loaders doc: added information about expanders/reducers declaration precendece, and other minor improvements

docs/experimental/loaders.rst

@@ -34,8 +34,9 @@ Then, you start adding values to the Loader, typically collecting them using
 same item field, the Loader will know how to "join" those values later using a
 Reducer.
 
-Here is a typical Loader usage in a :ref:`Spider <topics-spiders>` using the
-:ref:`Product item defined in the Items chapter <topics-newitems-declaring>`.::
+Here is a typical Loader usage in a :ref:`Spider <topics-spiders>`, using the
+:ref:`Product item <topics-newitems-declaring>` declared in the :ref:`Items
+section <topics-newitems>`::
 
     from scrapy.item.loader import XPathLoader
     from scrapy.xpath import HtmlXPathSelector
@@ -47,14 +48,14 @@ Here is a typical Loader usage in a :ref:`Spider <topics-spiders>` using the
         l.add_xpath('name', '//div[@class="product_title"]')
         l.add_xpath('price', '//p[@id="price"]')
         l.add_xpath('stock', '//p[@id="stock"]')
-        l.add_value('last_updated', 'today') # you can also literal values
+        l.add_value('last_updated', 'today') # you can also use literal values
         return l.get_item()
 
 By quickly looking at that code we can see the ``name`` field is being
 extracted from two different XPath locations in the page:
 
-* ``//div[@class="product_name"]``
-* ``//div[@class="product_title"]``
+1. ``//div[@class="product_name"]``
+2. ``//div[@class="product_title"]``
 
 In other words, data is being collected by extracting it from two XPath
 locations, using the :meth:`~XPathLoader.add_xpath` method. This is the data
@@ -74,14 +75,15 @@ previously extracted and collected with the :meth:`~XPathLoader.add_xpath` and
 Expanders and Reducers
 ======================
 
-A Loader is composed of one expander and one reducer for each item field. The
+A Loader is composed of one expander and one reducer for each (item) field. The
 Expander processes the extracted data as soon as it's received (through the
 :meth:`~XPathLoader.add_xpath` or :meth:`~Loader.add_value` methods) and the
 result of the expander is collected and kept inside the Loader. After
 collecting all data, the :meth:`Loader.get_item` method is called to actually
-populate and get the Item.  That's when the Reducers are called with the data
-previously collected (using the Expanders) and the output of the Reducers are
-the actual values that get assigned to the item.
+populate and get the :class:`~scrapy.newitem.Item` object.  That's when the
+Reducers are called with the data previously collected (using the Expanders)
+and the output of the Reducers are the actual values that get assigned to the
+item.
 
 Let's see an example to illustrate how Expanders and Reducers are called, for a
 particular field (the same applies for any other field)::
@@ -135,6 +137,45 @@ As you can see, expanders are declared using the ``_exp`` suffix while reducers
 are declared using the ``_red`` suffix. And you can also declare a default
 expander using the :attr:`Loader.default_expander` attribute.
 
+.. _topics-loader-expred-declaring:
+
+Declaring Extenders and Reducers
+================================
+
+As seen in the previous section, extenders and reducers can be declared in the
+Loader definition, and it's very common to declare expanders this way. However,
+there is one more place where you can specify the exanders and reducers to use:
+in the :ref:`Item Field <topics-newitem-fields` metadata. Here is an example::
+
+    from scrapy.newitem import Item, Field
+    from scrapy.newitem.loader.expanders import TreeExpander
+    from scrapy.newitem.loader.reducers import Join, TakeFirst
+
+    from scrapy.utils.markup import remove_entities
+    from myprojct.utils import filter_prices
+
+    class Product(Item):
+        name = Field(
+            expander=TreeExpander(remove_entities),
+            reducer=Join(),
+        )
+        price = Field(
+            default=0,
+            expander=TreeExpander(remove_entities, filter_prices),
+            reducer=TakeFirst(),
+        )
+
+The precendece order, for both expander and reducer declarations, is as
+follows:
+
+1. Loader field-specific attributes: ``field_exp`` and ``field_red`` (more
+   precedence)
+2. Field metadata (``expander`` and ``reducer`` key)
+3. Loader defaults: :meth:`Loader.default_expander` and
+   :meth:`Loader.default_reducer` (less precedence)
+
+See also: :ref:`topics-loader-extending`.
+
 .. _topics-loader-args:
 
 Item Loader arguments
@@ -260,22 +301,27 @@ Loader objects
         in which case this argument is ignored.
     :type response: :class:`~scrapy.http.Response` object
 
-    .. attribute:: default_selector_class
-
-        The class used to construct the selector, if only a response is given
-        in the constructor
-
     .. method:: add_xpath(field_name, xpath, \**new_loader_args)
 
         Similar to :meth:`Loader.add_value` but receives an XPath instead of a
         value, which is used to extract a list of unicode strings from the
         selector associated with this :class:`XPathLoader`.
 
+        Example::
+
+            loader.add_xpath('name', '//p[@class="product-name"]')
+
     .. method:: replace_xpath(field_name, xpath, \**new_loader_args)
 
         Similar to :meth:`add_xpath` but replaces collected data instead of
         adding it.
 
+    .. attribute:: default_selector_class
+
+        The class used to construct the selector, if only a response is given
+        in the constructor
+
+.. _topics-loader-extending:
 
 Reusing and extending Loaders
 =============================
@@ -312,6 +358,12 @@ want to remove ``CDATA`` occurrences. Here's an example of how to do it::
     class XmlLoader(ProductLoader):
         name_exp = TreeExpander(remove_cdata, ProductLoader.name_exp)
 
+And that's how you typically extend expanders.
+
+As for reducers, it is more common to declare them in the field metadata, as
+they usually depend only on the field and not on each specific site parsing
+rule (as expanders do). See also: :ref:`topics-loader-expred-declaring`.
+
 There are many other possible ways to extend, inherit and override your
 Loaders, and different Loader hierarchies may fit better for different
 projects. Scrapy only provides the mechanism, it doesn't impose any specific

docs/experimental/newitems.rst

@@ -40,6 +40,11 @@ objects. Here is an example::
 .. _Django: http://www.djangoproject.com/
 .. _Django Models: http://docs.djangoproject.com/en/dev/topics/db/models/
 
+.. _topics-newitems-fields:
+
+Item Fields
+===========
+
 :class:`Field` objects are used to specify metadata for each field. For
 example, the default value for the ``stock`` field illustrated in the example
 above.