diff --git a/python/protobuf.c b/python/protobuf.c
new file mode 100644
index 0000000..324b1ed
--- /dev/null
+++ b/python/protobuf.c
@@ -0,0 +1,431 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2023 Google LLC.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google LLC nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "python/protobuf.h"
+
+#include "python/descriptor.h"
+#include "python/descriptor_containers.h"
+#include "python/descriptor_pool.h"
+#include "python/extension_dict.h"
+#include "python/map.h"
+#include "python/message.h"
+#include "python/repeated.h"
+#include "python/unknown_fields.h"
+
+static upb_Arena* PyUpb_NewArena(void);
+
+static void PyUpb_ModuleDealloc(void* module) {
+  PyUpb_ModuleState* s = PyModule_GetState(module);
+  PyUpb_WeakMap_Free(s->obj_cache);
+  if (s->c_descriptor_symtab) {
+    upb_DefPool_Free(s->c_descriptor_symtab);
+  }
+}
+
+PyObject* PyUpb_SetAllowOversizeProtos(PyObject* m, PyObject* arg) {
+  if (!arg || !PyBool_Check(arg)) {
+    PyErr_SetString(PyExc_TypeError,
+                    "Argument to SetAllowOversizeProtos must be boolean");
+    return NULL;
+  }
+  PyUpb_ModuleState* state = PyUpb_ModuleState_Get();
+  state->allow_oversize_protos = PyObject_IsTrue(arg);
+  Py_INCREF(arg);
+  return arg;
+}
+
+static PyMethodDef PyUpb_ModuleMethods[] = {
+    {"SetAllowOversizeProtos", PyUpb_SetAllowOversizeProtos, METH_O,
+     "Enable/disable oversize proto parsing."},
+    {NULL, NULL}};
+
+static struct PyModuleDef module_def = {PyModuleDef_HEAD_INIT,
+                                        PYUPB_MODULE_NAME,
+                                        "Protobuf Module",
+                                        sizeof(PyUpb_ModuleState),
+                                        PyUpb_ModuleMethods,  // m_methods
+                                        NULL,                 // m_slots
+                                        NULL,                 // m_traverse
+                                        NULL,                 // m_clear
+                                        PyUpb_ModuleDealloc};
+
+// -----------------------------------------------------------------------------
+// ModuleState
+// -----------------------------------------------------------------------------
+
+PyUpb_ModuleState* PyUpb_ModuleState_MaybeGet(void) {
+  PyObject* module = PyState_FindModule(&module_def);
+  return module ? PyModule_GetState(module) : NULL;
+}
+
+PyUpb_ModuleState* PyUpb_ModuleState_GetFromModule(PyObject* module) {
+  PyUpb_ModuleState* state = PyModule_GetState(module);
+  assert(state);
+  assert(PyModule_GetDef(module) == &module_def);
+  return state;
+}
+
+PyUpb_ModuleState* PyUpb_ModuleState_Get(void) {
+  PyObject* module = PyState_FindModule(&module_def);
+  assert(module);
+  return PyUpb_ModuleState_GetFromModule(module);
+}
+
+PyObject* PyUpb_GetWktBases(PyUpb_ModuleState* state) {
+  if (!state->wkt_bases) {
+    PyObject* wkt_module = PyImport_ImportModule(PYUPB_PROTOBUF_INTERNAL_PACKAGE
+                                                 ".well_known_types");
+
+    if (wkt_module == NULL) {
+      return false;
+    }
+
+    state->wkt_bases = PyObject_GetAttrString(wkt_module, "WKTBASES");
+    PyObject* m = PyState_FindModule(&module_def);
+    // Reparent ownership to m.
