Issue #26331: Implement the parsing part of PEP 515.

Thanks to Georg Brandl for the patch.

Lib/test/test_float.py

@@ -1,4 +1,3 @@
-
 import fractions
 import operator
 import os
@@ -9,6 +8,8 @@ import time
 import unittest
 
 from test import support
+from test.test_grammar import (VALID_UNDERSCORE_LITERALS,
+                               INVALID_UNDERSCORE_LITERALS)
 from math import isinf, isnan, copysign, ldexp
 
 INF = float("inf")
@@ -60,6 +61,27 @@ class GeneralFloatCases(unittest.TestCase):
         float(b'.' + b'1'*1000)
         float('.' + '1'*1000)
 
+    def test_underscores(self):
+        for lit in VALID_UNDERSCORE_LITERALS:
+            if not any(ch in lit for ch in 'jJxXoObB'):
+                self.assertEqual(float(lit), eval(lit))
+                self.assertEqual(float(lit), float(lit.replace('_', '')))
+        for lit in INVALID_UNDERSCORE_LITERALS:
+            if lit in ('0_7', '09_99'):  # octals are not recognized here
+                continue
+            if not any(ch in lit for ch in 'jJxXoObB'):
+                self.assertRaises(ValueError, float, lit)
+        # Additional test cases; nan and inf are never valid as literals,
+        # only in the float() constructor, but we don't allow underscores
+        # in or around them.
+        self.assertRaises(ValueError, float, '_NaN')
+        self.assertRaises(ValueError, float, 'Na_N')
+        self.assertRaises(ValueError, float, 'IN_F')
+        self.assertRaises(ValueError, float, '-_INF')
+        self.assertRaises(ValueError, float, '-INF_')
+        # Check that we handle bytes values correctly.
+        self.assertRaises(ValueError, float, b'0_.\xff9')
+
     def test_non_numeric_input_types(self):
         # Test possible non-numeric types for the argument x, including
         # subclasses of the explicitly documented accepted types.