From adb881257c4ea8d36263ed574a5fd9b832f3eee8 Mon Sep 17 00:00:00 2001
From: "Eric V. Smith" <eric@trueblade.com>
Date: Mon, 24 Jan 2022 21:20:57 -0500
Subject: [PATCH 1/2] bpo-46503: Prevent an assert from firing.  Also fix one
 nearby tiny PEP-7 nit.

---
 Lib/test/test_fstring.py |  4 ++++
 Parser/string_parser.c   | 16 ++++++++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index bd1ca943c7c094..d0b1ade15137ba 100644
--- a/Lib/test/test_fstring.py
+++ b/Lib/test/test_fstring.py
@@ -746,12 +746,16 @@ def test_misformed_unicode_character_name(self):
         # differently inside f-strings.
         self.assertAllRaise(SyntaxError, r"\(unicode error\) 'unicodeescape' codec can't decode bytes in position .*: malformed \\N character escape",
                             [r"f'\N'",
+                             r"f'\N '",
+                             r"f'\N  '",  # See bpo-46503.
                              r"f'\N{'",
                              r"f'\N{GREEK CAPITAL LETTER DELTA'",
 
                              # Here are the non-f-string versions,
                              #  which should give the same errors.
                              r"'\N'",
+                             r"'\N '",
+                             r"'\N  '",
                              r"'\N{'",
                              r"'\N{GREEK CAPITAL LETTER DELTA'",
                              ])
diff --git a/Parser/string_parser.c b/Parser/string_parser.c
index 57d9b9ed3fdbbd..0b5e30ba2ca6a4 100644
--- a/Parser/string_parser.c
+++ b/Parser/string_parser.c
@@ -442,12 +442,23 @@ fstring_find_literal(Parser *p, const char **str, const char *end, int raw,
         if (!raw && ch == '\\' && s < end) {
             ch = *s++;
             if (ch == 'N') {
+                /* We need to look at and skip matching braces for "\N{name}"
+                   sequences because otherwise we'll think the opening '{'
+                   starts an expression, which is not the case with "\N".
+                   Keep looking for either a matched '{' '}' pair, or the end
+                   of the string. */
+
                 if (s < end && *s++ == '{') {
                     while (s < end && *s++ != '}') {
                     }
                     continue;
                 }
-                break;
+
+                /* This is an invalid "\N" sequence, since it's a "\N" not
+                   followed by a "{".  Just keep parsing this literal.  This
+                   error will be caught later by
+                   decode_unicode_with_escapes(). */
+                continue;
             }
             if (ch == '{' && warn_invalid_escape_sequence(p, ch, t) < 0) {
                 return -1;
@@ -491,7 +502,8 @@ fstring_find_literal(Parser *p, const char **str, const char *end, int raw,
             *literal = PyUnicode_DecodeUTF8Stateful(literal_start,
                                                     s - literal_start,
                                                     NULL, NULL);
-        } else {
+        }
+        else {
             *literal = decode_unicode_with_escapes(p, literal_start,
                                                    s - literal_start, t);
         }

From 0607094cde1c0c18bf7ef483c434ffaf7472841e Mon Sep 17 00:00:00 2001
From: "Eric V. Smith" <eric@trueblade.com>
Date: Mon, 24 Jan 2022 21:24:56 -0500
Subject: [PATCH 2/2] Added blurb.

---
 .../Core and Builtins/2022-01-24-21-24-41.bpo-46503.4UrPsE.rst   | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-01-24-21-24-41.bpo-46503.4UrPsE.rst

diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-01-24-21-24-41.bpo-46503.4UrPsE.rst b/Misc/NEWS.d/next/Core and Builtins/2022-01-24-21-24-41.bpo-46503.4UrPsE.rst
new file mode 100644
index 00000000000000..e48028d72ca8e9
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-01-24-21-24-41.bpo-46503.4UrPsE.rst	
@@ -0,0 +1 @@
+Fix an assert when parsing some invalid \N escape sequences in f-strings.