Merge branch 'lostinthefrost-develop' into develop

PR #151

* lostinthefrost-develop:
  bugfix: min_by and max_by given an empty array

Author: jamesls

jmespath/functions.py

@@ -331,14 +331,20 @@ def _func_min_by(self, array, expref):
         keyfunc = self._create_key_func(expref,
                                         ['number', 'string'],
                                         'min_by')
-        return min(array, key=keyfunc)
+        if array:
+            return min(array, key=keyfunc)
+        else:
+            return None
 
     @signature({'types': ['array']}, {'types': ['expref']})
     def _func_max_by(self, array, expref):
         keyfunc = self._create_key_func(expref,
                                         ['number', 'string'],
-                                        'min_by')
-        return max(array, key=keyfunc)
+                                        'max_by')
+        if array:
+            return max(array, key=keyfunc)
+        else:
+            return None
 
     def _create_key_func(self, expref, allowed_types, function_name):
         def keyfunc(x):

tests/compliance/functions.json

@@ -701,6 +701,10 @@
       "expression": "max_by(people, &to_number(age_str))",
       "result": {"age": 50, "age_str": "50", "bool": false, "name": "d"}
     },
+    {
+      "expression": "max_by(`[]`, &age)",
+      "result": null
+    },
     {
       "expression": "min_by(people, &age)",
       "result": {"age": 10, "age_str": "10", "bool": true, "name": 3}
@@ -720,6 +724,10 @@
     {
       "expression": "min_by(people, &to_number(age_str))",
       "result": {"age": 10, "age_str": "10", "bool": true, "name": 3}
+    },
+    {
+      "expression": "min_by(`[]`, &age)",
+      "result": null
     }
   ]
 }, {