Address commit review.

Author: serhiy-storchaka

Lib/test/test_binascii.py

@@ -240,17 +240,17 @@ def assertNonBase64Data(data, expected, ignorechars):
 
     def test_base64_excess_data(self):
         # Test excess data exceptions
-        def assertExcessData(data, non_strict_expected):
+        def assertExcessData(data, expected):
             assert_regex = r'(?i)Excess data'
             data = self.type2test(data)
             with self.assertRaisesRegex(binascii.Error, assert_regex):
                 binascii.a2b_base64(data, strict_mode=True)
             self.assertEqual(binascii.a2b_base64(data, strict_mode=False),
-                             non_strict_expected)
+                             expected)
             self.assertEqual(binascii.a2b_base64(data, strict_mode=True,
                                                  ignorechars=b'='),
-                             non_strict_expected)
-            self.assertEqual(binascii.a2b_base64(data), non_strict_expected)
+                             expected)
+            self.assertEqual(binascii.a2b_base64(data), expected)
 
         assertExcessData(b'ab==c=', b'i\xb7')
         assertExcessData(b'ab==cd', b'i\xb7\x1d')

Misc/NEWS.d/next/Library/2026-02-26-20-13-16.gh-issue-145264.4pggX_.rst

@@ -1,4 +1,4 @@
 Base64 decoder (see :func:`binascii.a2b_base64`, :func:`base64.b64decode`, etc) no
 longer ignores excess data after the first padded quad in non-strict
-(default) mode.  Instead, in conformance with :rfc:`4648`, it ignores
+(default) mode.  Instead, in conformance with :rfc:`4648`, section 3.3, it now ignores
 the pad character, "=", if it is present before the end of the encoded data.

Modules/binascii.c

@@ -643,6 +643,10 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
             if (quad_pos >= 2 && quad_pos + pads <= 4) {
                 continue;
             }
+            // See RFC 4648, section-3.3: "specifications MAY ignore the
+            // pad character, "=", treating it as non-alphabet data, if
+            // it is present before the end of the encoded data" and
+            // "the excess pad characters MAY also be ignored."
             if (!strict_mode || ignorechar(BASE64_PAD, ignorechars, ignorecache)) {
                 continue;
             }
@@ -662,6 +666,7 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
 
         unsigned char v = table_a2b_base64[this_ch];
         if (v >= 64) {
+            // See RFC 4648, section-3.3.
             if (strict_mode && !ignorechar(this_ch, ignorechars, ignorecache)) {
                 state = get_binascii_state(module);
                 if (state) {
@@ -672,7 +677,8 @@ binascii_a2b_base64_impl(PyObject *module, Py_buffer *data, int strict_mode,
             continue;
         }
 
-        // Characters that are not '=', in the middle of the padding, are not allowed
+        // Characters that are not '=', in the middle of the padding, are
+        // not allowed (except when they are). See RFC 4648, section-3.3.
         if (pads && strict_mode &&
             !ignorechar(BASE64_PAD, ignorechars, ignorecache))
         {