Pager helper to enable custom field getter (#287)

* Refactor to enable custom field getter

Example custom field getter for a response with 
```
def customGetter(message, attribute):
  del attribute # Unused argument
  for book in message.bookshelf:
     yield book
  for book in message.library:
     yield book
```

2 files changed, 31 insertions, 2 deletions

diff --git a/apitools/base/py/list_pager.py b/apitools/base/py/list_pager.py
index b755755..a2c1080 100644
--- a/apitools/base/py/list_pager.py
+++ b/apitools/base/py/list_pager.py
@@ -67,7 +67,8 @@ def YieldFromList(
         method='List', field='items', predicate=None,
         current_token_attribute='pageToken',
         next_token_attribute='nextPageToken',
-        batch_size_attribute='maxResults'):
+        batch_size_attribute='maxResults',
+        get_field_func=_GetattrNested):
     """Make a series of List requests, keeping track of page tokens.
 
     Args:
@@ -94,6 +95,8 @@ def YieldFromList(
           response message holding the maximum number of results to be
           returned. None if caller-specified batch size is unsupported.
           If a tuple, path to the attribute.
+      get_field_func: Function that returns the items to be yielded. Argument
+          is response message, and field.
 
     Yields:
       protorpc.message.Message, The resources listed by the service.
@@ -116,7 +119,7 @@ def YieldFromList(
             _SetattrNested(request, batch_size_attribute, request_batch_size)
         response = getattr(service, method)(request,
                                             global_params=global_params)
-        items = _GetattrNested(response, field)
+        items = get_field_func(response, field)
         if predicate:
             items = list(filter(predicate, items))
         for item in items:
diff --git a/apitools/base/py/list_pager_test.py b/apitools/base/py/list_pager_test.py
index f644146..e1141ae 100644
--- a/apitools/base/py/list_pager_test.py
+++ b/apitools/base/py/list_pager_test.py
@@ -268,6 +268,32 @@ class ListPagerTest(unittest2.TestCase):
 
         self._AssertInstanceSequence(results, 3)
 
+    def testYieldFromListWithCustomGetFieldFunction(self):
+        self.mocked_client.column.List.Expect(
+            messages.FusiontablesColumnListRequest(
+                maxResults=100,
+                pageToken=None,
+                tableId='mytable',
+            ),
+            messages.ColumnList(
+                items=[
+                    messages.Column(name='c0')
+                ]
+            ))
+        custom_getter_called = []
+
+        def Custom_Getter(message, attribute):
+            custom_getter_called.append(True)
+            return getattr(message, attribute)
+
+        client = fusiontables.FusiontablesV1(get_credentials=False)
+        request = messages.FusiontablesColumnListRequest(tableId='mytable')
+        results = list_pager.YieldFromList(
+            client.column, request, get_field_func=Custom_Getter)
+
+        self._AssertInstanceSequence(results, 1)
+        self.assertEquals(1, len(custom_getter_called))
+
 
 class ListPagerAttributeTest(unittest2.TestCase):