Configurable cell_class in DataTable

There are two changes in this proposal. One is to move the logic
for extracting cell data from the row datum (and handling auto columns)
to the Cell class, into a separate method for easy extending. The
second is to make that extending possible by making the cell class
configurable in the DataTable's meta.

Those changes are supposed to make it easier to extend the DataTable
with new kinds of cells -- for example, cells that contain an editable
widget, a progress bar, an icon, etc. as well as make it easier to
modify the behavior of the auto columns -- for example, by making it
possible to have some of the multi_select checkboxes preselected. Those
changes would also make it easier to integrate DataTable with a Django
FormSet.

Right now, in order to modify any of the proposed Cell.get_data
behavior, one has to make their own Row class and practically copy-paste
the whole Row.load_cells method, as there is no easy way to extend it.

Change-Id: I29277a9e77e1c413193fe80d3f8cfe001bf5d709
Closes-Bug: #1229677

horizon/tables/base.py

@@ -500,44 +500,7 @@ def load_cells(self, datum=None):
             datum = self.datum
         cells = []
         for column in table.columns.values():
-            if column.auto == "multi_select":
-                widget = forms.CheckboxInput(check_test=lambda value: False)
-                # Convert value to string to avoid accidental type conversion
-                data = widget.render('object_ids',
-                                     unicode(table.get_object_id(datum)),
-                                     {'class': 'table-row-multi-select'})
-                table._data_cache[column][table.get_object_id(datum)] = data
-            elif column.auto == "form_field":
-                widget = column.form_field
-                if issubclass(widget.__class__, forms.Field):
-                    widget = widget.widget
-
-                widget_name = "%s__%s" % \
-                    (column.name,
-                     unicode(table.get_object_id(datum)))
-
-                # Create local copy of attributes, so it don't change column
-                # class form_field_attributes
-                form_field_attributes = {}
-                form_field_attributes.update(column.form_field_attributes)
-                # Adding id of the input so it pairs with label correctly
-                form_field_attributes['id'] = widget_name
-
-                data = widget.render(widget_name,
-                                     column.get_data(datum),
-                                     form_field_attributes)
-                table._data_cache[column][table.get_object_id(datum)] = data
-            elif column.auto == "actions":
-                data = table.render_row_actions(datum)
-                table._data_cache[column][table.get_object_id(datum)] = data
-            else:
-                data = column.get_data(datum)
-
-            cell = Cell(datum, data, column, self)
-            if cell.inline_edit_available:
-                cell.attrs['data-cell-name'] = column.name
-                cell.attrs['data-update-url'] = cell.get_ajax_update_url()
-
+            cell = table._meta.cell_class(datum, column, self)
             cells.append((column.name or column.auto, cell))
         self.cells = SortedDict(cells)
 
@@ -609,21 +572,61 @@ def get_data(self, request, obj_id):
 
 class Cell(html.HTMLElement):
     """Represents a single cell in the table."""
-    def __init__(self, datum, data, column, row, attrs=None, classes=None):
+
+    def __init__(self, datum, column, row, attrs=None, classes=None):
         self.classes = classes or getattr(self, "classes", [])
         super(Cell, self).__init__()
         self.attrs.update(attrs or {})
 
         self.datum = datum
-        self.data = data
         self.column = column
         self.row = row
         self.wrap_list = column.wrap_list
         self.inline_edit_available = self.column.update_action is not None
         # initialize the update action if available
         if self.inline_edit_available:
             self.update_action = self.column.update_action()
+            self.attrs['data-cell-name'] = column.name
+            self.attrs['data-update-url'] = self.get_ajax_update_url()
         self.inline_edit_mod = False
+        self.data = self.get_data(datum, column, row)
+
+    def get_data(self, datum, column, row):
+        """Fetches the data to be displayed in this cell."""
+        table = row.table
+        if column.auto == "multi_select":
+            widget = forms.CheckboxInput(check_test=lambda value: False)
+            # Convert value to string to avoid accidental type conversion
+            data = widget.render('object_ids',
+                                 unicode(table.get_object_id(datum)),
+                                 {'class': 'table-row-multi-select'})
+            table._data_cache[column][table.get_object_id(datum)] = data
+        elif column.auto == "form_field":
+            widget = column.form_field
+            if issubclass(widget.__class__, forms.Field):
+                widget = widget.widget
+
+            widget_name = "%s__%s" % \
+                (column.name,
+                 unicode(table.get_object_id(datum)))
+
+            # Create local copy of attributes, so it don't change column
+            # class form_field_attributes
+            form_field_attributes = {}
+            form_field_attributes.update(column.form_field_attributes)
+            # Adding id of the input so it pairs with label correctly
+            form_field_attributes['id'] = widget_name
+
+            data = widget.render(widget_name,
+                                 column.get_data(datum),
+                                 form_field_attributes)
+            table._data_cache[column][table.get_object_id(datum)] = data
+        elif column.auto == "actions":
+            data = table.render_row_actions(datum)
+            table._data_cache[column][table.get_object_id(datum)] = data
+        else:
+            data = column.get_data(datum)
+        return data
 
     def __repr__(self):
         return '<%s: %s, %s>' % (self.__class__.__name__,
@@ -812,6 +815,11 @@ class DataTableOptions(object):
         The row status is used by other Horizon components to trigger tasks
         such as dynamic AJAX updating.
 
+    .. attribute:: cell_class
+
+        The class which should be used for rendering the cells of this table.
+        Optional. Default: :class:`~horizon.tables.Cell`.
+
     .. attribute:: row_class
 
         The class which should be used for rendering the rows of this table.
@@ -857,6 +865,7 @@ def __init__(self, options):
         self.status_columns = getattr(options, 'status_columns', [])
         self.table_actions = getattr(options, 'table_actions', [])
         self.row_actions = getattr(options, 'row_actions', [])
+        self.cell_class = getattr(options, 'cell_class', Cell)
         self.row_class = getattr(options, 'row_class', Row)
         self.column_class = getattr(options, 'column_class', Column)
         self.pagination_param = getattr(options, 'pagination_param', 'marker')