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