Add new sort field for length of a todo item

test/data/SorterTest14-result.txt

+1 No length (zero)
+2016-04-25 2 No length (zero)
+3 Length of 0 days t:2016-04-25 due:2016-04-25
+4 Length of 0 days t:2016-04-25 due:2016-04-25
+5 Length of 0 days tomorrow t:2016-04-26 due:2016-04-26
+2016-04-25 1 Length of 1 day with creation due:2016-04-26
+2 Length of 1 day t:2016-04-25 due:2016-04-26
+2016-04-25 Length of 2 days with creation due:2016-04-27

test/data/SorterTest14.txt

+4 Length of 0 days t:2016-04-25 due:2016-04-25
+2016-04-25 2 No length (zero)
+3 Length of 0 days t:2016-04-25 due:2016-04-25
+5 Length of 0 days tomorrow t:2016-04-26 due:2016-04-26
+2 Length of 1 day t:2016-04-25 due:2016-04-26
+2016-04-25 Length of 2 days with creation due:2016-04-27
+2016-04-25 1 Length of 1 day with creation due:2016-04-26
+1 No length (zero)

test/test_sorter.py

@@ -14,6 +14,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+from freezegun import freeze_time
 import unittest
 
 from test.facilities import (load_file, load_file_to_todolist, print_view,
@@ -23,6 +24,7 @@ from topydo.lib.Config import config
 from topydo.lib.Sorter import Sorter
 
 
+@freeze_time("2016, 04, 25")
 class SorterTest(TopydoTest):
     def sort_file(self, p_filename, p_filename_ref, p_sorter):
         """
@@ -189,5 +191,14 @@ class SorterTest(TopydoTest):
                        'test/data/SorterTest13-result-context.txt', sorter)
 
 
+    def test_sort18(self):
+        """
+        Check sorting by length.
+        """
+        sorter = Sorter('length,text')
+
+        self.sort_file('test/data/SorterTest14.txt',
+                       'test/data/SorterTest14-result.txt', sorter)
+
 if __name__ == '__main__':
     unittest.main()

topydo/lib/Sorter.py

@@ -52,6 +52,8 @@ def get_field_function(p_field):
         result = importance
     elif p_field == 'importance-avg' or p_field == 'importance-average':
         result = average_importance
+    elif p_field == 'length':
+        result = lambda a: a.length()
     elif p_field == 'project' or p_field == 'projects':
         result = lambda a: sorted([c.lower() for c in a.projects()])
     elif p_field == 'text':