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我构建了复杂的项目，其中字段可能是其他项目类型的列表。当我使用默认值导出它时，XmlItemExporter子列表项以<value>标签为前缀。我正在寻找如何将子项目标识符分配给这些值标签的示例。

文档的 Item Exporters 页面解释了这句话：

除非在方法中被覆盖，否则通过序列化元素serialize_field()内的每个值来导出多值字段。<value>这是为了方便，因为多值字段很常见。

文档页面还提供了在字段中声明序列化程序和覆盖 Serialize_Field() 方法的简单示例，但两者都适用于单值字段，没有建议如何为多值字段自定义它们。

我在网上搜索了一个如何完成的示例，但我没有找到任何示例。

这是我用于测试的示例项目树：

class Course(scrapy.Item):
    title = scrapy.Field()
    lessons = scrapy.Field()

class Lesson(scrapy.Item):
    session = scrapy.Field()
    topic = scrapy.Field()
    assignment = scrapy.Field()

class ReadingAssignment(scrapy.Item):
    textBook = scrapy.Field()
    pages = scrapy.Field()

course = Course()
course['title'] = 'Greatness'
course['lessons'] = []

lesson = Lesson()
lesson['session'] = 'Week 1'
lesson['topic'] = 'Think Great'
lesson['assignment'] = []

reading =  ReadingAssignment()
reading['textBook'] = 'Great Book 1'
reading['pages'] = '1-20'
lesson['assignment'].append(reading)
course['lessons'].append(lesson)

lesson = Lesson()
lesson['session'] = 'Week 2'
lesson['topic'] = 'Act Great'
lesson['assignment'] = []

reading =  ReadingAssignment()
reading['textBook'] = 'Great Book 2'
reading['pages'] = '21-40'
lesson['assignment'].append(reading)
course['lessons'].append(lesson)

lesson = Lesson()
lesson['session'] = 'Week 3'
lesson['topic'] = 'Look Great'
lesson['assignment'] = []

reading =  ReadingAssignment()
reading['textBook'] = 'Great Book 3'
reading['pages'] = '41-60'
lesson['assignment'].append(reading)
course['lessons'].append(lesson)

lesson = Lesson()
lesson['session'] = 'Week 4'
lesson['topic'] = 'Be Great'
lesson['assignment'] = []

reading =  ReadingAssignment()
reading['textBook'] = 'Great Book 4'
reading['pages'] = '61-80'
lesson['assignment'].append(reading)
course['lessons'].append(lesson)

输出：

>>> course
{'lessons': [{'assignment': [{'pages': '1-20', 'textBook': 'Great Book 1'}],
              'session': 'Week 1',
              'topic': 'Think Great'},
             {'assignment': [{'pages': '21-40', 'textBook': 'Great Book 2'}],
              'session': 'Week 2',
              'topic': 'Act Great'},
             {'assignment': [{'pages': '41-60', 'textBook': 'Great Book 3'}],
              'session': 'Week 3',
              'topic': 'Look Great'},
             {'assignment': [{'pages': '61-80', 'textBook': 'Great Book 4'}],
              'session': 'Week 4',
              'topic': 'Be Great'}],
 'title': 'Greatness'}

当我运行它时，XmlItemExporter我得到：

<?xml version="1.0" encoding="utf-8"?>
<items>
  <course>
    <title>Greatness</title>
    <lessons>
      <value>
        <session>Week 1</session>
        <topic>Think Great</topic>
        <assignment>
          <value>
            <textBook>Great Book 1</textBook>
            <pages>1-20</pages>
          </value>
        </assignment>
      </value>
      <value>
        <session>Week 2</session>
        <topic>Act Great</topic>
        <assignment>
          <value>
            <textBook>Great Book 2</textBook>
            <pages>21-40</pages>
          </value>
        </assignment>
      </value>
      <value>
        <session>Week 3</session>
        <topic>Look Great</topic>
        <assignment>
          <value>
            <textBook>Great Book 3</textBook>
            <pages>41-60</pages>
          </value>
        </assignment>
      </value>
      <value>
        <session>Week 4</session>
        <topic>Be Great</topic>
        <assignment>
          <value>
            <textBook>Great Book 4</textBook>
            <pages>61-80</pages>
          </value>
        </assignment>
      </value>
    </lessons>
  </course>
</items>

我想做的是将这些<value>标签更改为附加到列表中的项目的名称。像这样：

<items>
  <course>
    <title>Greatness</title>
    <lessons>
      <lesson>
        <session>Week 1</session>
        <topic>Think Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 1</textBook>
            <pages>1-20</pages>
          </reading>
        </assignment>
      </lesson>
      <lesson>
        <session>Week 2</session>
        <topic>Act Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 2</textBook>
            <pages>21-40</pages>
          </reading>
        </assignment>
      </lesson>
      <lesson>
        <session>Week 3</session>
        <topic>Look Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 3</textBook>
            <pages>41-60</pages>
          </reading>
        </assignment>
      </lesson>
      <lesson>
        <session>Week 4</session>
        <topic>Be Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 4</textBook>
            <pages>61-80</pages>
          </reading>
        </assignment>
      </lesson>
    </lessons>
  </course>
</items>
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1 回答 1

