Only emit a warning if validate=False.

Author: serhiy-storchaka

Lib/base64.py

@@ -78,12 +78,27 @@ def b64decode(s, altchars=None, validate=False):
     https://docs.python.org/3.11/library/binascii.html#binascii.a2b_base64
     """
     s = _bytes_from_decode_data(s)
+    badchar = None
     if altchars is not None:
         altchars = _bytes_from_decode_data(altchars)
         if len(altchars) != 2:
             raise ValueError(f'invalid altchars: {altchars!r}')
-        s = s.translate(bytes.maketrans(b'+/' + altchars, altchars + b'+/'))
-    return binascii.a2b_base64(s, strict_mode=validate)
+        if validate:
+            s = s.translate(bytes.maketrans(b'+/' + altchars, altchars + b'+/'))
+        else:
+            for b in set(b'+/') - set(altchars):
+                if b in s:
+                    badchar = b
+                    break
+            s = s.translate(bytes.maketrans(altchars, b'+/'))
+    result = binascii.a2b_base64(s, strict_mode=validate)
+    if badchar is not None:
+        import warnings
+        warnings.warn(f'invalid character {chr(badchar)!a} in base64 data '
+                      f'with altchars={altchars!r} will be discarded in '
+                      f'future Python versions',
+                      FutureWarning, stacklevel=2)
+    return result
 
 
 def standard_b64encode(s):

Lib/test/test_base64.py

@@ -301,13 +301,21 @@ def test_b64decode_invalid_chars(self):
             with self.assertRaises(binascii.Error):
                 base64.b64decode(bstr.decode('ascii'), validate=True)
 
-        # Normal alphabet characters are discarded when alternative given
-        self.assertEqual(base64.b64decode(b'++//', altchars=b'-_'), b'')
-        self.assertEqual(base64.urlsafe_b64decode(b'++//'), b'')
+        # Normal alphabet characters will be discarded when alternative given
+        with self.assertWarns(FutureWarning):
+            self.assertEqual(base64.b64decode(b'++++', altchars=b'-_'),
+                             b'\xfb\xef\xbe')
+        with self.assertWarns(FutureWarning):
+            self.assertEqual(base64.b64decode(b'////', altchars=b'-_'),
+                             b'\xff\xff\xff')
+        self.assertEqual(base64.urlsafe_b64decode(b'++++'), b'')
+        self.assertEqual(base64.urlsafe_b64decode(b'////'), b'')
         with self.assertRaises(binascii.Error):
             base64.b64decode(b'++++', altchars=b'-_', validate=True)
         with self.assertRaises(binascii.Error):
             base64.b64decode(b'////', altchars=b'-_', validate=True)
+        with self.assertRaises(binascii.Error):
+            base64.b64decode(b'+/!', altchars=b'-_')
 
     def _altchars_strategy():
         """Generate 'altchars' for base64 encoding."""