supports functions with no arguments

jmespath/functions.py

@@ -88,9 +88,14 @@ def call_function(self, function_name, resolved_args, *args, **kwargs):
         return function(self, *resolved_args)
 
     def _validate_arguments(self, args, signature, function_name):
-        required_arguments_count = len([param for param in signature if not param.get('optional') or not param['optional']])
-        optional_arguments_count = len([param for param in signature if param.get('optional') and param['optional']])
+
+        if len(signature) == 0:
+            return self._type_check(args, signature, function_name)
+
+        required_arguments_count = len([param for param in signature if param and (not param.get('optional') or not param['optional'])])
+        optional_arguments_count = len([param for param in signature if param and param.get('optional') and param['optional']])
         has_variadic = signature[-1].get('variadic') if signature != None else False
+
         if has_variadic:
             if len(args) < len(signature):
                 raise exceptions.VariadictArityError(

tests/test_signatures.py

@@ -40,6 +40,10 @@ def test_signature_with_variadic_arguments(self):
 			self._functions._validate_arguments(['string'], signature, function_name)
 		)
 
+	def test_signature_with_empty_argument(self):
+		(function_name, signature) = self._make_test("_function_with_empty_argument")
+		self._functions._validate_arguments([], signature, function_name)
+
 	def test_signature_with_no_arguments(self):
 		(function_name, signature) = self._make_test("_function_with_no_arguments")
 		self._functions._validate_arguments([], signature, function_name)
@@ -69,6 +73,10 @@ def _function_with_optional_arguments(self, arg1, opt1, opt2):
 		return None
 
 	@functions.signature({})
+	def _function_with_empty_argument(self):
+		return None
+
+	@functions.signature()
 	def _function_with_no_arguments(self):
 		return None