Add more comments nd tests.

serhiy-storchaka · Jan 14, 2026 · Dec 26, 2025 · Dec 27, 2025 · Dec 27, 2025 · Dec 27, 2025
commit 5062ae55c286429e68d5b1271bdab5c9472d6476
diff --git a/Lib/test/test_base64.py b/Lib/test/test_base64.py
@@ -2,6 +2,7 @@
 import base64
 import binascii
 import string
+import sys
 import os
 from array import array
 from test.support import cpython_only
@@ -178,6 +179,18 @@ def test_b64encode(self):
         eq(base64.b64encode(b"www.python.org", wrapcol=76), b'd3d3LnB5dGhvbi5vcmc=')
         eq(base64.b64encode(b"www.python.org", wrapcol=1),
            b'd\n3\nd\n3\nL\nn\nB\n5\nd\nG\nh\nv\nb\ni\n5\nv\nc\nm\nc\n=')
+        eq(base64.b64encode(b"www.python.org", wrapcol=sys.maxsize),
+           b'd3d3LnB5dGhvbi5vcmc=')
+        eq(base64.b64encode(b"www.python.org", wrapcol=sys.maxsize*2),
+           b'd3d3LnB5dGhvbi5vcmc=')
+        with self.assertRaises(OverflowError):
+            base64.b64encode(b"www.python.org", wrapcol=2**1000)
+        with self.assertRaises(ValueError):
+            base64.b64encode(b"www.python.org", wrapcol=-8)
+        with self.assertRaises(TypeError):
+            base64.b64encode(b"www.python.org", wrapcol=8.0)
+        with self.assertRaises(TypeError):
+            base64.b64encode(b"www.python.org", wrapcol='8')
         eq(base64.b64encode(b"", wrapcol=0), b'')
         eq(base64.b64encode(b"", wrapcol=8), b'')
 

diff --git a/Lib/test/test_binascii.py b/Lib/test/test_binascii.py
@@ -4,6 +4,7 @@
 import binascii
 import array
 import re
+import sys
 from test.support import bigmemtest, _1G, _4G
 from test.support.hypothesis_helper import hypothesis
 
@@ -498,6 +499,18 @@ def test_b2a_base64_wrapcol(self):
                          b'd3d3LnB5\ndGhvbi5v\ncmc=')
         self.assertEqual(binascii.b2a_base64(b, wrapcol=1),
             b'd\n3\nd\n3\nL\nn\nB\n5\nd\nG\nh\nv\nb\ni\n5\nv\nc\nm\nc\n=\n')
+        self.assertEqual(binascii.b2a_base64(b, wrapcol=sys.maxsize),
+                         b'd3d3LnB5dGhvbi5vcmc=\n')
+        self.assertEqual(binascii.b2a_base64(b, wrapcol=sys.maxsize*2),
+                         b'd3d3LnB5dGhvbi5vcmc=\n')
+        with self.assertRaises(OverflowError):
+            binascii.b2a_base64(b, wrapcol=2**1000)
+        with self.assertRaises(ValueError):
+            binascii.b2a_base64(b, wrapcol=-8)
+        with self.assertRaises(TypeError):
+            binascii.b2a_base64(b, wrapcol=8.0)
+        with self.assertRaises(TypeError):
+            binascii.b2a_base64(b, wrapcol='8')
         b = self.type2test(b'')
         self.assertEqual(binascii.b2a_base64(b), b'\n')
         self.assertEqual(binascii.b2a_base64(b, wrapcol=0), b'\n')

diff --git a/Modules/binascii.c b/Modules/binascii.c
@@ -189,6 +189,10 @@ ascii_buffer_converter(PyObject *arg, Py_buffer *buf)
     return Py_CLEANUP_SUPPORTED;
 }
 
+/* The function inserts '\n' each width characters, moving the data right.
+ * It assumes that we allocated enough space for all of the newlines in data.
+ * Returns the size of the data including the newlines.
+ */
 static Py_ssize_t
 wraplines(unsigned char *data, Py_ssize_t size, size_t width)
 {
@@ -564,12 +568,18 @@ binascii_b2a_base64_impl(PyObject *module, Py_buffer *data, size_t wrapcol,
 
     assert(bin_len >= 0);
 
+    /* Each group of 3 bytes (rounded up) gets encoded as 4 characters,
+     * not counting newlines.
+     * Note that 'b' gets encoded as 'Yg==' (1 in, 4 out).
+     *
+     * Use unsigned integer arithmetic to avoid signed integer overflow.
+     */
     size_t out_len = ((size_t)bin_len + 2u) / 3u * 4u;
     if (out_len > PY_SSIZE_T_MAX) {
         goto toolong;
     }
     if (wrapcol && out_len) {
-        out_len += (out_len - 1) / wrapcol;
+        out_len += (out_len - 1u) / wrapcol;
         if (out_len > PY_SSIZE_T_MAX) {
             goto toolong;
         }