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======= ==========================
SEP 3
Title List fields API
Author Pablo Hoffman
Created 2009-07-21
Status Obsolete by :ref:`sep-008`
======= ==========================
=========================
SEP-002 - List fields API
=========================
This page presents different usage scenarios for the new multi-valued field,
called !ListField.
Proposed Implementation
=======================
.. code-block:: python
#!python
from scrapy.item.fields import BaseField
class ListField(BaseField):
def __init__(self, field, default=None):
self._field = field
super(ListField, self).__init__(default)
def to_python(self, value):
if hasattr(value, "__iter__"): # str/unicode not allowed
return [self._field.to_python(v) for v in value]
else:
raise TypeError("Expected iterable, got %s" % type(value).__name__)
def get_default(self):
# must return a new copy to avoid unexpected behaviors with mutable defaults
return list(self._default)
Usage Scenarios
===============
Defining a list field
---------------------
.. code-block:: python
#!python
from scrapy.item.models import Item
from scrapy.item.fields import ListField, TextField, DateField, IntegerField
class Article(Item):
categories = ListField(TextField)
dates = ListField(DateField, default=[])
numbers = ListField(IntegerField, [])
Another case of products and variants which highlights the fact that it's
important to instantiate !ListField with field instances, not classes:
.. code-block:: python
#!python
from scrapy.item.models import Item
from scrapy.item.fields import ListField, TextField
class Variant(Item):
name = TextField()
class Product(Variant):
variants = ListField(ItemField(Variant))
Assigning a list field
----------------------
.. code-block:: python
#!python
i = Article()
i["categories"] = []
i["categories"] = ["politics", "sport"]
i["categories"] = ["test", 1] # -> raises TypeError
i["categories"] = asd # -> raises TypeError
i["dates"] = []
i["dates"] = ["2009-01-01"] # raises TypeError? (depends on TextField)
i["numbers"] = ["1", 2, "3"]
i["numbers"] # returns [1, 2, 3]
Default values
--------------
.. code-block:: python
#!python
i = Article()
i["categories"] # raises KeyError
i.get("categories") # returns None
i["numbers"] # returns []
Appending values
----------------
.. code-block:: python
#!python
i = Article()
i["categories"] = ["one", "two"]
i["categories"].append(3) # XXX: should this fail?
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