add FieldKeeper filter

saucebrush/filters.py

@@ -215,6 +215,24 @@ class FieldModifier(FieldFilter):
                                  str(self._target_keys), str(self._filter_func))
 
 
+class FieldKeeper(Filter):
+    """ Filter that removes all but the given set of fields.
+    
+        FieldKeeper(('spam', 'eggs')) removes all bu tthe spam and eggs
+        fields from every record filtered.
+    """
+    
+    def __init__(self, keys):
+        super(FieldKeeper, self).__init__()
+        self._target_keys = utils.str_or_list(keys)
+    
+    def process_record(self, record):
+        for key in record.keys():
+            if key not in self._target_keys:
+                del record[key]
+        return record
+
+
 class FieldRemover(Filter):
     """ Filter that removes a given set of fields.
 

saucebrush/tests/filters.py

@@ -3,7 +3,7 @@ import operator
 import types
 from saucebrush.filters import (Filter, YieldFilter, FieldFilter,
                                 SubrecordFilter, ConditionalPathFilter,
-                                ConditionalFilter, FieldModifier,
+                                ConditionalFilter, FieldModifier, FieldKeeper,
                                 FieldRemover, FieldMerger, FieldAdder,
                                 FieldCopier, FieldRenamer, Unique)
 
@@ -200,6 +200,13 @@ class FilterTestCase(unittest.TestCase):
                          {'a':2, 'b':10, 'c':200}]
         self.assert_filter_result(fm, expected_data)
 
+    def test_field_keeper(self):
+        fk = FieldKeeper(['c'])
+        
+        # check against expected results
+        expected_data = [{'c':3}, {'c':5}, {'c':100}]
+        self.assert_filter_result(fk, expected_data)
+
     def test_field_remover(self):
         fr = FieldRemover(['a', 'b'])