diff --git a/wafo/covariance/estimation.py b/wafo/covariance/estimation.py
index 1a2a195..c058ad2 100644
--- a/wafo/covariance/estimation.py
+++ b/wafo/covariance/estimation.py
@@ -123,10 +123,10 @@ class CovarianceEstimator(object):
         except Exception:
             x = timeseries[:, 1:].flatten('F')
             dt = sampling_period(timeseries[:, 0])
-        if not (self.dt is None):
+        if self.dt is not None:
             dt = self.dt
 
-        if not (self.tr is None):
+        if self.tr is not None:
             x = self.tr.dat2gauss(x)
 
         n = len(x)
diff --git a/wafo/stats/tests/test_distributions.py b/wafo/stats/tests/test_distributions.py
index 9025317..b436f1a 100644
--- a/wafo/stats/tests/test_distributions.py
+++ b/wafo/stats/tests/test_distributions.py
@@ -1732,7 +1732,7 @@ class TestNct(TestCase):
                           [0.00153078, 0.00291093, 0.00525206, 0.00900815]])
         assert_allclose(res, expected, rtol=1e-5)
 
-    def text_variance_gh_issue_2401(self):
+    def test_variance_gh_issue_2401(self):
         # Computation of the variance of a non-central t-distribution resulted
         # in a TypeError: ufunc 'isinf' not supported for the input types,
         # and the inputs could not be safely coerced to any supported types
diff --git a/wafo/transform/models.py b/wafo/transform/models.py
index 3efe47c..831ad0c 100644
--- a/wafo/transform/models.py
+++ b/wafo/transform/models.py
@@ -219,7 +219,7 @@ class TrHermite(TrCommon2):
         return
 
     def check_forward(self, x):
-        if not (self._x_limit is None):
+        if self._x_limit is not None:
             x00 = self._x_limit
             txt2 = 'for the given interval x = [%g, %g]' % (x[0], x[-1])
 
@@ -306,7 +306,7 @@ class TrHermite(TrCommon2):
 #            Km1 = np.sqrt(1.+2.*c3**2+6*c4**2)
 #            q0 = x0**3-1.5*b1*(x0+xn*Km1)
             # q0 = x0**3-1.5*b1*(x0+xn)
-            if not (self._x_limit is None):  # % Three real roots
+            if self._x_limit is not None:  # % Three real roots
                 d = sqrt(-p1)
                 theta1 = arccos(-q0 / d ** 3) / 3
                 th2 = np.r_[0, -2 * pi / 3, 2 * pi / 3]