lib2to3: Support named assignment expressions (GH-12702)

There are two copies of the grammar -- the one used by Python itself as
Grammar/Grammar, and the one used by lib2to3 which has necessarily diverged at
Lib/lib2to3/Grammar.txt because it needs to support older syntax an we want it
to be reasonable stable to avoid requiring fixer rewrites.

This brings suport for syntax like `if x:= foo():` to match what the live
Python grammar does.

This should've been added at the time of the walrus operator itself, but lib2to3 being
independent is often overlooked.  So we do consider this a bugfix rather than enhancement.
(cherry picked from commit 3c3aa4516c70753de06bb142b6793d01330fcf0f)

Co-authored-by: Tim Hatch <tim@timhatch.com>

6 files changed, 27 insertions, 6 deletions

diff --git a/Lib/lib2to3/Grammar.txt b/Lib/lib2to3/Grammar.txt
index 68b7386..8ce7fd8 100644
--- a/Lib/lib2to3/Grammar.txt
+++ b/Lib/lib2to3/Grammar.txt
@@ -67,8 +67,8 @@ 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)+
@@ -91,6 +91,7 @@ testlist_safe: old_test [(',' old_test)+ [',']]
 old_test: or_test | old_lambdef
 old_lambdef: 'lambda' [varargslist] ':' old_test
 
+namedexpr_test: test [':=' test]
 test: or_test ['if' or_test 'else' test] | lambdef
 or_test: and_test ('or' and_test)*
 and_test: not_test ('and' not_test)*
@@ -111,8 +112,8 @@ atom: ('(' [yield_expr|testlist_gexp] ')' |
        '{' [dictsetmaker] '}' |
        '`' testlist1 '`' |
        NAME | NUMBER | STRING+ | '.' '.' '.')
-listmaker: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )
-testlist_gexp: (test|star_expr) ( comp_for | (',' (test|star_expr))* [','] )
+listmaker: (namedexpr_test|star_expr) ( comp_for | (',' (namedexpr_test|star_expr))* [','] )
+testlist_gexp: (namedexpr_test|star_expr) ( comp_for | (',' (namedexpr_test|star_expr))* [','] )
 lambdef: 'lambda' [varargslist] ':' test
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
 subscriptlist: subscript (',' subscript)* [',']
@@ -137,6 +138,7 @@ 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 )
diff --git a/Lib/lib2to3/pgen2/grammar.py b/Lib/lib2to3/pgen2/grammar.py
index a1da546..6a4d575 100644
--- a/Lib/lib2to3/pgen2/grammar.py
+++ b/Lib/lib2to3/pgen2/grammar.py
@@ -178,6 +178,7 @@ opmap_raw = """
 // DOUBLESLASH
 //= DOUBLESLASHEQUAL
 -> RARROW
+:= COLONEQUAL
 """
 
 opmap = {}
diff --git a/Lib/lib2to3/pgen2/token.py b/Lib/lib2to3/pgen2/token.py
index 1a67955..5f6612f 100755
--- a/Lib/lib2to3/pgen2/token.py
+++ b/Lib/lib2to3/pgen2/token.py
@@ -65,7 +65,8 @@ RARROW = 55
 AWAIT = 56
 ASYNC = 57
 ERRORTOKEN = 58
-N_TOKENS = 59
+COLONEQUAL = 59
+N_TOKENS = 60
 NT_OFFSET = 256
 #--end constants--
 
diff --git a/Lib/lib2to3/pgen2/tokenize.py b/Lib/lib2to3/pgen2/tokenize.py
index 7924ff3..0e2685d 100644
--- a/Lib/lib2to3/pgen2/tokenize.py
+++ b/Lib/lib2to3/pgen2/tokenize.py
@@ -93,7 +93,7 @@ Operator = group(r"\*\*=?", r">>=?", r"<<=?", r"<>", r"!=",
                  r"~")
 
 Bracket = '[][(){}]'
-Special = group(r'\r?\n', r'[:;.,`@]')
+Special = group(r'\r?\n', r':=', r'[:;.,`@]')
 Funny = group(Operator, Bracket, Special)
 
 PlainToken = group(Number, Funny, String, Name)
diff --git a/Lib/lib2to3/tests/test_parser.py b/Lib/lib2to3/tests/test_parser.py
index 868ada7..3a0e7f4 100644
--- a/Lib/lib2to3/tests/test_parser.py
+++ b/Lib/lib2to3/tests/test_parser.py
@@ -629,6 +629,21 @@ class TestLiterals(GrammarTest):
         self.validate(s)
 
 
+class TestNamedAssignments(GrammarTest):
+
+    def test_named_assignment_if(self):
+        driver.parse_string("if f := x(): pass\n")
+
+    def test_named_assignment_while(self):
+        driver.parse_string("while f := x(): pass\n")
+
+    def test_named_assignment_generator(self):
+        driver.parse_string("any((lastNum := num) == 1 for num in [1, 2, 3])\n")
+
+    def test_named_assignment_listcomp(self):
+        driver.parse_string("[(lastNum := num) == 1 for num in [1, 2, 3]]\n")
+
+
 class TestPickleableException(unittest.TestCase):
     def test_ParseError(self):
         err = ParseError('msg', 2, None, (1, 'context'))
diff --git a/Misc/NEWS.d/next/Library/2019-06-18-19-38-27.bpo-36541.XI8mi1.rst b/Misc/NEWS.d/next/Library/2019-06-18-19-38-27.bpo-36541.XI8mi1.rst
new file mode 100644
index 0000000..e7b9dd6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-06-18-19-38-27.bpo-36541.XI8mi1.rst
@@ -0,0 +1,2 @@
+lib2to3 now recognizes named assignment expressions (the walrus operator,
+``:=``)