[3.8] bpo-32912: Revert SyntaxWarning on invalid escape sequences (GH-15142)

* bpo-32912: Revert warnings for invalid escape sequences.

DeprecationWarning will continue to be emitted for invalid escape sequences in string and bytes literals in 3.8 just as it did in 3.7.

SyntaxWarning may be emitted in the future. But per mailing list discussion, we don't yet know when because we haven't settled on how to do so in a non-disruptive manner.

Doc/reference/lexical_analysis.rst

@@ -594,11 +594,9 @@ escape sequences only recognized in string literals fall into the category of
 unrecognized escapes for bytes literals.
 
    .. versionchanged:: 3.6
-      Unrecognized escape sequences produce a :exc:`DeprecationWarning`.
-
-   .. versionchanged:: 3.8
-      Unrecognized escape sequences produce a :exc:`SyntaxWarning`.  In
-      some future version of Python they will be a :exc:`SyntaxError`.
+      Unrecognized escape sequences produce a :exc:`DeprecationWarning`.  In
+      a future Python version they will be a :exc:`SyntaxWarning` and
+      eventually a :exc:`SyntaxError`.
 
 Even in a raw literal, quotes can be escaped with a backslash, but the
 backslash remains in the result; for example, ``r"\""`` is a valid string

Doc/whatsnew/3.8.rst

@@ -414,11 +414,6 @@ Other Language Changes
   and :keyword:`return` statements.
   (Contributed by David Cuthbert and Jordan Chapman in :issue:`32117`.)
 
-* A backslash-character pair that is not a valid escape sequence generates
-  a :exc:`DeprecationWarning` since Python 3.6. In Python 3.8 it generates
-  a :exc:`SyntaxWarning` instead.
-  (Contributed by Serhiy Storchaka in :issue:`32912`.)
-
 * The compiler now produces a :exc:`SyntaxWarning` in some cases when a comma
   is missed before tuple or list.  For example::
 

Lib/test/test_fstring.py

@@ -649,7 +649,7 @@ non-important content
         self.assertEqual(f'2\x203', '2 3')
         self.assertEqual(f'\x203', ' 3')
 
-        with self.assertWarns(SyntaxWarning):  # invalid escape sequence
+        with self.assertWarns(DeprecationWarning):  # invalid escape sequence
             value = eval(r"f'\{6*7}'")
         self.assertEqual(value, '\\42')
         self.assertEqual(f'\\{6*7}', '\\42')

Lib/test/test_string_literals.py

@@ -32,6 +32,7 @@ import sys
 import shutil
 import tempfile
 import unittest
+import warnings
 
 
 TEMPLATE = r"""# coding: %s
@@ -110,10 +111,24 @@ class TestLiterals(unittest.TestCase):
         for b in range(1, 128):
             if b in b"""\n\r"'01234567NU\\abfnrtuvx""":
                 continue
-            with self.assertWarns(SyntaxWarning):
+            with self.assertWarns(DeprecationWarning):
                 self.assertEqual(eval(r"'\%c'" % b), '\\' + chr(b))
 
-        self.check_syntax_warning("'''\n\\z'''")
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always', category=DeprecationWarning)
+            eval("'''\n\\z'''")
+        self.assertEqual(len(w), 1)
+        self.assertEqual(w[0].filename, '<string>')
+        self.assertEqual(w[0].lineno, 1)
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('error', category=DeprecationWarning)
+            with self.assertRaises(SyntaxError) as cm:
+                eval("'''\n\\z'''")
+            exc = cm.exception
+        self.assertEqual(w, [])
+        self.assertEqual(exc.filename, '<string>')
+        self.assertEqual(exc.lineno, 1)
 
     def test_eval_str_raw(self):
         self.assertEqual(eval(""" r'x' """), 'x')
@@ -145,10 +160,24 @@ class TestLiterals(unittest.TestCase):
         for b in range(1, 128):
             if b in b"""\n\r"'01234567\\abfnrtvx""":
                 continue
-            with self.assertWarns(SyntaxWarning):
+            with self.assertWarns(DeprecationWarning):
                 self.assertEqual(eval(r"b'\%c'" % b), b'\\' + bytes([b]))
 
-        self.check_syntax_warning("b'''\n\\z'''")
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always', category=DeprecationWarning)
+            eval("b'''\n\\z'''")
+        self.assertEqual(len(w), 1)
+        self.assertEqual(w[0].filename, '<string>')
+        self.assertEqual(w[0].lineno, 1)
+
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('error', category=DeprecationWarning)
+            with self.assertRaises(SyntaxError) as cm:
+                eval("b'''\n\\z'''")
+            exc = cm.exception
+        self.assertEqual(w, [])
+        self.assertEqual(exc.filename, '<string>')
+        self.assertEqual(exc.lineno, 1)
 
     def test_eval_bytes_raw(self):
         self.assertEqual(eval(""" br'x' """), b'x')

Misc/NEWS.d/next/Core and Builtins/2019-08-06-14-03-59.bpo-32912.UDwSMJ.rst

@@ -0,0 +1,3 @@
+Reverted :issue:`32912`: emitting :exc:`SyntaxWarning` instead of
+:exc:`DeprecationWarning` for invalid escape sequences in string and bytes
+literals.

Python/ast.c

@@ -4674,12 +4674,12 @@ warn_invalid_escape_sequence(struct compiling *c, const node *n,
     if (msg == NULL) {
         return -1;
     }
-    if (PyErr_WarnExplicitObject(PyExc_SyntaxWarning, msg,
+    if (PyErr_WarnExplicitObject(PyExc_DeprecationWarning, msg,
                                    c->c_filename, LINENO(n),
                                    NULL, NULL) < 0)
     {
-        if (PyErr_ExceptionMatches(PyExc_SyntaxWarning)) {
-            /* Replace the SyntaxWarning exception with a SyntaxError
+        if (PyErr_ExceptionMatches(PyExc_DeprecationWarning)) {
+            /* Replace the DeprecationWarning exception with a SyntaxError
                to get a more accurate error report */
             PyErr_Clear();
             ast_error(c, n, "%U", msg);