bpo-30682: Removed a too-strict assertion that failed for certain f-strings. (#2232)

This caused a segfault on eval("f'\\\n'") and eval("f'\\\r'") in debug build.

Lib/test/test_fstring.py

@@ -780,5 +780,11 @@ f'{a * x()}'"""
         self.assertEqual(f'{d["foo"]}', 'bar')
         self.assertEqual(f"{d['foo']}", 'bar')
 
+    def test_backslash_char(self):
+        # Check eval of a backslash followed by a control char.
+        # See bpo-30682: this used to raise an assert in pydebug mode.
+        self.assertEqual(eval('f"\\\n"'), '')
+        self.assertEqual(eval('f"\\\r"'), '')
+
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-30682: Removed a too-strict assertion that failed for certain f-strings,
+  such as eval("f'\\\n'") and eval("f'\\\r'").
+  
 - bpo-30501: The compiler now produces more optimal code for complex condition
   expressions in the "if", "while" and "assert" statement, the "if" expression,
   and generator expressions and comprehensions.

Python/ast.c

@@ -4914,6 +4914,8 @@ FstringParser_ConcatFstring(FstringParser *state, const char **str,
             /* Do nothing. Just leave last_str alone (and possibly
                NULL). */
         } else if (!state->last_str) {
+            /*  Note that the literal can be zero length, if the
+                input string is "\\\n" or "\\\r", among others. */
             state->last_str = literal;
             literal = NULL;
         } else {
@@ -4923,8 +4925,6 @@ FstringParser_ConcatFstring(FstringParser *state, const char **str,
                 return -1;
             literal = NULL;
         }
-        assert(!state->last_str ||
-               PyUnicode_GET_LENGTH(state->last_str) != 0);
 
         /* We've dealt with the literal now. It can't be leaked on further
            errors. */