diff --git a/examples/variational_classifier/main.py b/examples/variational_classifier/main.py
index a8d7ef5d4e..20bcb5f432 100644
--- a/examples/variational_classifier/main.py
+++ b/examples/variational_classifier/main.py
@@ -111,9 +111,7 @@ def main(nclasses, nqubits, nlayers, nshots, training, RxRzRx, method):
                 method=method,
             )
             path_angles = (
-                LOCAL_FOLDER
-                / "data"
-                / f"optimal_angles_ry_{nqubits}q_{nlayers}l.npy"
+                LOCAL_FOLDER / "data" / f"optimal_angles_ry_{nqubits}q_{nlayers}l.npy"
             )
             np.save(
                 path_angles,
diff --git a/src/qibo/callbacks.py b/src/qibo/callbacks.py
index 65bc124144..2313e42e95 100644
--- a/src/qibo/callbacks.py
+++ b/src/qibo/callbacks.py
@@ -310,9 +310,7 @@ def __init__(self, mode: Union[str, int] = "gap", check_degenerate: bool = True)
                 f"Gap callback mode should be integer or string but is {type(mode)}.",
             )
         elif isinstance(mode, str) and mode != "gap":
-            raise_error(
-                ValueError, f"Unsupported mode {mode} for gap callback."
-            )
+            raise_error(ValueError, f"Unsupported mode {mode} for gap callback.")
         self.mode = mode
         self.check_degenerate = check_degenerate
         self.evolution = None