+    PyModule_AddObject(m, "__internal_wktbases", state->wkt_bases);
+    Py_DECREF(wkt_module);
+  }
+
+  return state->wkt_bases;
+}
+
+// -----------------------------------------------------------------------------
+// WeakMap
+// -----------------------------------------------------------------------------
+
+struct PyUpb_WeakMap {
+  upb_inttable table;
+  upb_Arena* arena;
+};
+
+PyUpb_WeakMap* PyUpb_WeakMap_New(void) {
+  upb_Arena* arena = PyUpb_NewArena();
+  PyUpb_WeakMap* map = upb_Arena_Malloc(arena, sizeof(*map));
+  map->arena = arena;
+  upb_inttable_init(&map->table, map->arena);
+  return map;
+}
+
+void PyUpb_WeakMap_Free(PyUpb_WeakMap* map) { upb_Arena_Free(map->arena); }
+
+// To give better entropy in the table key, we shift away low bits that are
+// always zero.
+static const int PyUpb_PtrShift = (sizeof(void*) == 4) ? 2 : 3;
+
+uintptr_t PyUpb_WeakMap_GetKey(const void* key) {
+  uintptr_t n = (uintptr_t)key;
+  assert((n & ((1 << PyUpb_PtrShift) - 1)) == 0);
+  return n >> PyUpb_PtrShift;
+}
+
+void PyUpb_WeakMap_Add(PyUpb_WeakMap* map, const void* key, PyObject* py_obj) {
+  upb_inttable_insert(&map->table, PyUpb_WeakMap_GetKey(key),
+                      upb_value_ptr(py_obj), map->arena);
+}
+
+void PyUpb_WeakMap_Delete(PyUpb_WeakMap* map, const void* key) {
+  upb_value val;
+  bool removed =
+      upb_inttable_remove(&map->table, PyUpb_WeakMap_GetKey(key), &val);
+  (void)removed;
+  assert(removed);
+}
+
+void PyUpb_WeakMap_TryDelete(PyUpb_WeakMap* map, const void* key) {
+  upb_inttable_remove(&map->table, PyUpb_WeakMap_GetKey(key), NULL);
+}
+
+PyObject* PyUpb_WeakMap_Get(PyUpb_WeakMap* map, const void* key) {
+  upb_value val;
+  if (upb_inttable_lookup(&map->table, PyUpb_WeakMap_GetKey(key), &val)) {
+    PyObject* ret = upb_value_getptr(val);
+    Py_INCREF(ret);
+    return ret;
+  } else {
+    return NULL;
+  }
+}
+
+bool PyUpb_WeakMap_Next(PyUpb_WeakMap* map, const void** key, PyObject** obj,
+                        intptr_t* iter) {
+  uintptr_t u_key;
+  upb_value val;
+  if (!upb_inttable_next(&map->table, &u_key, &val, iter)) return false;
+  *key = (void*)(u_key << PyUpb_PtrShift);
+  *obj = upb_value_getptr(val);
+  return true;
+}
+
+void PyUpb_WeakMap_DeleteIter(PyUpb_WeakMap* map, intptr_t* iter) {
+  upb_inttable_removeiter(&map->table, iter);
+}
+
+// -----------------------------------------------------------------------------
+// ObjCache
+// -----------------------------------------------------------------------------
+
+PyUpb_WeakMap* PyUpb_ObjCache_Instance(void) {
+  PyUpb_ModuleState* state = PyUpb_ModuleState_Get();
+  return state->obj_cache;
+}
+
+void PyUpb_ObjCache_Add(const void* key, PyObject* py_obj) {
+  PyUpb_WeakMap_Add(PyUpb_ObjCache_Instance(), key, py_obj);
+}
+
+void PyUpb_ObjCache_Delete(const void* key) {
+  PyUpb_ModuleState* state = PyUpb_ModuleState_MaybeGet();
+  if (!state) {
+    // During the shutdown sequence, our object's Dealloc() methods can be
+    // called *after* our module Dealloc() method has been called.  At that
+    // point our state will be NULL and there is nothing to delete out of the
+    // map.
