allow the keyword else immediately after (no space) an integer (closes #21642)

Lib/test/test_grammar.py

@@ -75,6 +75,12 @@ class TokenTests(unittest.TestCase):
         x = .3e14
         x = 3.1e4
 
+    def test_float_exponent_tokenization(self):
+        # See issue 21642.
+        self.assertEqual(1 if 1else 0, 1)
+        self.assertEqual(1 if 0else 0, 0)
+        self.assertRaises(SyntaxError, eval, "0 if 1Else 0")
+
     def testStringLiterals(self):
         x = ''; y = ""; self.assertTrue(len(x) == 0 and x == y)
         x = '\''; y = "'"; self.assertTrue(len(x) == 1 and x == y and ord(x) == 39)

Misc/NEWS

@@ -13,6 +13,10 @@ Core and Builtins
 - Issue #19656: Running Python with the -3 option now also warns about
   non-ascii bytes literals.
 
+- Issue #21642: If the conditional if-else expression, allow an integer written
+  with no space between itself and the ``else`` keyword (e.g. ``True if 42else
+  False``) to be valid syntax.
+
 - Issue #21523: Fix over-pessimistic computation of the stack effect of
   some opcodes in the compiler.  This also fixes a quadratic compilation
   time issue noticeable when compiling code with a large number of "and"

Parser/tokenizer.c

@@ -1500,16 +1500,25 @@ tok_get(register struct tok_state *tok, char **p_start, char **p_end)
                     } while (isdigit(c));
                 }
                 if (c == 'e' || c == 'E') {
+                    int e;
                   exponent:
+                    e = c;
                     /* Exponent part */
                     c = tok_nextc(tok);
-                    if (c == '+' || c == '-')
+                    if (c == '+' || c == '-') {
                         c = tok_nextc(tok);
                         if (!isdigit(c)) {
                             tok->done = E_TOKEN;
                             tok_backup(tok, c);
                             return ERRORTOKEN;
                         }
+                    } else if (!isdigit(c)) {
+                        tok_backup(tok, c);
+                        tok_backup(tok, e);
+                        *p_start = tok->start;
+                        *p_end = tok->cur;
+                        return NUMBER;
+                    }
                     do {
                         c = tok_nextc(tok);
                     } while (isdigit(c));