do not catch import error when reader is not installed, return '' for empty ods cells instead of nan

Author: davidovitch

pandas/io/excel.py

@@ -107,8 +107,6 @@ def load_engine(self):
         try:
             self._load_engine()
             _readers[self.engine] = True
-        except ImportError:
-            _readers[self.engine] = False
         except AttributeError:
             _readers[self.engine] = False
             msg = 'Excel engine "%s" is not implemented' % self.engine
@@ -574,8 +572,12 @@ def _parse_cell(cell):
                     value = cell.value
             elif isinstance(cell.value, bool):
                 value = cell.value
-#            elif isinstance(cell.value, type(None)):
-#                value = np.nan
+            # empty cells have None as value, type, currency, formula.
+            # xlrd assigns empty string to empty cells, ezodf assigns None
+            # test_excel.ExcelReaderTests.test_reader_converters expects empty
+            # cells to be an empty string
+            elif isinstance(cell.value, type(None)):
+                value = ''
             else:
                 value = np.nan
             return value