Add bugfix grammar changes for Python 3.8.

libcst/_parser/conversions/expression.py

@@ -1459,7 +1459,8 @@ def convert_yield_expr(
     return WithLeadingWhitespace(yield_node, yield_token.whitespace_before)
 
 
-@with_production("yield_arg", "'from' test | testlist")
+@with_production("yield_arg", "'from' test | testlist", version="<=3.7")
+@with_production("yield_arg", "'from' test | testlist_star_expr", version=">=3.8")
 def convert_yield_arg(
     config: ParserConfig, children: typing.Sequence[typing.Any]
 ) -> typing.Any:

libcst/_parser/conversions/statement.py

@@ -262,7 +262,8 @@ def convert_expr_stmt(config: ParserConfig, children: Sequence[Any]) -> Any:
     )
 
 
-@with_production("annassign", "':' test ['=' test]", version=">=3.6")
+@with_production("annassign", "':' test ['=' test]", version=">=3.6,<3.8")
+@with_production("annassign", "':' test ['=' (yield_expr|testlist_star_expr)]", version=">=3.8")
 def convert_annassign(config: ParserConfig, children: Sequence[Any]) -> Any:
     if len(children) == 2:
         # Variable annotation only
@@ -362,7 +363,8 @@ def convert_break_stmt(config: ParserConfig, children: Sequence[Any]) -> Any:
     return WithLeadingWhitespace(Break(), name.whitespace_before)
 
 
-@with_production("return_stmt", "'return' [testlist]")
+@with_production("return_stmt", "'return' [testlist]", version="<=3.7")
+@with_production("return_stmt", "'return' [testlist_star_expr]", version=">=3.8")
 def convert_return_stmt(config: ParserConfig, children: Sequence[Any]) -> Any:
     if len(children) == 1:
         (keyword,) = children

libcst/_parser/types/config.py

@@ -83,7 +83,7 @@ class PartialParserConfig:
     #: run LibCST. For example, you can parse code as 3.7 with a CPython 3.6
     #: interpreter.
     #:
-    #: Currently, only Python 3.5, 3.6 and 3.7 syntax is supported.
+    #: Currently, only Python 3.5, 3.6, 3.7 and 3.8 syntax is supported.
     python_version: Union[str, AutoConfig] = AutoConfig.token
 
     #: A named tuple with the ``major`` and ``minor`` Python version numbers. This is
@@ -121,11 +121,12 @@ class PartialParserConfig:
             PythonVersionInfo(3, 5),
             PythonVersionInfo(3, 6),
             PythonVersionInfo(3, 7),
+            PythonVersionInfo(3, 8),
         ):
             raise ValueError(
                 "LibCST can only parse code using one of the following versions of "
-                + "Python's grammar: 3.5, 3.6, 3.7. More versions may be supported "
-                + "by future releases."
+                + "Python's grammar: 3.5, 3.6, 3.7, 3.8. More versions may be "
+                + "supported by future releases."
             )
 
         object.__setattr__(self, "parsed_python_version", parsed_python_version)

libcst/_parser/types/tests/test_config.py

@@ -33,7 +33,7 @@ class TestConfig(UnitTest):
                 "The given version is not in the right format",
             ),
             "python_version_unsupported": (
-                lambda: PartialParserConfig(python_version="3.8"),
+                lambda: PartialParserConfig(python_version="3.4"),
                 "LibCST can only parse code using one of the following versions of Python's grammar",
             ),
             "encoding": (