diff --git a/src/environs/__init__.py b/src/environs/__init__.py
index d918666..f4072ff 100644
--- a/src/environs/__init__.py
+++ b/src/environs/__init__.py
@@ -382,10 +382,12 @@ def _deserialize(self, value, *args, **kwargs) -> timedelta:
             seconds = int(value)
             return timedelta(
                 seconds=seconds,
-                microseconds=value-seconds,
+                microseconds=value - seconds,
             )
         if not isinstance(value, str):
-            raise ValueError("Not a valid period of time (must be either a int, float, timedelta or string).")
+            raise ValueError(
+                "Not a valid period of time (must be either a int, float, timedelta or string)."
+            )
         match = _TIMEDELTA_PATTERN.match(value)
         if match is not None and match.group(0):  # disallow "", allow "0s"
             return timedelta(
diff --git a/tests/test_environs.py b/tests/test_environs.py
index 7f8fa1d..752e28f 100644
--- a/tests/test_environs.py
+++ b/tests/test_environs.py
@@ -233,9 +233,15 @@ def test_timedelta_cast(self, set_env, env):
         assert env.timedelta("NO_VALUE", default=0) == dt.timedelta()
         assert env.timedelta("NO_VALUE", default=42) == dt.timedelta(seconds=42)
         assert env.timedelta("NO_VALUE", default=-42) == dt.timedelta(seconds=-42)
-        assert env.timedelta("NO_VALUE", default=42.3) == dt.timedelta(seconds=42, microseconds=3)
-        assert env.timedelta("NO_VALUE", default=dt.timedelta(seconds=42)) == dt.timedelta(seconds=42)
-        assert env.timedelta("NO_VALUE", default=dt.timedelta(seconds=42, microseconds=3)) == dt.timedelta(seconds=42, microseconds=3)
+        assert env.timedelta("NO_VALUE", default=42.3) == dt.timedelta(
+            seconds=42, microseconds=3
+        )
+        assert env.timedelta(
+            "NO_VALUE", default=dt.timedelta(seconds=42)
+        ) == dt.timedelta(seconds=42)
+        assert env.timedelta(
+            "NO_VALUE", default=dt.timedelta(seconds=42, microseconds=3)
+        ) == dt.timedelta(seconds=42, microseconds=3)
         # seconds as integer string
         set_env({"TIMEDELTA": "0"})
         assert env.timedelta("TIMEDELTA") == dt.timedelta()
@@ -249,7 +255,9 @@ def test_timedelta_cast(self, set_env, env):
         # seconds as a float string
         set_env({"TIMEDELTA": "42.3"})
         assert env.timedelta("TIMEDELTA") == dt.timedelta(seconds=42, microseconds=3)
-        assert env.timedelta("NO_VALUE", default="42.3") == dt.timedelta(seconds=42, microseconds=3)
+        assert env.timedelta("NO_VALUE", default="42.3") == dt.timedelta(
+            seconds=42, microseconds=3
+        )
         # seconds as duration string
         set_env({"TIMEDELTA": "0s"})
         assert env.timedelta("TIMEDELTA") == dt.timedelta()
@@ -263,11 +271,15 @@ def test_timedelta_cast(self, set_env, env):
         # whitespaces, units subselection (but descending ordering)
         set_env({"TIMEDELTA": " 42 d  -42s "})
         assert env.timedelta("TIMEDELTA") == dt.timedelta(days=42, seconds=-42)
-        assert env.timedelta("NO_VALUE", default=" 42 d  -42s ") == dt.timedelta(days=42, seconds=-42)
+        assert env.timedelta("NO_VALUE", default=" 42 d  -42s ") == dt.timedelta(
+            days=42, seconds=-42
+        )
         # unicode µs (in addition to us below)
         set_env({"TIMEDELTA": "42µs"})
         assert env.timedelta("TIMEDELTA") == dt.timedelta(microseconds=42)
-        assert env.timedelta("NO_VALUE", default="42µs") == dt.timedelta(microseconds=42)
+        assert env.timedelta("NO_VALUE", default="42µs") == dt.timedelta(
+            microseconds=42
+        )
         # all supported units
         set_env({"TIMEDELTA": "42w 42d 42h 42m 42s 42ms 42us"})
         assert env.timedelta("TIMEDELTA") == dt.timedelta(
@@ -279,7 +291,9 @@ def test_timedelta_cast(self, set_env, env):
             milliseconds=42,
             microseconds=42,
         )
-        assert env.timedelta("NO_VALUE", default="42w 42d 42h 42m 42s 42ms 42us") == dt.timedelta(
+        assert env.timedelta(
+            "NO_VALUE", default="42w 42d 42h 42m 42s 42ms 42us"
+        ) == dt.timedelta(
             weeks=42,
             days=42,
             hours=42,