color_scale uses by default the shorter path considering the hue cycle, long path as an option

README.rst

@@ -187,7 +187,9 @@ easily. Here, is the color scale of the rainbow between red and blue::
 
     >>> red = Color("red")
     >>> blue = Color("blue")
-    >>> list(red.range_to(blue, 5))
+    >>> list(red.range_to(blue, 3))
+    [<Color red>, <Color magenta>, <Color blue>]
+    >>> list(red.range_to(blue, 5, longer=True))
     [<Color red>, <Color yellow>, <Color lime>, <Color cyan>, <Color blue>]
 
 Or the different amount of gray between black and white::

colour.py

@@ -690,16 +690,52 @@ def web2hex(web, force_long=False):
 hsl2web = lambda x: rgb2web(hsl2rgb(x))
 
 
-def color_scale(begin_hsl, end_hsl, nb):
+def color_scale(begin_hsl, end_hsl, nb, longer=False):
     """Returns a list of nb color HSL tuples between begin_hsl and end_hsl
 
     >>> from colour import color_scale
 
-    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale((0, 1, 0.5),
-    ...                                               (1, 1, 0.5), 3)]
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale((0, 1, 0.5), # longer, as is
+    ...                                               (1, 1, 0.5), 3, longer=True)]
     ['#f00', '#0f0', '#00f', '#f00']
 
-    >>> [rgb2hex(hsl2rgb(hsl))
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale( # longer, as is, backwards
+    ...     (1, 1, 0.5),
+    ...     (0, 1, 0.5),
+    ...     3, longer=True)]
+    ['#f00', '#00f', '#0f0', '#f00']
+
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale( # shorter, h1 incremented
+    ...     (0, 1, 0.5), # lower red
+    ...     (1, 1, 0.5), # higher red
+    ...     3)]
+    ['#f00', '#f00', '#f00', '#f00']
+
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale( # shorter, h2 incremented
+    ...     (1, 1, 0.5), # lower red
+    ...     (0, 1, 0.5), # higher red
+    ...     3)]
+    ['#f00', '#f00', '#f00', '#f00']
+
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale( # shorter, as is
+    ...     (1./3, 1, 0.5), # green
+    ...     (2./3, 1, 0.5), # blue
+    ...     3)]
+    ['#0f0', '#0fa', '#0af', '#00f']
+
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale( # longer, h1 incremented
+    ...     (1./3, 1, 0.5), # green
+    ...     (2./3, 1, 0.5), # blue
+    ...     3, longer=True)]
+    ['#0f0', '#fa0', '#f0a', '#00f']
+
+    >>> [rgb2hex(hsl2rgb(hsl)) for hsl in color_scale( # longer, h2 incremented
+    ...     (2./3, 1, 0.5), # green
+    ...     (1./3, 1, 0.5), # blue
+    ...     3, longer=True)]
+    ['#00f', '#f0a', '#fa0', '#0f0']
+
+    >>> [rgb2hex(hsl2rgb(hsl)) #luminosity interpolation
     ...  for hsl in color_scale((0, 0, 0),
     ...                         (0, 0, 1),
     ...                         15)]  # doctest: +ELLIPSIS
@@ -718,16 +754,19 @@ def color_scale(begin_hsl, end_hsl, nb):
         raise ValueError(
             "Unsupported negative number of colors (nb=%r)." % nb)
 
-    step = tuple([float(end_hsl[i] - begin_hsl[i]) / nb for i in range(0, 3)]) \
-           if nb > 0 else (0, 0, 0)
-
-    def mul(step, value):
-        return tuple([v * value for v in step])
-
-    def add_v(step, step2):
-        return tuple([v + step2[i] for i, v in enumerate(step)])
-
-    return [add_v(begin_hsl, mul(step, r)) for r in range(0, nb + 1)]
+    h1,s1,l1 = begin_hsl
+    h2,s2,l2 = end_hsl
+    if longer == (abs(h1-h2)<0.5):
+        if h1<h2:
+            h1+=1
+        else:
+            h2+=1
+    return [(
+        (h1*(1-v) + h2*v)%1,
+        s1*(1-v) + s2*v,
+        l1*(1-v) + l2*v)
+        for v in (float(step)/(nb) for step in range(nb+1))
+    ]
 
 
 ##
@@ -1085,8 +1124,8 @@ def set_web(self, value):
 
     ## range of color generation
 
-    def range_to(self, value, steps):
-        for hsl in color_scale(self._hsl, Color(value).hsl, steps - 1):
+    def range_to(self, value, steps, longer=False):
+        for hsl in color_scale(self._hsl, Color(value).hsl, steps - 1, longer=longer):
             yield Color(hsl=hsl)
 
     ##