Raise KeyError for missing keys in RepositoryEmpty

decouple.py

@@ -115,7 +115,7 @@ def __contains__(self, key):
         return False
 
     def __getitem__(self, key):
-        return None
+        raise KeyError(key)
 
 
 class RepositoryIni(RepositoryEmpty):

tests/test_empty.py

@@ -0,0 +1,34 @@
+import pytest
+
+from decouple import Config, RepositoryEmpty, UndefinedValueError
+
+
+@pytest.mark.parametrize('key', ['MISSING', '', None, 42])
+def test_empty_repository_missing_key(key):
+    repository = RepositoryEmpty()
+
+    assert key not in repository
+    with pytest.raises(KeyError) as error:
+        repository[key]
+    assert error.value.args == (key,)
+
+
+def test_empty_repository_config_missing(monkeypatch):
+    monkeypatch.delenv('DECOUPLE_EMPTY_TEST', raising=False)
+    config = Config(RepositoryEmpty())
+
+    with pytest.raises(UndefinedValueError):
+        config('DECOUPLE_EMPTY_TEST')
+    assert config('DECOUPLE_EMPTY_TEST', default=None) is None
+    assert config('DECOUPLE_EMPTY_TEST', default='42', cast=int) == 42
+
+
+@pytest.mark.parametrize('value', ['', 'from environment'])
+def test_empty_repository_config_environment(monkeypatch, value):
+    monkeypatch.setenv('DECOUPLE_EMPTY_TEST', value)
+    repository = RepositoryEmpty()
+
+    assert 'DECOUPLE_EMPTY_TEST' not in repository
+    with pytest.raises(KeyError):
+        repository['DECOUPLE_EMPTY_TEST']
+    assert Config(repository)('DECOUPLE_EMPTY_TEST', default='fallback') == value