2

这确实没有很好的记录，我们将不得不求助于阅读XmlItemExporter源代码，结果发现<value>标签选择已在XmlItemExporter._export_xml_field()方法中进行了硬编码：

elif is_listlike(serialized_value):
    self._beautify_newline()
    for value in serialized_value:
        self._export_xml_field('value', value, depth=depth+1)
    self._beautify_indent(depth=depth)

幸运的是，在前面的几行中，有一条出路：

if hasattr(serialized_value, 'items'):
    self._beautify_newline()
    for subname, value in serialized_value.items():
        self._export_xml_field(subname, value, depth=depth+1)
    self._beautify_indent(depth=depth)

这意味着处理字典，但实际上它会接受任何具有.items()返回字符串和项目元组的方法的东西！

但是，导出器中缺少一个重要步骤：递归。您基本上只能serializer在顶级项目字段上设置标志，当前 Scrapy 实现完全忽略顶级项目之外的子类Field()上的任何元素。每个导出器在如何驱动内部方法Item方面都有自己的特点，因此我们不能预先处理递归，因为每个特定的导出器（JSON、XML 等）在需要序列化字段的方式上有所不同。我们可以使用该类的子类来解决这个问题，更多内容如下。BaseItemExporter._get_serialized_fields()XmlItemExporter

所以这里的第一个技巧是创建一个具有.items()方法并为您提供<container>标签的专用对象。请注意，您必须自己处理序列化的递归！Scrapy 序列化程序本身并不处理递归到嵌套结构：

class CustomXMLValuesSerializer:
    @classmethod
    def serialize_as(cls, name):
        def serializer(items, serialize):
            return cls(name, items, serialize)
        return serializer

    def __init__(self, name, items, serialize=None):
        self._name = name
        self._items = items
        self._serialize = serialize if serialise is not None else lambda x: x

    def items(self):
        for item in self._items:
            yield (self._name, self._serialize(item))

然后使用CustomXMLValuesSerializer.serialize_as()类方法为您的列表字段创建自定义序列化程序：

class Course(scrapy.Item):
    title = scrapy.Field()
    lessons = scrapy.Field(
        serializer=CustomXMLValuesSerializer.serialize_as("lesson")
    )

class Lesson(scrapy.Item):
    session = scrapy.Field()
    topic = scrapy.Field()
    assignment = scrapy.Field(
        serializer=CustomXMLValuesSerializer.serialize_as("reading")
    )

class ReadingAssignment(scrapy.Item):
    textBook = scrapy.Field()
    pages = scrapy.Field()

最后，我们需要一个稍微定制的导出器，它实际上可以让我们递归处理嵌套项：

from functools import partial

class RecursingXmlItemExporter(XmlItemExporter):
    def _recursive_serialized_fields(self, item):
        if isinstance(item, scrapy.Item):
            return dict(self._get_serialized_fields(item, default_value=''))
        return item

    def serialize_field(self, field, name, value):
        serializer = field.get('serializer', lambda x: x)
        try:
            return serializer(value, self._recursive_serialized_fields)
        except TypeError:
            return serializer(value)

请注意，这是传入的default_value=''，因为这是基本XmlItemExporter.export_item()实现使用的。

确保使用此自定义导出器，因为它传入所需的上下文以序列化嵌套项：

exporter = RecursingXmlItemExporter(some_file, indent=2, item_element='course')
exporter.start_exporting()
exporter.export_item(course)
exporter.finish_exporting()

现在容器实际上是使用name字符串作为容器元素导出的：

<?xml version="1.0" encoding="utf-8"?>
<items>
  <course>
    <title>Greatness</title>
    <lessons>
      <lesson>
        <session>Week 1</session>
        <topic>Think Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 1</textBook>
            <pages>1-20</pages>
          </reading>
        </assignment>
      </lesson>
      <lesson>
        <session>Week 2</session>
        <topic>Act Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 2</textBook>
            <pages>21-40</pages>
          </reading>
        </assignment>
      </lesson>
      <lesson>
        <session>Week 3</session>
        <topic>Look Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 3</textBook>
            <pages>41-60</pages>
          </reading>
        </assignment>
      </lesson>
      <lesson>
        <session>Week 4</session>
        <topic>Be Great</topic>
        <assignment>
          <reading>
            <textBook>Great Book 4</textBook>
            <pages>61-80</pages>
          </reading>
        </assignment>
      </lesson>
    </lessons>
  </course>
</items>

我使用 Scrapy解决问题 #3888以查看该项目是否有兴趣Item更好地支持嵌套结构。

另一种方法是通过对该方法的单独调用来导出嵌套项 XmlItemExporter.export_item()，但这要求导出器可以作为与序列化程序相同的命名空间中的全局访问，或者您将导出器子类化并...传递导出器到序列化器。XmlItemExporter.export_item()然后你必须满足于硬编码缩进的事实。

于 2019-07-19T11:03:14.970 回答