SEP | 3 |
Title | List fields API |
Author | Pablo Hoffman |
Created | 2009-07-21 |
Status | Obsolete by :ref:`sep-008` |
This page presents different usage scenarios for the new multi-valued field, called !ListField.
#!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)
#!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:
#!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))
#!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]
#!python i = Article() i['categories'] # raises KeyError i.get('categories') # returns None i['numbers'] # returns []
#!python i = Article() i['categories'] = ['one', 'two'] i['categories'].append(3) # XXX: should this fail?