commit 1518e8713d1e372284c653f254fb29cecc66d764
author Benjamin Peterson <benjamin@python.org> Wed Nov 23 10:44:52 2011 -0600
committer Benjamin Peterson <benjamin@python.org> Wed Nov 23 10:44:52 2011 -0600
tree c0fe49c380c525da05f06ff5b33c2297e555b5e6
parent d2b0217944926a0f63b5dec1d91193cc999222ee

and back to the "magic" formula (with a comment) it is

Objects/unicodeobject.c

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index ab59e21..2fefdbe 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -6164,21 +6164,9 @@
     kind = PyUnicode_KIND(unicode);
     data = PyUnicode_DATA(unicode);
     len = PyUnicode_GET_LENGTH(unicode);
-    expandsize = 0;
-    switch (kind) {
-    case PyUnicode_1BYTE_KIND:
-        expandsize = 4;
-        break;
-    case PyUnicode_2BYTE_KIND:
-        expandsize = 6;
-        break;
-    case PyUnicode_4BYTE_KIND:
-        expandsize = 10;
-        break;
-    default:
-        assert(0);
-        break;
-    }
+    /* 4 byte characters can take up 10 bytes, 2 byte characters can take up 6
+       bytes, and 1 byte characters 4. */
+    expandsize = kind * 2 + 2;
 
     if (len > PY_SSIZE_T_MAX / expandsize)
         return PyErr_NoMemory();