bpo-32912: Revert SyntaxWarning on invalid escape sequences. (GH-15195)

DeprecationWarning will continue to be emitted for invalid escape
sequences in string and bytes literals 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.

(Applies 4c5b6bac2408f879231c7cd38d67657dd4804e7c to the master branch).
(This is https://github.com/python/cpython/pull/15142 for master/3.9)


https://bugs.python.org/issue32912



Automerge-Triggered-By: @gpshead

6 files changed, 43 insertions, 18 deletions

diff --git a/Doc/reference/lexical_analysis.rst b/Doc/reference/lexical_analysis.rst
index cc1b2f57a7..7e1e17edb2 100644
--- a/Doc/reference/lexical_analysis.rst
+++ b/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
diff --git a/Doc/whatsnew/3.8.rst b/Doc/whatsnew/3.8.rst
index d8062e772f..82da10cc3b 100644
--- a/Doc/whatsnew/3.8.rst
+++ b/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::
 
diff --git a/Lib/test/test_fstring.py b/Lib/test/test_fstring.py
index fb761441fc..49663923e7 100644
--- a/Lib/test/test_fstring.py
+++ b/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')
diff --git a/Lib/test/test_string_literals.py b/Lib/test/test_string_literals.py
index 5961d591c4..0cea2edc32 100644
--- a/Lib/test/test_string_literals.py
+++ b/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')
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-08-06-14-03-59.bpo-32912.UDwSMJ.rst b/Misc/NEWS.d/next/Core and Builtins/2019-08-06-14-03-59.bpo-32912.UDwSMJ.rst
new file mode 100644
index 0000000000..e18d8adfbe
--- /dev/null
+++ b/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.
diff --git a/Python/ast.c b/Python/ast.c
index 976be70d4d..8b3dbead2f 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -4671,12 +4671,12 @@ 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);