[#469] Implement cppbind helper for loading of C++ optimized code

compiler/extensions/python/runtime/src/zserio/cppbind.py

@@ -0,0 +1,41 @@
+"""
+The module provides helper for importing of optimized C++ classes.
+"""
+
+import importlib
+import os
+import typing
+
+from zserio.exception import PythonRuntimeException
+
+ZSERIO_PYTHON_IMPLEMENTATION_ENV = "ZSERIO_PYTHON_IMPLEMENTATION"
+ZSERIO_CPP_MODULE = "zserio_cpp"
+
+def import_cpp_class(cppname: str) -> typing.Optional[typing.Type[typing.Any]]:
+    """
+    Tries to import optimized C++ implementation of the given python class if 'ZSERIO_PYTHON_IMPLEMENTATION'
+    environment variable is either unset or set to 'cpp'.
+
+    Depending on the content of the 'ZSERIO_PYTHON_IMPLEMENTATION' environment variable,
+    it either fails when no C++ implementation is available ('cpp') or ignores missing implementation
+    and just return the original python class (None) or even does not try to load the C++ implementation if
+    the variable is set to anyhing else (e.g. 'python').
+
+    :param pyclass: Pure python class implemenation for which the C++ optimized version should be loaded.
+    :param cppname: Name of optimized C++ class in case that it differs from the pyclass name.
+    :returns: Requested implemenation of the given pyclass.
+    :raises PythonRuntimeException: When the requested implementation is not available.
+    """
+
+    impl = os.getenv(ZSERIO_PYTHON_IMPLEMENTATION_ENV)
+    if not impl in [None, "python", "cpp", "c++"]:
+        raise PythonRuntimeException(f"Zserio Python runtime implementation '{impl}' is not available!")
+
+    if impl != "python":
+        try:
+            return getattr(importlib.import_module(ZSERIO_CPP_MODULE), cppname)
+        except (ImportError, AttributeError) as err:
+            if impl in ["cpp", "c++"]:
+                message = f"Zserio C++ implementation of '{cppname}' is not available!"
+                raise PythonRuntimeException(message) from err
+    return None

compiler/extensions/python/runtime/tests/test_bitposition.py

@@ -1,7 +1,7 @@
 import unittest
 
-from zserio.bitposition import (alignto, bits_to_bytes, bytes_to_bits, bitsize_to_bytesize,
-                                PythonRuntimeException)
+from zserio.bitposition import (alignto, bits_to_bytes, bytes_to_bits, bitsize_to_bytesize)
+from zserio.exception import PythonRuntimeException
 
 class BitPositionTest(unittest.TestCase):
 

compiler/extensions/python/runtime/tests/test_cppbind.py

@@ -0,0 +1,30 @@
+import unittest
+import os
+
+from zserio.cppbind import import_cpp_class
+from zserio.exception import PythonRuntimeException
+
+class MissingCppClass:
+    pass
+
+class CppBindTest(unittest.TestCase):
+
+    def test_invalid_env(self):
+        os.environ["ZSERIO_PYTHON_IMPLEMENTATION"] = "invalid"
+        with self.assertRaises(PythonRuntimeException):
+            import_cpp_class(MissingCppClass)
+
+    def test_missing_cpp_default(self):
+        os.environ.pop("ZSERIO_PYTHON_IMPLEMENTATION")
+        imported = import_cpp_class(MissingCppClass)
+        self.assertEqual(MissingCppClass, imported)
+
+    def test_missing_cpp_python(self):
+        os.environ["ZSERIO_PYTHON_IMPLEMENTATION"] = "python"
+        imported = import_cpp_class(MissingCppClass)
+        self.assertEqual(MissingCppClass, imported)
+
+    def test_missing_cpp_cpp(self):
+        os.environ["ZSERIO_PYTHON_IMPLEMENTATION"] = "cpp"
+        with self.assertRaises(PythonRuntimeException):
+            import_cpp_class(MissingCppClass)