Use singular argument when expected arity is 1

Before:

invalid-arity: Expected 1 arguments for function length(), received 2

Now:

invalid-arity: Expected 1 argument for function length(), received 2

Author: jamesls

jmespath/exceptions.py

@@ -71,19 +71,29 @@ def __init__(self, expected, actual, name):
         self.expression = None
 
     def __str__(self):
-        return ("Expected %s arguments for function %s(), "
-                "received %s" % (self.expected_arity,
-                                 self.function_name,
-                                 self.actual_arity))
+        return ("Expected %s %s for function %s(), "
+                "received %s" % (
+                    self.expected_arity,
+                    self._pluralize('argument', self.expected_arity),
+                    self.function_name,
+                    self.actual_arity))
+
+    def _pluralize(self, word, count):
+        if count == 1:
+            return word
+        else:
+            return word + 's'
 
 
 @with_str_method
 class VariadictArityError(ArityError):
     def __str__(self):
-        return ("Expected at least %s arguments for function %s, "
-                "received %s" % (self.expected_arity,
-                                 self.function_name,
-                                 self.actual_arity))
+        return ("Expected at least %s %s for function %s(), "
+                "received %s" % (
+                    self.expected_arity,
+                    self._pluralize('argument', self.expected_arity),
+                    self.function_name,
+                    self.actual_arity))
 
 
 @with_str_method

tests/test_functions.py

@@ -4,7 +4,7 @@
 import json
 
 import jmespath
-from jmespath.exceptions import JMESPathTypeError
+from jmespath import exceptions
 
 
 class TestFunctions(unittest.TestCase):
@@ -19,7 +19,7 @@ def test_can_max_datetimes(self):
         self.assertEqual(json.loads(result), str(data[-1]))
 
     def test_type_error_messages(self):
-        with self.assertRaises(JMESPathTypeError) as e:
+        with self.assertRaises(exceptions.JMESPathTypeError) as e:
             jmespath.search('length(@)', 2)
         exception = e.exception
         # 1. Function name should be in error message
@@ -31,3 +31,27 @@ def test_type_error_messages(self):
                       str(exception))
         # 4. Mention the actual type.
         self.assertIn('received: "number"', str(exception))
+
+    def test_singular_in_error_message(self):
+        with self.assertRaises(exceptions.ArityError) as e:
+            jmespath.search('length(@, @)', [0, 1])
+        exception = e.exception
+        self.assertEqual(
+            str(exception),
+            'Expected 1 argument for function length(), received 2')
+
+    def test_error_message_is_pluralized(self):
+        with self.assertRaises(exceptions.ArityError) as e:
+            jmespath.search('sort_by(@)', [0, 1])
+        exception = e.exception
+        self.assertEqual(
+            str(exception),
+            'Expected 2 arguments for function sort_by(), received 1')
+
+    def test_variadic_is_pluralized(self):
+        with self.assertRaises(exceptions.VariadictArityError) as e:
+            jmespath.search('not_null()', 'foo')
+        exception = e.exception
+        self.assertEqual(
+            str(exception),
+            'Expected at least 1 argument for function not_null(), received 0')