Fix lexer issue with empty string (#18)

Author: Cito

README.md

@@ -13,7 +13,7 @@ a query language for APIs created by Facebook.
 [![Code Style](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)
 
 The current version 1.0.1 of GraphQL-core-next is up-to-date with GraphQL.js version
-14.0.2. All parts of the API are covered by an extensive test suite of currently 1657
+14.0.2. All parts of the API are covered by an extensive test suite of currently 1659
 unit tests.
 
 

graphql/language/lexer.py

@@ -52,9 +52,9 @@ def __init__(
         self.kind = kind
         self.start, self.end = start, end
         self.line, self.column = line, column
-        self.prev: Optional[Token] = prev or None
+        self.prev: Optional[Token] = prev
         self.next: Optional[Token] = None
-        self.value: Optional[str] = value or None
+        self.value: Optional[str] = value
 
     def __repr__(self):
         return "<Token {} at {}-{} ({}/{})>".format(

tests/language/test_lexer.py

@@ -95,6 +95,12 @@ def updates_column_numbers_in_error_for_file_context():
             """
         )
 
+    # noinspection PyArgumentEqualDefault
+    def lexes_empty_string():
+        token = lex_one('""')
+        assert token == Token(TokenKind.STRING, 0, 2, 1, 1, None, "")
+        assert token.value == ""
+
     # noinspection PyArgumentEqualDefault
     def lexes_strings():
         assert lex_one('"simple"') == Token(

tests/language/test_parser.py

@@ -388,6 +388,12 @@ def parses_null_value():
         assert isinstance(result, NullValueNode)
         assert result.loc == (0, 4)
 
+    def parses_empty_strings():
+        result = parse_value('""')
+        assert isinstance(result, StringValueNode)
+        assert result.value == ""
+        assert result.loc == (0, 2)
+
     def parses_list_values():
         result = parse_value('[123 "abc"]')
         assert isinstance(result, ListValueNode)