+    return;
+  }
+  PyUpb_WeakMap_Delete(state->obj_cache, key);
+}
+
+PyObject* PyUpb_ObjCache_Get(const void* key) {
+  return PyUpb_WeakMap_Get(PyUpb_ObjCache_Instance(), key);
+}
+
+// -----------------------------------------------------------------------------
+// Arena
+// -----------------------------------------------------------------------------
+
+typedef struct {
+  PyObject_HEAD;
+  upb_Arena* arena;
+} PyUpb_Arena;
+
+// begin:google_only
+// static upb_alloc* global_alloc = &upb_alloc_global;
+// end:google_only
+
+// begin:github_only
+#ifdef __GLIBC__
+#include <malloc.h>  // malloc_trim()
+#endif
+
+// A special allocator that calls malloc_trim() periodically to release
+// memory to the OS.  Without this call, we appear to leak memory, at least
+// as measured in RSS.
+//
+// We opt not to use this instead of PyMalloc (which would also solve the
+// problem) because the latter requires the GIL to be held.  This would make
+// our messages unsafe to share with other languages that could free at
+// unpredictable
+// times.
+static void* upb_trim_allocfunc(upb_alloc* alloc, void* ptr, size_t oldsize,
+                                size_t size) {
+  (void)alloc;
+  (void)oldsize;
+  if (size == 0) {
+    free(ptr);
+#ifdef __GLIBC__
+    static int count = 0;
+    if (++count == 10000) {
+      malloc_trim(0);
+      count = 0;
+    }
+#endif
+    return NULL;
+  } else {
+    return realloc(ptr, size);
+  }
+}
+static upb_alloc trim_alloc = {&upb_trim_allocfunc};
+static const upb_alloc* global_alloc = &trim_alloc;
+// end:github_only
+
+static upb_Arena* PyUpb_NewArena(void) {
+  return upb_Arena_Init(NULL, 0, global_alloc);
+}
+
+PyObject* PyUpb_Arena_New(void) {
+  PyUpb_ModuleState* state = PyUpb_ModuleState_Get();
+  PyUpb_Arena* arena = (void*)PyType_GenericAlloc(state->arena_type, 0);
+  arena->arena = PyUpb_NewArena();
+  return &arena->ob_base;
+}
+
+static void PyUpb_Arena_Dealloc(PyObject* self) {
+  upb_Arena_Free(PyUpb_Arena_Get(self));
+  PyUpb_Dealloc(self);
+}
+
+upb_Arena* PyUpb_Arena_Get(PyObject* arena) {
+  return ((PyUpb_Arena*)arena)->arena;
+}
+
+static PyType_Slot PyUpb_Arena_Slots[] = {
+    {Py_tp_dealloc, PyUpb_Arena_Dealloc},
+    {0, NULL},
+};
+
+static PyType_Spec PyUpb_Arena_Spec = {
+    PYUPB_MODULE_NAME ".Arena",
+    sizeof(PyUpb_Arena),
+    0,  // itemsize
+    Py_TPFLAGS_DEFAULT,
+    PyUpb_Arena_Slots,
+};
+
+static bool PyUpb_InitArena(PyObject* m) {
+  PyUpb_ModuleState* state = PyUpb_ModuleState_GetFromModule(m);
+  state->arena_type = PyUpb_AddClass(m, &PyUpb_Arena_Spec);
+  return state->arena_type;
+}
+
+// -----------------------------------------------------------------------------
+// Utilities
+// -----------------------------------------------------------------------------
+
+PyTypeObject* AddObject(PyObject* m, const char* name, PyType_Spec* spec) {
+  PyObject* type = PyType_FromSpec(spec);
+  return type && PyModule_AddObject(m, name, type) == 0 ? (PyTypeObject*)type
+                                                        : NULL;
+}
+
+static const char* PyUpb_GetClassName(PyType_Spec* spec) {
+  // spec->name contains a fully-qualified name, like:
+  //   google.protobuf.pyext._message.FooBar
+  //
+  // Find the rightmost '.' to get "FooBar".
