add NullValue ast node

- addresses graphql-python#118
- initial implementation by @yen223 in PR graphql-python#119

Author: jaemk

graphql/language/ast.py

@@ -534,6 +534,27 @@ def __hash__(self):
         return id(self)
 
 
+class NullValue(Value):
+    __slots__ = ('loc', 'value')
+    _fields = ('value',)
+
+    def __init__(self, value=None, loc=None):
+        self.value = None
+        self.loc = loc
+
+    def __eq__(self, other):
+        return isinstance(other, NullValue)
+
+    def __repr__(self):
+        return 'NullValue'
+
+    def __copy__(self):
+        return type(self)(self.value, self.loc)
+
+    def __hash__(self):
+        return id(self)
+
+
 class EnumValue(Value):
     __slots__ = ('loc', 'value',)
     _fields = ('value',)

graphql/language/parser.py

@@ -414,13 +414,14 @@ def parse_value_literal(parser, is_const):
         return ast.StringValue(value=token.value, loc=loc(parser, token.start))
 
     elif token.kind == TokenKind.NAME:
+        advance(parser)
         if token.value in ('true', 'false'):
-            advance(parser)
             return ast.BooleanValue(value=token.value == 'true', loc=loc(parser, token.start))
 
-        if token.value != 'null':
-            advance(parser)
-            return ast.EnumValue(value=token.value, loc=loc(parser, token.start))
+        if token.value == 'null':
+            return ast.NullValue(loc=loc(parser, token.start))
+
+        return ast.EnumValue(value=token.value, loc=loc(parser, token.start))
 
     elif token.kind == TokenKind.DOLLAR:
         if not is_const:

graphql/language/printer.py

@@ -81,6 +81,9 @@ def leave_StringValue(self, node, *args):
     def leave_BooleanValue(self, node, *args):
         return json.dumps(node.value)
 
+    def leave_NullValue(self, node, *args):
+        return 'null'
+
     def leave_EnumValue(self, node, *args):
         return node.value
 

graphql/language/visitor_meta.py

@@ -19,6 +19,7 @@
     ast.FloatValue: (),
     ast.StringValue: (),
     ast.BooleanValue: (),
+    ast.NullValue: (),
     ast.EnumValue: (),
     ast.ListValue: ('values',),
     ast.ObjectValue: ('fields',),