Parse named expressions

conftest.py

@@ -14,7 +14,7 @@
 collect_ignore = ["setup.py"]
 
 VERSIONS_2 = '2.6', '2.7'
-VERSIONS_3 = '3.3', '3.4', '3.5', '3.6', '3.7'
+VERSIONS_3 = '3.3', '3.4', '3.5', '3.6', '3.7', '3.8'
 
 
 @pytest.fixture(scope='session')
@@ -155,3 +155,9 @@ def works_ge_py3(each_version):
 def works_ge_py35(each_version):
     version_info = parse_version_string(each_version)
     return Checker(each_version, version_info >= (3, 5))
+
+
+@pytest.fixture
+def works_ge_py38(each_version):
+    version_info = parse_version_string(each_version)
+    return Checker(each_version, version_info >= (3, 8))

parso/python/grammar38.txt

@@ -69,8 +69,8 @@ assert_stmt: 'assert' test [',' test]
 
 compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt
 async_stmt: 'async' (funcdef | with_stmt | for_stmt)
-if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
-while_stmt: 'while' test ':' suite ['else' ':' suite]
+if_stmt: 'if' namedexpr_test ':' suite ('elif' namedexpr_test ':' suite)* ['else' ':' suite]
+while_stmt: 'while' namedexpr_test ':' suite ['else' ':' suite]
 for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]
 try_stmt: ('try' ':' suite
            ((except_clause ':' suite)+
@@ -83,6 +83,7 @@ with_item: test ['as' expr]
 except_clause: 'except' [test ['as' NAME]]
 suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT
 
+namedexpr_test: test [':=' test]
 test: or_test ['if' or_test 'else' test] | lambdef
 test_nocond: or_test | lambdef_nocond
 lambdef: 'lambda' [varargslist] ':' test
@@ -108,7 +109,7 @@ atom: ('(' [yield_expr|testlist_comp] ')' |
        '[' [testlist_comp] ']' |
        '{' [dictorsetmaker] '}' |
        NAME | NUMBER | strings | '...' | 'None' | 'True' | 'False')
-testlist_comp: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )
+testlist_comp: (namedexpr_test|star_expr) ( comp_for | (',' (namedexpr_test|star_expr))* [','] )
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
 subscriptlist: subscript (',' subscript)* [',']
 subscript: test | [test] ':' [test] [sliceop]
@@ -134,6 +135,7 @@ arglist: argument (',' argument)*  [',']
 # multiple (test comp_for) arguments are blocked; keyword unpackings
 # that precede iterable unpackings are blocked; etc.
 argument: ( test [comp_for] |
+            test ':=' test |
             test '=' test |
             '**' test |
             '*' test )

parso/python/tokenize.py

@@ -190,9 +190,13 @@ def _create_token_collection(version_info):
 
     Bracket = '[][(){}]'
 
-    special_args = [r'\r\n?', r'\n', r'[:;.,@]']
+    special_args = [r'\r\n?', r'\n', r'[;.,@]']
     if version_info >= (3, 0):
         special_args.insert(0, r'\.\.\.')
+    if version_info >= (3, 8):
+        special_args.insert(0, ":=?")
+    else:
+        special_args.insert(0, ":")
     Special = group(*special_args)
 
     Funny = group(Operator, Bracket, Special)

test/test_parser.py

@@ -189,3 +189,7 @@ def check(node):
                 check(child)
 
     check(parse("if foo:\n bar", version=each_version))
+
+
+def test_named_expression(works_ge_py38):
+    works_ge_py38.parse("(a := 1, a + 1)")