2,12 +2,18 @@
 from .api import *
 
 import pkg_resources
-# Load in primitives registered by other libraries into Featuretools namespace
+# Load in a list of primitives registered by other libraries into Featuretools
+# Example entry_points definition for a library using this entry point:
+#    entry_points={
+#        "featuretools_primitives": [
+#            other_library = other_library:LIST_OF_PRIMITIVES
+#        ]
+#    }
 for entry_point in pkg_resources.iter_entry_points('featuretools_primitives'):
     try:
         loaded = entry_point.load()
-        if hasattr(loaded, 'primitives'):
-            for name, obj in loaded.primitives.items():
-                globals()[name] = obj
+        for primitive in loaded:
+            if issubclass(primitive, (AggregationPrimitive, TransformPrimitive)):
+                globals()[primitive.__name__] = primitive
     except Exception:
         pass