diff --git a/examples/square.py b/examples/square.py
index c61089cdb56c0f1f7472e4a22896befc2b37bf4b..580df113f33af5743bc32140a460aba003aa6ca8 100644
--- a/examples/square.py
+++ b/examples/square.py
@@ -9,6 +9,7 @@ from kwant.physics.leads import square_selfenergy
 
 __all__ = ['System']
 
+
 class Lead(object):
     def __init__(self, width, t, potential):
         self.width = width
diff --git a/kwant/builder.py b/kwant/builder.py
index b8a37d175cd53eb53ddc2d85b8d2d1df1e0c2c5d..cd83747df7112a8fb35169dd644db138050f5d89 100644
--- a/kwant/builder.py
+++ b/kwant/builder.py
@@ -6,8 +6,6 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-
-
 __all__ = ['Builder', 'Site', 'SiteFamily', 'SimpleSiteFamily', 'Symmetry',
            'HoppingKind', 'Lead', 'BuilderLead', 'SelfEnergyLead', 'ModesLead']
 
diff --git a/kwant/digest.py b/kwant/digest.py
index 7d2d16d957acd9edb57281a9db511c583fcbc494..0b502d5f2992477ee12d721111a104b0fa481536 100644
--- a/kwant/digest.py
+++ b/kwant/digest.py
@@ -19,8 +19,6 @@ good enough to pass the "dieharder" battery of tests: see the function `test` of
 this module.
 """
 
-
-
 from math import pi, log, sqrt, cos
 from hashlib import md5
 from struct import unpack
diff --git a/kwant/graph/tests/test_scotch.py b/kwant/graph/tests/test_scotch.py
index ca7e25af34064e3bbb8ba0d7e891af82e1179611..2ded31ae2f863eceee4a371b0e2db6a35eb70788 100644
--- a/kwant/graph/tests/test_scotch.py
+++ b/kwant/graph/tests/test_scotch.py
@@ -17,9 +17,9 @@ def _DISABLED_test_bisect():
     size = 5
     graph = Graph()
 
-    for i in range(size-1):
+    for i in range(size - 1):
         offset = i * size
-        for j in range(size-1):
+        for j in range(size - 1):
             graph.add_edge(offset + j, offset + j + 1)
             graph.add_edge(offset + j + 1, offset + j)
         if i > 0:
@@ -36,9 +36,9 @@ def _DISABLED_test_reset():
     size = 5
     graph = Graph()
 
-    for i in range(size-1):
+    for i in range(size - 1):
         offset = i * size
-        for j in range(size-1):
+        for j in range(size - 1):
             graph.add_edge(offset + j, offset + j + 1)
             graph.add_edge(offset + j + 1, offset + j)
         if i > 0:
diff --git a/kwant/graph/tests/test_utils.py b/kwant/graph/tests/test_utils.py
index ae7f3d2277505e016f62d777c7d9c8ad4b1f8343..0906765e5fd587dfe7fa2345bf06cb44c6dc8748 100644
--- a/kwant/graph/tests/test_utils.py
+++ b/kwant/graph/tests/test_utils.py
@@ -83,7 +83,7 @@ def test_induced_subgraph():
     num_nodes = 6
 
     graph = Graph()
-    for i in range(num_nodes-1):
+    for i in range(num_nodes - 1):
         graph.add_edge(i, i + 1)
     graph.add_edge(1, 0)
     g = graph.compressed()
diff --git a/kwant/lattice.py b/kwant/lattice.py
index ff74ecd28cbdf4440e424e97bb38645be89d3fd8..e93228980cc7392bcd5f9842f15bae745eb584eb 100644
--- a/kwant/lattice.py
+++ b/kwant/lattice.py
@@ -6,8 +6,6 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-
-
 __all__ = ['TranslationalSymmetry', 'general', 'Polyatomic', 'Monatomic',
            'chain', 'square', 'triangular', 'honeycomb', 'kagome']
 
diff --git a/kwant/linalg/lll.py b/kwant/linalg/lll.py
index 9a29b473db3ca7ab36ffdd48f8a5fe37d4ff9943..c9f2540fc68056234bc03056ae3f89d862483590 100644
--- a/kwant/linalg/lll.py
+++ b/kwant/linalg/lll.py
@@ -6,13 +6,12 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-
-
 __all__ = ['lll', 'cvp', 'voronoi']
 
 import numpy as np
 from itertools import product
 
+
 def gs_coefficient(a, b):
     """Gram-Schmidt coefficient."""
     return np.dot(a, b) / np.linalg.norm(b)**2
diff --git a/kwant/rmt.py b/kwant/rmt.py
index 1d307f66a54b7a518682d1da8664191747a16fff..6442fc0a90e220277086f35a9a07576f327e4299 100644
--- a/kwant/rmt.py
+++ b/kwant/rmt.py
@@ -6,8 +6,6 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-
-
 __all__ = ['gaussian', 'circular']
 
 import numpy as np
diff --git a/kwant/system.py b/kwant/system.py
index 45d0b473395b2a189ecf60de62078e815ebca4e8..ca541b4ad9c832d3971b75fe61afe707b7e27230 100644
--- a/kwant/system.py
+++ b/kwant/system.py
@@ -8,7 +8,6 @@
 
 """Low-level interface of systems"""
 
-
 __all__ = ['System', 'FiniteSystem', 'InfiniteSystem']
 
 import abc
diff --git a/kwant/tests/test_builder.py b/kwant/tests/test_builder.py
index 920bc2ffa803b8a05b273484de461ae7e8b2a52b..11b95c4b3be0d1e0572ffee7f65d5eb8892a60f2 100644
--- a/kwant/tests/test_builder.py
+++ b/kwant/tests/test_builder.py
@@ -6,7 +6,6 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-
 import warnings
 from random import Random
 from nose.tools import assert_raises
diff --git a/kwant/tests/test_lattice.py b/kwant/tests/test_lattice.py
index a9f8a94ae489253789d89ec96d27faf270f35176..f2169863a75a48aa91fc6c7c2d3229433fec08ba 100644
--- a/kwant/tests/test_lattice.py
+++ b/kwant/tests/test_lattice.py
@@ -6,7 +6,6 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-
 from math import sqrt
 import numpy as np
 import tinyarray as ta