+  const char* name = strrchr(spec->name, '.');
+  assert(name);
+  return name + 1;
+}
+
+PyTypeObject* PyUpb_AddClass(PyObject* m, PyType_Spec* spec) {
+  PyObject* type = PyType_FromSpec(spec);
+  const char* name = PyUpb_GetClassName(spec);
+  if (PyModule_AddObject(m, name, type) < 0) {
+    Py_XDECREF(type);
+    return NULL;
+  }
+  return (PyTypeObject*)type;
+}
+
+PyTypeObject* PyUpb_AddClassWithBases(PyObject* m, PyType_Spec* spec,
+                                      PyObject* bases) {
+  PyObject* type = PyType_FromSpecWithBases(spec, bases);
+  const char* name = PyUpb_GetClassName(spec);
+  if (PyModule_AddObject(m, name, type) < 0) {
+    Py_XDECREF(type);
+    return NULL;
+  }
+  return (PyTypeObject*)type;
+}
+
+const char* PyUpb_GetStrData(PyObject* obj) {
+  if (PyUnicode_Check(obj)) {
+    return PyUnicode_AsUTF8AndSize(obj, NULL);
+  } else if (PyBytes_Check(obj)) {
+    return PyBytes_AsString(obj);
+  } else {
+    return NULL;
+  }
+}
+
+const char* PyUpb_VerifyStrData(PyObject* obj) {
+  const char* ret = PyUpb_GetStrData(obj);
+  if (ret) return ret;
+  PyErr_Format(PyExc_TypeError, "Expected string: %S", obj);
+  return NULL;
+}
+
+PyObject* PyUpb_Forbidden_New(PyObject* cls, PyObject* args, PyObject* kwds) {
+  PyObject* name = PyObject_GetAttrString(cls, "__name__");
+  PyErr_Format(PyExc_RuntimeError,
+               "Objects of type %U may not be created directly.", name);
+  Py_XDECREF(name);
+  return NULL;
+}
+
+bool PyUpb_IndexToRange(PyObject* index, Py_ssize_t size, Py_ssize_t* i,
+                        Py_ssize_t* count, Py_ssize_t* step) {
+  assert(i && count && step);
+  if (PySlice_Check(index)) {
+    Py_ssize_t start, stop;
+    if (PySlice_Unpack(index, &start, &stop, step) < 0) return false;
+    *count = PySlice_AdjustIndices(size, &start, &stop, *step);
+    *i = start;
+  } else {
+    *i = PyNumber_AsSsize_t(index, PyExc_IndexError);
+
+    if (*i == -1 && PyErr_Occurred()) {
+      PyErr_SetString(PyExc_TypeError, "list indices must be integers");
+      return false;
+    }
+
+    if (*i < 0) *i += size;
+    *step = 0;
+    *count = 1;
+
+    if (*i < 0 || size <= *i) {
+      PyErr_Format(PyExc_IndexError, "list index out of range");
+      return false;
+    }
+  }
+  return true;
+}
+
+// -----------------------------------------------------------------------------
+// Module Entry Point
+// -----------------------------------------------------------------------------
+
+__attribute__((visibility("default"))) PyMODINIT_FUNC PyInit__message(void) {
+  PyObject* m = PyModule_Create(&module_def);
+  if (!m) return NULL;
+
+  PyUpb_ModuleState* state = PyUpb_ModuleState_GetFromModule(m);
+
+  state->allow_oversize_protos = false;
+  state->wkt_bases = NULL;
+  state->obj_cache = PyUpb_WeakMap_New();
+  state->c_descriptor_symtab = NULL;
+
+  if (!PyUpb_InitDescriptorContainers(m) || !PyUpb_InitDescriptorPool(m) ||
+      !PyUpb_InitDescriptor(m) || !PyUpb_InitArena(m) ||
+      !PyUpb_InitExtensionDict(m) || !PyUpb_Map_Init(m) ||
+      !PyUpb_InitMessage(m) || !PyUpb_Repeated_Init(m) ||
+      !PyUpb_UnknownFields_Init(m)) {
+    Py_DECREF(m);
+    return NULL;
+  }
+
+  // Temporary: an cookie we can use in the tests to ensure we are testing upb
+  // and not another protobuf library on the system.
+  PyModule_AddIntConstant(m, "_IS_UPB", 1);
+
+  return m;
+}