From daa0fe1f51d07a3560b56d6f0523bf4023b42e6d Mon Sep 17 00:00:00 2001
From: Brett Sun <qisheng.brett.sun@gmail.com>
Date: Fri, 2 Dec 2016 17:29:14 +0100
Subject: [PATCH] Add utility decorator for registering backend dispatches
 based on modules

---
 bigchaindb/backend/util.py | 20 +++++++++++
 tests/backend/test_util.py | 73 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+)
 create mode 100644 bigchaindb/backend/util.py
 create mode 100644 tests/backend/test_util.py

diff --git a/bigchaindb/backend/util.py b/bigchaindb/backend/util.py
new file mode 100644
index 00000000..c30f7b53
--- /dev/null
+++ b/bigchaindb/backend/util.py
@@ -0,0 +1,20 @@
+class BackendModuleDispatchRegisterError(Exception):
+    """Raised when there is a problem registering dispatched functions for a
+    backend module"""
+
+
+def make_module_dispatch_registrar(module):
+    def dispatch_wrapper(obj_type):
+        def wrapper(func):
+            func_name = func.__name__
+            try:
+                dispatch_registrar = getattr(module, func_name)
+                return dispatch_registrar.register(obj_type)(func)
+            except AttributeError as ex:
+                raise BackendModuleDispatchRegisterError(
+                    ("'{module}' does not contain a single-dispatchable "
+                     "function named '{func}'. The backend being used has not "
+                     "been implemented correctly!").format(
+                         func=func_name, module=module.__name__)) from ex
+        return wrapper
+    return dispatch_wrapper
diff --git a/tests/backend/test_util.py b/tests/backend/test_util.py
new file mode 100644
index 00000000..ad3f7351
--- /dev/null
+++ b/tests/backend/test_util.py
@@ -0,0 +1,73 @@
+import pytest
+
+from types import ModuleType
+
+
+@pytest.fixture
+def mock_module():
+    return ModuleType('mock_module')
+
+
+def test_module_dispatch_registers(mock_module):
+    from functools import singledispatch
+    from bigchaindb.backend.utils import make_module_dispatch_registrar
+
+    @singledispatch
+    def dispatcher(t):
+        pass
+    mock_module.dispatched = dispatcher
+    mock_dispatch = make_module_dispatch_registrar(mock_module)
+
+    @mock_dispatch(str)
+    def dispatched(t):
+        pass
+
+    assert mock_module.dispatched.registry[str] == dispatched
+
+
+def test_module_dispatch_dispatches(mock_module):
+    from functools import singledispatch
+    from bigchaindb.backend.utils import make_module_dispatch_registrar
+
+    @singledispatch
+    def dispatcher(t):
+        return False
+    mock_module.dispatched = dispatcher
+    mock_dispatch = make_module_dispatch_registrar(mock_module)
+
+    @mock_dispatch(str)
+    def dispatched(t):
+        return True
+
+    assert mock_module.dispatched(1) is False  # Goes to dispatcher()
+    assert mock_module.dispatched('1') is True  # Goes to dispatched()
+
+
+def test_module_dispatch_errors_on_missing_func(mock_module):
+    from bigchaindb.backend.utils import (
+        make_module_dispatch_registrar,
+        BackendModuleDispatchRegisterError,
+    )
+    mock_dispatch = make_module_dispatch_registrar(mock_module)
+
+    with pytest.raises(BackendModuleDispatchRegisterError):
+        @mock_dispatch(str)
+        def dispatched():
+            pass
+
+
+def test_module_dispatch_errors_on_non_dispatchable_func(mock_module):
+    from bigchaindb.backend.utils import (
+        make_module_dispatch_registrar,
+        BackendModuleDispatchRegisterError,
+    )
+
+    def dispatcher():
+        pass
+    mock_module.dispatched = dispatcher
+    mock_dispatch = make_module_dispatch_registrar(mock_module)
+
+    with pytest.raises(BackendModuleDispatchRegisterError):
+        @mock_dispatch(str)
+        def dispatched():
+            pass