fix dict iterator methods for flask DynaconfConfig (#581)

Co-authored-by: Frank Battaglia <frank.battaglia@gmail.com>
Co-authored-by: Bruno Rocha <rochacbruno@users.noreply.github.com>

dynaconf/contrib/flask_dynaconf.py

@@ -1,4 +1,5 @@
 import warnings
+from collections import ChainMap
 from contextlib import suppress
 
 try:
@@ -165,6 +166,21 @@ def __setitem__(self, key, value):
         """
         return self._settings.__setitem__(key, value)
 
+    def _chain_map(self):
+        return ChainMap(self._settings, dict(dict.items(self)))
+
+    def keys(self):
+        return self._chain_map().keys()
+
+    def values(self):
+        return self._chain_map().values()
+
+    def items(self):
+        return self._chain_map().items()
+
+    def __iter__(self):
+        return self._chain_map().__iter__()
+
     def __getattr__(self, name):
         """
         First try to get value from dynaconf then from Flask Config

tests/test_flask.py

@@ -101,6 +101,14 @@ def test_flask_dynaconf(settings):
 
     assert "MY_VAR" in app.config
     assert "MY_VAR2" in app.config
+    assert "MY_VAR" in app.config.keys()
+    assert "MY_VAR2" in app.config.keys()
+    assert ("MY_VAR", "foo") in app.config.items()
+    assert ("MY_VAR2", "bar") in app.config.items()
+    assert "foo" in app.config.values()
+    assert "bar" in app.config.values()
+    assert "MY_VAR" in list(app.config)
+    assert "MY_VAR2" in list(app.config)
 
     with pytest.raises(KeyError):
         app.config["NONEXISTENETVAR"]