diff --git a/wafo/wave_theory/dispersion_relation.py b/wafo/wave_theory/dispersion_relation.py
index 86b5d64..138a362 100644
--- a/wafo/wave_theory/dispersion_relation.py
+++ b/wafo/wave_theory/dispersion_relation.py
@@ -9,13 +9,23 @@ import numpy as np
 from wafo.misc import lazywhere
 from numpy import (atleast_1d, sqrt, ones_like, zeros_like, arctan2, where,
                    tanh, sin, cos, sign, inf,
-                   flatnonzero, finfo, cosh, abs)
+                   flatnonzero, finfo, cosh)
 
 __all__ = ['k2w', 'w2k']
 
 
+def _assert(cond, msg):
+    if not cond:
+        raise ValueError(msg)
+
+
+def _assert_warn(cond, msg):
+    if not cond:
+        warnings.warn(msg)
+
+
 def k2w(k1, k2=0e0, h=inf, g=9.81, u1=0e0, u2=0e0):
-    ''' Translates from wave number to frequency
+    """ Translates from wave number to frequency
         using the dispersion relation
 
     Parameters
@@ -61,7 +71,7 @@ def k2w(k1, k2=0e0, h=inf, g=9.81, u1=0e0, u2=0e0):
     array([ 0.3132092 ,  1.43530485,  2.00551739])
     >>> wsd.k2w(arange(0.01,.5,0.2),h=20)[0]
     array([ 0.13914927,  1.43498213,  2.00551724])
-    '''
+    """
 
     k1i, k2i, hi, gi, u1i, u2i = atleast_1d(k1, k2, h, g, u1, u2)
 
@@ -75,21 +85,19 @@ def k2w(k1, k2=0e0, h=inf, g=9.81, u1=0e0, u2=0e0):
     k = sqrt(k1i ** 2 + k2i ** 2)
     w = where(k > 0, ku1 + ku2 + sqrt(gi * k * tanh(k * hi)), 0.0)
 
-    cond = (w < 0)
-    if np.any(cond):
-        txt0 = '''
-               Waves and current are in opposite directions
-               making some of the frequencies negative.
-               Here we are forcing the negative frequencies to zero.
-               '''
-        warnings.warn(txt0)
-        w = where(cond, 0.0, w)  # force w to zero
+    cond = (0 <= w)
+    _assert_warn(np.all(cond), """
+        Waves and current are in opposite directions
+        making some of the frequencies negative.
+        Here we are forcing the negative frequencies to zero.
+        """)
 
+    w = where(cond, w, 0.0)  # force w to zero
     return w, theta
 
 
 def w2k(w, theta=0.0, h=inf, g=9.81, count_limit=100):
-    '''
+    """
     Translates from frequency to wave number
       using the dispersion relation
 
@@ -136,7 +144,7 @@ def w2k(w, theta=0.0, h=inf, g=9.81, count_limit=100):
     See also
     --------
     k2w
-    '''
+    """
     wi, th, hi, gi = atleast_1d(w, theta, h, g)
 
     if wi.size == 0:
@@ -147,10 +155,8 @@ def w2k(w, theta=0.0, h=inf, g=9.81, count_limit=100):
         k2 = k * sin(th) * gi[0] / gi[-1]  # size np x nf
         k1 = k * cos(th)
         return k1, k2
-
-    if gi.size > 1:
-        raise ValueError('Finite depth in combination with 3D normalization' +
-                         ' (len(g)=2) is not implemented yet.')
+    _assert(gi.size == 1, 'Finite depth in combination with 3D normalization'
+            ' (len(g)=2) is not implemented yet.')
 
     find = flatnonzero
     eps = finfo(float).eps
@@ -193,9 +199,8 @@ def w2k(w, theta=0.0, h=inf, g=9.81, count_limit=100):
                   np.abs(hn) > sqrt(eps))
         count += 1
 
-    if count == count_limit:
-        warnings.warn('W2K did not converge. The maximum error in the ' +
-                      'last step was: %13.8f' % max(hn[ix]))
+    _assert_warn(count < count_limit, 'W2K did not converge. '
+                 'Maximum error in the last step was: %13.8f' % max(hn[ix]))
 
     k.shape = oshape