Add try...finally to prefixed method

environs.py

@@ -226,12 +226,16 @@ def read_env(path=None, recurse=True, stream=None, verbose=False, override=False
     @contextlib.contextmanager
     def prefixed(self, prefix):
         """Context manager for parsing envvars with a common prefix."""
-        old_prefix = self._prefix
-        if old_prefix is None:
-            self._prefix = prefix
-        else:
-            self._prefix = "{}{}".format(old_prefix, prefix)
-        yield self
+        try:
+            old_prefix = self._prefix
+            if old_prefix is None:
+                self._prefix = prefix
+            else:
+                self._prefix = "{}{}".format(old_prefix, prefix)
+            yield self
+        finally:
+            # explicitly reset the stored prefix on completion and exceptions
+            self._prefix = None
         self._prefix = old_prefix
 
     def __getattr__(self, name, **kwargs):

tests/test_environs.py

@@ -38,6 +38,10 @@ def env():
     return environs.Env()
 
 
+class FauxTestException(Exception):
+    pass
+
+
 class TestCasting:
     def test_call(self, set_env, env):
         set_env({"STR": "foo", "INT": "42"})
@@ -434,6 +438,54 @@ def test_dump_with_nested_prefixed(self, env):
         }
 
 
+class TestFailedNestedPrefix:
+    @pytest.fixture(autouse=True)
+    def default_environ(self, set_env):
+        set_env({"APP_STR": "foo", "APP_NESTED_INT": "42"})
+
+    def test_failed_nested_prefixed(self, env):
+
+        # define repeated prefixed steps
+        def nested_prefixed(env, fail=False):
+            with env.prefixed("APP_"):
+                with env.prefixed("NESTED_"):
+                    assert env.int("INT") == 42
+                    assert env("NOT_FOUND", "mydefault") == "mydefault"
+                assert env.str("STR") == "foo"
+                assert env("NOT_FOUND", "mydefault") == "mydefault"
+                if fail:
+                    raise FauxTestException
+
+        try:
+            nested_prefixed(env, fail=True)
+        except FauxTestException:
+            nested_prefixed(env, fail=False)
+
+    def test_failed_dump_with_nested_prefixed(self, env):
+
+        # define repeated prefixed steps
+        def dump_with_nested_prefixed(env, fail=False):
+            with env.prefixed("APP_"):
+                with env.prefixed("NESTED_"):
+                    env.int("INT") == 42
+                    env("NOT_FOUND", "mydefault") == "mydefault"
+                env.str("STR") == "foo"
+                env("NOT_FOUND", "mydefault") == "mydefault"
+                if fail:
+                    raise FauxTestException
+            assert env.dump() == {
+                "APP_STR": "foo",
+                "APP_NOT_FOUND": "mydefault",
+                "APP_NESTED_INT": 42,
+                "APP_NESTED_NOT_FOUND": "mydefault",
+            }
+
+        try:
+            dump_with_nested_prefixed(env, fail=True)
+        except FauxTestException:
+            dump_with_nested_prefixed(env, fail=False)
+
+
 class TestDjango:
     def test_dj_db_url(self, env, set_env):
         db_url = "postgresql://localhost:5432/mydb"