diff --git a/qucumber/utils/data.py b/qucumber/utils/data.py
index 80ca039b..302b9c16 100644
--- a/qucumber/utils/data.py
+++ b/qucumber/utils/data.py
@@ -58,7 +58,7 @@ def load_data(tr_samples_path, tr_psi_path=None, tr_bases_path=None, bases_path=
         for i in range(len(bases_data)):
             tmp = ""
             for j in range(len(bases_data[i])):
-                if bases_data[i][j] is not " ":
+                if bases_data[i][j] != " ":
                     tmp += bases_data[i][j]
             bases.append(tmp)
         data.append(bases)
diff --git a/setup.py b/setup.py
index 99d06f5c..be554812 100644
--- a/setup.py
+++ b/setup.py
@@ -50,7 +50,7 @@
 style_requires = [
     "radon>=2.2.0",
     "black==18.6b4; python_version>='3.6'",
-    "flake8>=3.5.0",
+    "flake8>=3.7.5",
     "flake8-per-file-ignores>=0.6",
     "flake8-bugbear>=18.2.0",
 ]
diff --git a/tests/grads_utils.py b/tests/grads_utils.py
index 9457828c..bbda131e 100644
--- a/tests/grads_utils.py
+++ b/tests/grads_utils.py
@@ -131,7 +131,7 @@ def transform_bases(self, bases_data):
         for i in range(len(bases_data)):
             tmp = ""
             for j in range(len(bases_data[i])):
-                if bases_data[i][j] is not " ":
+                if bases_data[i][j] != " ":
                     tmp += bases_data[i][j]
             bases.append(tmp)
         return bases
@@ -151,7 +151,7 @@ def rotate_psi(self, basis, unitary_dict, vis):
             num_nontrivial_U = 0
             nontrivial_sites = []
             for j in range(N):
-                if basis[j] is not "Z":
+                if basis[j] != "Z":
                     num_nontrivial_U += 1
                     nontrivial_sites.append(j)
             sub_state = self.nn_state.generate_hilbert_space(num_nontrivial_U)
@@ -159,7 +159,7 @@ def rotate_psi(self, basis, unitary_dict, vis):
             for xp in range(1 << num_nontrivial_U):
                 cnt = 0
                 for j in range(N):
-                    if basis[j] is not "Z":
+                    if basis[j] != "Z":
                         v[j] = sub_state[xp][cnt]
                         cnt += 1
                     else: