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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  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 Inc. 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.
+
+// This file can be included by other C++ libraries, typically extension modules
+// which want to interact with the Python Messages coming from the "cpp"
+// implementation of protocol buffers.
+//
+// Usage:
+// Declare a (probably static) variable to hold the API:
+//    const PyProto_API* py_proto_api;
+// In some initialization function, write:
+//    py_proto_api = static_cast<const PyProto_API*>(PyCapsule_Import(
+//        PyProtoAPICapsuleName(), 0));
+//    if (!py_proto_api) { ...handle ImportError... }
+// Then use the methods of the returned class:
+//    py_proto_api->GetMessagePointer(...);
+
+#ifndef GOOGLE_PROTOBUF_PYTHON_PROTO_API_H__
+#define GOOGLE_PROTOBUF_PYTHON_PROTO_API_H__
+
+#include <Python.h>
+
+#include <google/protobuf/message.h>
+
+namespace google {
+namespace protobuf {
+namespace python {
+
+// Note on the implementation:
+// This API is designed after
+// https://docs.python.org/3/extending/extending.html#providing-a-c-api-for-an-extension-module
+// The class below contains no mutable state, and all methods are "const";
+// we use a C++ class instead of a C struct with functions pointers just because
+// the code looks more readable.
+struct PyProto_API {
+  // The API object is created at initialization time and never freed.
+  // This destructor is never called.
+  virtual ~PyProto_API() {}
+
+  // Operations on Messages.
+
+  // If the passed object is a Python Message, returns its internal pointer.
+  // Otherwise, returns NULL with an exception set.
+  virtual const Message* GetMessagePointer(PyObject* msg) const = 0;
+
+  // If the passed object is a Python Message, returns a mutable pointer.
+  // Otherwise, returns NULL with an exception set.
+  // This function will succeed only if there are no other Python objects
+  // pointing to the message, like submessages or repeated containers.
+  // With the current implementation, only empty messages are in this case.
+  virtual Message* GetMutableMessagePointer(PyObject* msg) const = 0;
+};
+
+inline const char* PyProtoAPICapsuleName() {
+  static const char kCapsuleName[] =
+      "google.protobuf.pyext._message.proto_API";
+  return kCapsuleName;
+}
+
+}  // namespace python
+}  // namespace protobuf
+}  // namespace google
+
+#endif  // GOOGLE_PROTOBUF_PYTHON_PROTO_API_H__