diff --git a/src/qibotn/eval_qu.py b/src/qibotn/eval_qu.py
index 4d06034c..df9425f6 100644
--- a/src/qibotn/eval_qu.py
+++ b/src/qibotn/eval_qu.py
@@ -46,8 +46,9 @@ def dense_vector_tn_qu(qasm: str, initial_state, mps_opts, backend="numpy"):
     return amplitudes
 
 
-
-def expectation_qu(qasm: str, pauli_string_pattern, initial_state, mps_opts, backend="numpy"):
+def expectation_qu(
+    qasm: str, pauli_string_pattern, initial_state, mps_opts, backend="numpy"
+):
 
     qasm_mod, nqubits = modify_qasm(qasm)
     if initial_state is not None:
@@ -58,32 +59,34 @@ def expectation_qu(qasm: str, pauli_string_pattern, initial_state, mps_opts, bac
         qasm, psi0=initial_state, gate_opts=mps_opts
     )
 
-    obs = pauli_string_gen(nqubits-1, pauli_string_pattern)
+    obs = pauli_string_gen(nqubits - 1, pauli_string_pattern)
 
     opt = ctg.ReusableHyperOptimizer(
-    # just do a few runs
+        # just do a few runs
         max_repeats=32,
-    # only use the basic greedy optimizer ...
-        methods=['greedy'],
-    # ... but pair it with reconfiguration
+        # only use the basic greedy optimizer ...
+        methods=["greedy"],
+        # ... but pair it with reconfiguration
         reconf_opts={},
-    # just uniformly sample the space
-        optlib='random',
-    # terminate search if contraction is cheap
-        max_time='rate:1e6',
-    # account for both flops and write - usually wise for practical performance
-        minimize='combo-64',
-    # persist paths found in here
-        directory='cotengra_cache_eco',
+        # just uniformly sample the space
+        optlib="random",
+        # terminate search if contraction is cheap
+        max_time="rate:1e6",
+        # account for both flops and write - usually wise for practical performance
+        minimize="combo-64",
+        # persist paths found in here
+        directory="cotengra_cache_eco",
+    )
+    expectation = circ_quimb.local_expectation(
+        obs, where=list(range(nqubits - 1)), optimize=opt, simplify_sequence="DRC"
     )
-    expectation = circ_quimb.local_expectation(obs,  where=list(range(nqubits-1)), optimize=opt, 
-        simplify_sequence ="DRC")
-  
+
     return expectation
 
+
 def modify_qasm(qasm_circ):
-    lines =qasm_circ.split("\n")
-    qasm_circ_mod =[]
+    lines = qasm_circ.split("\n")
+    qasm_circ_mod = []
     while lines:
         line = lines.pop(0).strip()
         sta = re.compile(r"qreg\s+(\w+)\s*\[(\d+)\];")
@@ -94,7 +97,8 @@ def modify_qasm(qasm_circ):
         else:
             qasm_circ_mod.append(line)
     qasm_circ_mod = "\n".join(qasm_circ_mod)
-    return qasm_circ_mod, int(nqubits)+1
+    return qasm_circ_mod, int(nqubits) + 1
+
 
 def pauli_string_gen(nqubits, pauli_string_pattern):
     """Used internally to generate the string based on given pattern and number
@@ -117,11 +121,10 @@ def pauli_string_gen(nqubits, pauli_string_pattern):
     for i in range(nqubits):
         char_to_add = pauli_string_pattern[i % len(pauli_string_pattern)]
         result += char_to_add
-    
 
     for i, c in enumerate(result):
-        if (i==0):
+        if i == 0:
             obs = qu.pauli(c)
         else:
-            obs = obs&qu.pauli(c)
+            obs = obs & qu.pauli(c)
     return obs