Moved type check on keys to ensure that python2.7 and python3.3 have the same errors.

theano/compile/function.py

@@ -189,12 +189,14 @@ def function(inputs, outputs=None, mode=None, updates=None, givens=None,
     if isinstance(outputs, dict):
         output_items = outputs.items()
 
+        for item_pair in output_items: 
+            assert isinstance(item_pair[0], basestring)
+
         output_items_sorted = sorted(output_items)
 
         output_keys = []
         outputs = []
         for pair in output_items_sorted: 
-            assert isinstance(pair[0], basestring)
             output_keys.append(pair[0])
             outputs.append(pair[1])
 

theano/tests/test_dictionary_output.py

@@ -135,3 +135,25 @@ class dictionary_output_checker(unittest.TestCase):
         assert result[0] == 5.0
         assert result[1] == 10.0
         assert result[2] == 15.0
+
+    def test_key_string_requirement(self):
+        '''
+        Tests that an exception is thrown if a non-string key is used in
+        the outputs dictionary.
+        '''
+        x = T.scalar('x')
+        try:
+            theano.function([x], outputs={1.0: x})
+            raise Exception("Did not throw exception with 1.0 as only key")
+        except AssertionError:
+            pass
+        try:
+            theano.function([x], outputs={1.0: x, "a": x**2})
+            raise Exception("Did not throw exception with 1.0 as one key")
+        except AssertionError:
+            pass
+        try:
+            theano.function([x], outputs={(1, "b"): x, 1.0: x**2})
+            raise Exception("Did not throw exception with tuple as key")
+        except AssertionError:
+            pass