Merge pull request #3671 from Gallaecio/item-deepcopy

Implement Item.deepcopy()

docs/topics/items.rst

@@ -40,6 +40,7 @@ objects. Here is an example::
         name = scrapy.Field()
         price = scrapy.Field()
         stock = scrapy.Field()
+        tags = scrapy.Field()
         last_updated = scrapy.Field(serializer=str)
 
 .. note:: Those familiar with `Django`_ will notice that Scrapy Items are
@@ -155,19 +156,42 @@ To access all populated values, just use the typical `dict API`_::
     >>> product.items()
     [('price', 1000), ('name', 'Desktop PC')]
 
+
+Copying items
+-------------
+
+To copy an item, you must first decide whether you want a shallow copy or a
+deep copy.
+
+If your item contains mutable_ values like lists or dictionaries, a shallow
+copy will keep references to the same mutable values across all different
+copies.
+
+.. _mutable: https://docs.python.org/glossary.html#term-mutable
+
+For example, if you have an item with a list of tags, and you create a shallow
+copy of that item, both the original item and the copy have the same list of
+tags. Adding a tag to the list of one of the items will add the tag to the
+other item as well.
+
+If that is not the desired behavior, use a deep copy instead.
+
+See the `documentation of the copy module`_ for more information.
+
+.. _documentation of the copy module: https://docs.python.org/library/copy.html
+
+To create a shallow copy of an item, you can either call
+:meth:`~scrapy.item.Item.copy` on an existing item
+(``product2 = product.copy()``) or instantiate your item class from an existing
+item (``product2 = Product(product)``).
+
+To create a deep copy, call :meth:`~scrapy.item.Item.deepcopy` instead
+(``product2 = product.deepcopy()``).
+
+
 Other common tasks
 ------------------
 
-Copying items::
-
-    >>> product2 = Product(product)
-    >>> print(product2)
-    Product(name='Desktop PC', price=1000)
-
-    >>> product3 = product2.copy()
-    >>> print(product3)
-    Product(name='Desktop PC', price=1000)
-
 Creating dicts from items::
 
     >>> dict(product) # create a dict from all populated values

scrapy/item.py

@@ -6,6 +6,7 @@ See documentation in docs/topics/item.rst
 
 from pprint import pformat
 from collections import MutableMapping
+from copy import deepcopy
 
 from abc import ABCMeta
 import six
@@ -96,6 +97,13 @@ class DictItem(MutableMapping, BaseItem):
     def copy(self):
         return self.__class__(self)
 
+    def deepcopy(self):
+        """Return a `deep copy`_ of this item.
+
+        .. _deep copy: https://docs.python.org/library/copy.html#copy.deepcopy
+        """
+        return deepcopy(self)
+
 
 @six.add_metaclass(ItemMeta)
 class Item(DictItem):

tests/test_item.py

@@ -249,6 +249,14 @@ class ItemTest(unittest.TestCase):
         copied_item['name'] = copied_item['name'].upper()
         self.assertNotEqual(item['name'], copied_item['name'])
 
+    def test_deepcopy(self):
+        class TestItem(Item):
+            tags = Field()
+        item = TestItem({'tags': ['tag1']})
+        copied_item = item.deepcopy()
+        item['tags'].append('tag2')
+        assert item['tags'] != copied_item['tags']
+
 
 class ItemMetaTest(unittest.TestCase):