python · StanFromIreland · Jul 14, 2025 · Jul 14, 2025 · Jul 14, 2025 · Jul 14, 2025
diff --git a/Lib/encodings/__init__.py b/Lib/encodings/__init__.py
@@ -30,6 +30,7 @@
 
 import codecs
 import sys
+from _codecs import _normalize_encoding
 from . import aliases
 
 _cache = {}
@@ -55,18 +56,7 @@ def normalize_encoding(encoding):
     if isinstance(encoding, bytes):
         encoding = str(encoding, "ascii")
 
-    chars = []
-    punct = False
-    for c in encoding:
-        if c.isalnum() or c == '.':
-            if punct and chars:
-                chars.append('_')
-            if c.isascii():
-                chars.append(c)
-            punct = False
-        else:
-            punct = True
-    return ''.join(chars)
+    return _normalize_encoding(encoding)
 
 def search_function(encoding):
 

diff --git a/Misc/NEWS.d/next/Library/2025-07-14-09-33-17.gh-issue-55531.Gt2e12.rst b/Misc/NEWS.d/next/Library/2025-07-14-09-33-17.gh-issue-55531.Gt2e12.rst
@@ -0,0 +1,4 @@
+:mod:`encodings`: Improve :func:`~encodings.normalize_encoding` performance
+by implementing the function in C using the private
+``_Py_normalize_encoding`` which has been modified to make lowercase
+conversion optional.
diff --git a/Modules/_codecsmodule.c b/Modules/_codecsmodule.c
@@ -1022,6 +1022,58 @@ _codecs_lookup_error_impl(PyObject *module, const char *name)
     return PyCodec_LookupError(name);
 }
 
+extern int _Py_normalize_encoding(const char *, char *, size_t, int);
+
+/*[clinic input]
+_codecs._normalize_encoding
+    encoding: unicode
+
+Normalize an encoding name *encoding*.
+
+Used for encodings.normalize_encoding. Does not convert to lower case.
+[clinic start generated code]*/
+
+static PyObject *
+_codecs__normalize_encoding_impl(PyObject *module, PyObject *encoding)
+/*[clinic end generated code: output=d27465d81e361f8e input=3ff3f4d64995b988]*/
+{
+    Py_ssize_t len;
+    const char *cstr = PyUnicode_AsUTF8AndSize(encoding, &len);
+    if (cstr == NULL) {
+        return NULL;
+    }
+
+    if (len > PY_SSIZE_T_MAX) {
+        PyErr_SetString(PyExc_OverflowError, "encoding is too large");
+        return NULL;
+    }
+
+    PyUnicodeWriter *writer = PyUnicodeWriter_Create(len + 1);
+    if (writer == NULL) {
+        return NULL;
+    }
+
+    char *normalized = PyMem_Malloc(len + 1);
+    if (normalized == NULL) {
+        PyUnicodeWriter_Discard(writer);
+        return PyErr_NoMemory();
+    }
+
+    if (!_Py_normalize_encoding(cstr, normalized, len + 1, 0)) {
+        PyMem_Free(normalized);
+        PyUnicodeWriter_Discard(writer);
+        return NULL;
+    }
+
+    if (PyUnicodeWriter_WriteUTF8(writer, normalized, (Py_ssize_t)strlen(normalized)) < 0) {
+        PyUnicodeWriter_Discard(writer);
+        PyMem_Free(normalized);
+        return NULL;
+    }
+    PyMem_Free(normalized);
+    return PyUnicodeWriter_Finish(writer);
+}
+
 /* --- Module API --------------------------------------------------------- */
 
 static PyMethodDef _codecs_functions[] = {
@@ -1071,6 +1123,7 @@ static PyMethodDef _codecs_functions[] = {
     _CODECS_REGISTER_ERROR_METHODDEF
     _CODECS__UNREGISTER_ERROR_METHODDEF
     _CODECS_LOOKUP_ERROR_METHODDEF
+    _CODECS__NORMALIZE_ENCODING_METHODDEF
     {NULL, NULL}                /* sentinel */
 };
 

diff --git a/Modules/clinic/_codecsmodule.c.h b/Modules/clinic/_codecsmodule.c.h
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
@@ -3587,13 +3587,14 @@ PyUnicode_FromEncodedObject(PyObject *obj,
     return v;
 }
 
