TST: Add parser for date values from answer key.

Refactor the reading of values from the Excel spreadsheet so that
parsers are configurable by index.

Needed so that we can parse columns that have dates, in addition
to floats as previously.

tests/risk/answer_key.py

@@ -12,10 +12,10 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 import hashlib
 import os
 
+import numpy as np
 import xlrd
 import requests
 
@@ -117,11 +117,13 @@ class DataIndex(object):
     The python-excel libraries use 0 index, while the spreadsheet in a GUI
     uses a 1 index.
     """
-    def __init__(self, sheet_name, col, row_start, row_end):
+    def __init__(self, sheet_name, col, row_start, row_end,
+                 value_type='float'):
         self.sheet_name = sheet_name
         self.col = col
         self.row_start = row_start
         self.row_end = row_end
+        self.value_type = value_type
 
     @property
     def col_index(self):
@@ -230,8 +232,25 @@ class AnswerKey(object):
             else:
                 setattr(self, name, self.get_values(index))
 
-    def get_values(self, data_index):
+    def parse_date_value(self, value):
+        return xlrd.xldate_as_tuple(value, 0)
+
+    def parse_float_value(self, value):
+        return value if value != '' else np.nan
+
+    def get_raw_values(self, data_index):
         return self.sheets[data_index.sheet_name].col_values(
             data_index.col_index,
             data_index.row_start_index,
             data_index.row_end_index + 1)
+
+    @property
+    def value_type_to_value_func(self):
+        return {
+            'float': self.parse_float_value,
+            'date': self.parse_date_value,
+        }
+
+    def get_values(self, data_index):
+        value_parser = self.value_type_to_value_func[data_index.value_type]
+        return map(value_parser, self.get_raw_values(data_index))