diff --git a/devito/types/basic.py b/devito/types/basic.py
index 9f85f1273a..03e6c4f7c0 100644
--- a/devito/types/basic.py
+++ b/devito/types/basic.py
@@ -985,8 +985,9 @@ def __init_finalize__(self, *args, **kwargs):
 
         # Averaging mode for off the grid evaluation
         self._avg_mode = kwargs.get('avg_mode', 'arithmetic')
-        assert self._avg_mode in ['arithmetic', 'harmonic'], "Accepted avg_mode " \
-            "values are 'arithmetic' or 'harmonic', invalid %s" % self._avg_mode
+        if self._avg_mode not in ['arithmetic', 'harmonic']:
+            raise ValueError("Invalid averaging mode_mode %s, accepted values are"
+                             " arithmetic or harmonic" % self._avg_mode)
 
     @classmethod
     def __args_setup__(cls, *args, **kwargs):
@@ -1152,10 +1153,13 @@ def _evaluate(self, **kwargs):
             return self
 
         # Base function
-        retval = 1 / self.function if self._avg_mode == 'harmonic' else self.function
+        if self._avg_mode == 'harmonic':
+            retval = 1 / self.function
+        else:
+            retval = self.function
         # Apply interpolation from inner most dim
         for d, i in self._grid_map.items():
-            retval = retval.diff(d, deriv_order=0, fd_order=2, x0={d: i}).evaluate
+            retval = retval.diff(d, deriv_order=0, fd_order=2, x0={d: i})
         if self._avg_mode == 'harmonic':
             retval = 1 / retval
 
diff --git a/examples/seismic/acoustic/acoustic_example.py b/examples/seismic/acoustic/acoustic_example.py
index 7d563d42d9..d45d935b8b 100644
--- a/examples/seismic/acoustic/acoustic_example.py
+++ b/examples/seismic/acoustic/acoustic_example.py
@@ -1,7 +1,7 @@
 import numpy as np
 try:
     import pytest
-except:
+except ImportError:
     pass
 
 from devito.logger import info
diff --git a/examples/seismic/elastic/elastic_example.py b/examples/seismic/elastic/elastic_example.py
index 66b26540ae..fa1bc546a9 100644
--- a/examples/seismic/elastic/elastic_example.py
+++ b/examples/seismic/elastic/elastic_example.py
@@ -1,7 +1,7 @@
 import numpy as np
 try:
     import pytest
-except:
+except ImportError:
     pass
 from devito import norm
 from devito.logger import info
diff --git a/examples/seismic/tti/tti_example.py b/examples/seismic/tti/tti_example.py
index de7d61eee9..d65a8ce88b 100644
--- a/examples/seismic/tti/tti_example.py
+++ b/examples/seismic/tti/tti_example.py
@@ -1,7 +1,7 @@
 import numpy as np
 try:
     import pytest
-except:
+except ImportError:
     pass
 
 from devito import Function, smooth, norm, info, Constant
diff --git a/examples/seismic/viscoacoustic/viscoacoustic_example.py b/examples/seismic/viscoacoustic/viscoacoustic_example.py
index bd64922295..3b065d50f9 100755
--- a/examples/seismic/viscoacoustic/viscoacoustic_example.py
+++ b/examples/seismic/viscoacoustic/viscoacoustic_example.py
@@ -1,7 +1,7 @@
 import numpy as np
 try:
     import pytest
-except:
+except ImportError:
     pass
 
 from devito.logger import info
diff --git a/examples/seismic/viscoelastic/viscoelastic_example.py b/examples/seismic/viscoelastic/viscoelastic_example.py
index 22de2e11d7..910462953a 100644
--- a/examples/seismic/viscoelastic/viscoelastic_example.py
+++ b/examples/seismic/viscoelastic/viscoelastic_example.py
@@ -1,7 +1,7 @@
 import numpy as np
 try:
     import pytest
-except:
+except ImportError:
     pass
 
 from devito import norm