-/* Normalize an encoding name: similar to encodings.normalize_encoding(), but
-   also convert to lowercase. Return 1 on success, or 0 on error (encoding is
-   longer than lower_len-1). */
+/* Normalize an encoding name like encodings.normalize_encoding()
+   but allow to convert to lowercase if *to_lower* is true.
+   Return 1 on success, or 0 on error (encoding is longer than lower_len-1). */
 int
 _Py_normalize_encoding(const char *encoding,
                        char *lower,
-                       size_t lower_len)
+                       size_t lower_len,
+                       int to_lower)
 {
     const char *e;
     char *l;
@@ -3624,7 +3625,7 @@ _Py_normalize_encoding(const char *encoding,
             if (l == l_end) {
                 return 0;
             }
-            *l++ = Py_TOLOWER(c);
+            *l++ = to_lower ? Py_TOLOWER(c) : c;
         }
         else {
             punct = 1;
@@ -3659,7 +3660,7 @@ PyUnicode_Decode(const char *s,
     }
 
     /* Shortcuts for common default encodings */
-    if (_Py_normalize_encoding(encoding, buflower, sizeof(buflower))) {
+    if (_Py_normalize_encoding(encoding, buflower, sizeof(buflower), 1)) {
         char *lower = buflower;
 
         /* Fast paths */
@@ -3916,7 +3917,7 @@ PyUnicode_AsEncodedString(PyObject *unicode,
     }
 
     /* Shortcuts for common default encodings */
-    if (_Py_normalize_encoding(encoding, buflower, sizeof(buflower))) {
+    if (_Py_normalize_encoding(encoding, buflower, sizeof(buflower), 1)) {
         char *lower = buflower;
 
         /* Fast paths */

diff --git a/Python/codecs.c b/Python/codecs.c
@@ -90,7 +90,7 @@ PyCodec_Unregister(PyObject *search_function)
     return 0;
 }
 
-extern int _Py_normalize_encoding(const char *, char *, size_t);
+extern int _Py_normalize_encoding(const char *, char *, size_t, int);
 
 /* Convert a string to a normalized Python string(decoded from UTF-8): all characters are
    converted to lower case, spaces and hyphens are replaced with underscores. */
@@ -108,10 +108,11 @@ PyObject *normalizestring(const char *string)
     }
 
     encoding = PyMem_Malloc(len + 1);
-    if (encoding == NULL)
+    if (encoding == NULL) {
         return PyErr_NoMemory();
+    }
 
-    if (!_Py_normalize_encoding(string, encoding, len + 1))
+    if (!_Py_normalize_encoding(string, encoding, len + 1, 1))
     {
         PyErr_SetString(PyExc_RuntimeError, "_Py_normalize_encoding() failed");
         PyMem_Free(encoding);

diff --git a/Python/fileutils.c b/Python/fileutils.c
@@ -180,7 +180,7 @@ _Py_mbrtowc(wchar_t *pwc, const char *str, size_t len, mbstate_t *pmbs)
 
 #define USE_FORCE_ASCII
 
-extern int _Py_normalize_encoding(const char *, char *, size_t);
+extern int _Py_normalize_encoding(const char *, char *, size_t, int);
 
 /* Workaround FreeBSD and OpenIndiana locale encoding issue with the C locale
    and POSIX locale. nl_langinfo(CODESET) announces an alias of the
@@ -231,7 +231,7 @@ check_force_ascii(void)
     }
 
     char encoding[20];   /* longest name: "iso_646.irv_1991\0" */
-    if (!_Py_normalize_encoding(codeset, encoding, sizeof(encoding))) {
+    if (!_Py_normalize_encoding(codeset, encoding, sizeof(encoding), 1)) {
         goto error;
     }