diff --git a/src/qibo/hamiltonians/abstract.py b/src/qibo/hamiltonians/abstract.py
index a393ab78da..749ad0b21e 100644
--- a/src/qibo/hamiltonians/abstract.py
+++ b/src/qibo/hamiltonians/abstract.py
@@ -16,13 +16,9 @@ def nqubits(self):
     @nqubits.setter
     def nqubits(self, n):
         if not isinstance(n, int):
-            raise_error(
-                RuntimeError, f"nqubits must be an integer but is {type(n)}."
-            )
+            raise_error(RuntimeError, f"nqubits must be an integer but is {type(n)}.")
         if n < 1:
-            raise_error(
-                ValueError, f"nqubits must be a positive integer but is {n}"
-            )
+            raise_error(ValueError, f"nqubits must be a positive integer but is {n}")
         self._nqubits = n
 
     @abstractmethod
diff --git a/src/qibo/hamiltonians/adiabatic.py b/src/qibo/hamiltonians/adiabatic.py
index 951c024128..4f69be8bf1 100644
--- a/src/qibo/hamiltonians/adiabatic.py
+++ b/src/qibo/hamiltonians/adiabatic.py
@@ -28,7 +28,7 @@ def __new__(cls, h0, h1):
         else:
             raise_error(
                 TypeError,
-                f"h0 should be a hamiltonians.Hamiltonian object but is {type(h0)}."
+                f"h0 should be a hamiltonians.Hamiltonian object but is {type(h0)}.",
             )
 
     def __init__(self, h0, h1):  # pragma: no cover
diff --git a/src/qibo/hamiltonians/terms.py b/src/qibo/hamiltonians/terms.py
index d2fd014b92..f1d2f7cda0 100644
--- a/src/qibo/hamiltonians/terms.py
+++ b/src/qibo/hamiltonians/terms.py
@@ -28,9 +28,7 @@ def __init__(self, matrix, *q):
                     f"Invalid qubit id {qi} < 0 was given in Hamiltonian term.",
                 )
         if not isinstance(matrix, np.ndarray):
-            raise_error(
-                TypeError, f"Invalid type {type(matrix)} of symbol matrix."
-            )
+            raise_error(TypeError, f"Invalid type {type(matrix)} of symbol matrix.")
         dim = int(matrix.shape[0])
         if 2 ** len(q) != dim:
             raise_error(
diff --git a/src/qibo/models/evolution.py b/src/qibo/models/evolution.py
index ee4fc7df78..0a53e4917e 100644
--- a/src/qibo/models/evolution.py
+++ b/src/qibo/models/evolution.py
@@ -55,9 +55,7 @@ def __init__(self, hamiltonian, dt, solver="exp", callbacks=[], accelerators=Non
         else:
             ham = hamiltonian(0)
             if not isinstance(ham, AbstractHamiltonian):
-                raise TypeError(
-                    f"Hamiltonian type {type(ham)} not understood."
-                )
+                raise TypeError(f"Hamiltonian type {type(ham)} not understood.")
         self.nqubits = ham.nqubits
         self.backend = ham.backend
         if dt <= 0:
diff --git a/src/qibo/models/variational.py b/src/qibo/models/variational.py
index 304d8a7d44..d379cf386c 100644
--- a/src/qibo/models/variational.py
+++ b/src/qibo/models/variational.py
@@ -245,9 +245,7 @@ def schedule(self, t):
 
         s = self._schedule(t / self._t_max)
         if (abs(s) - 1) > self.ATOL:  # pragma: no cover
-            raise_error(
-                ValueError, f"s cannot be greater than 1 but it is {s}."
-            )
+            raise_error(ValueError, f"s cannot be greater than 1 but it is {s}.")
         return s
 
     def hamiltonian(self, t):
@@ -359,9 +357,7 @@ def __init__(
         self.params = None
         # problem hamiltonian
         if not isinstance(hamiltonian, AbstractHamiltonian):
-            raise_error(
-                TypeError, f"Invalid Hamiltonian type {type(hamiltonian)}."
-            )
+            raise_error(TypeError, f"Invalid Hamiltonian type {type(hamiltonian)}.")
         self.hamiltonian = hamiltonian
         self.nqubits = hamiltonian.nqubits
         # mixer hamiltonian (default = -sum(sigma_x))
diff --git a/src/qibo/symbols.py b/src/qibo/symbols.py
index 9d27910618..9aa12a1dcc 100644
--- a/src/qibo/symbols.py
+++ b/src/qibo/symbols.py
@@ -63,9 +63,7 @@ def __init__(self, q, matrix=None, name="Symbol", commutative=False):
                 ),
             )
         ):
-            raise_error(
-                TypeError, f"Invalid type {type(matrix)} of symbol matrix."
-            )
+            raise_error(TypeError, f"Invalid type {type(matrix)} of symbol matrix.")
         self.matrix = matrix
 
     def __getstate__(self):