Question

Item Loaders提供了一种便捷的方式填充抓取到的 :Items 。 虽然Items可以使用自带的类字典形式API填充，但是Items Loaders提供了更便捷的API， 可以分析原始数据并对Item进行赋值。

从另一方面来说， Items 提供保存抓取数据的 容器 ， 而 Item Loaders提供的是 填充 容器的机制。

Item Loaders提供的是一种灵活，高效的机制，可以更方便的被spider或source format (HTML, XML, etc)扩展，并override更易于维护的、不同的内容分析规则。

Using Item Loaders to populate items

要使用Item Loader, 你必须先将它实例化. 你可以使用类似字典的对象(例如: Item or dict)来进行实例化, 或者不使用对象也可以, 当不用对象进行实例化的时候,Item会自动使用 ItemLoader.default_item_class 属性中指定的Item 类在Item Loader constructor中实例化.

然后,你开始收集数值到Item Loader时,通常使用 Selectors. 你可以在同一个item field 里面添加多个数值;Item Loader将知道如何用合适的处理函数来“添加”这些数值.

下面是在 Spider 中典型的Item Loader的用法, 使用 Items chapter 中声明的 Product item:

from scrapy.contrib.loader import ItemLoader
from myproject.items import Product

def parse(self, response):
    l = ItemLoader(item=Product(), response=response)
    l.add_xpath('name', '//div[@class="product_name"]')
    l.add_xpath('name', '//div[@class="product_title"]')
    l.add_xpath('price', '//p[@id="price"]')
    l.add_css('stock', 'p#stock]')
    l.add_value('last_updated', 'today') # you can also use literal values
    return l.load_item()

快速查看这些代码之后,我们可以看到发现 name 字段被从页面中两个不同的XPath位置提取:

1. //div[@class="product_name"]
2. //div[@class="product_title"]

换言之,数据通过用 add_xpath() 的方法,把从两个不同的XPath位置提取的数据收集起来. 这是将在以后分配给 name 字段中的数据｡

之后,类似的请求被用于 price 和 stock 字段 (后者使用 CSS selector 和 add_css() 方法), 最后使用不同的方法 add_value() 对 last_update 填充文本值( today ).

最终, 当所有数据被收集起来之后, 调用 ItemLoader.load_item() 方法, 实际上填充并且返回了之前通过调用 add_xpath(), add_css(), and add_value() 所提取和收集到的数据的Item.

Input and Output processors

Item Loader在每个(Item)字段中都包含了一个输入处理器和一个输出处理器｡ 输入处理器收到数据时立刻提取数据 (通过 add_xpath(), add_css() 或者 add_value() 方法) 之后输入处理器的结果被收集起来并且保存在ItemLoader内. 收集到所有的数据后, 调用 ItemLoader.load_item() 方法来填充,并得到填充后的 Item 对象. 这是当输出处理器被和之前收集到的数据(和用输入处理器处理的)被调用.输出处理器的结果是被分配到Item的最终值｡

让我们看一个例子来说明如何输入和输出处理器被一个特定的字段调用(同样适用于其他field)::

l = ItemLoader(Product(), some_selector)
l.add_xpath('name', xpath1) # (1)
l.add_xpath('name', xpath2) # (2)
l.add_css('name', css) # (3)
l.add_value('name', 'test') # (4)
return l.load_item() # (5)

发生了这些事情:

1. 从 xpath1 提取出的数据,传递给 输入处理器 的 name 字段.输入处理器的结果被收集和保存在Item Loader中(但尚未分配给该Item)｡
2. 从 xpath2 提取出来的数据,传递给(1)中使用的相同的 输入处理器 .输入处理器的结果被附加到在(1)中收集的数据(如果有的话) ｡
3. 和之前相似，只不过这里的数据是通过 css CSS selector抽取，之后传输到在(1)和(2)使用 的 input processor 中。最终输入处理器的结果被附加到在(1)和(2)中收集的数据之后 (如果存在数据的话)。
4. 这里的处理方式也和之前相似，但是此处的值是通过add_value直接赋予的， 而不是利用XPath表达式或CSS selector获取。得到的值仍然是被传送到输入处理器。 在这里例程中，因为得到的值并非可迭代，所以在传输到输入处理器之前需要将其 转化为可迭代的单个元素，这才是它所接受的形式。
5. 在之前步骤中所收集到的数据被传送到 output processor 的 name field中。 输出处理器的结果就是赋到item中 name field的值。

需要注意的是，输入和输出处理器都是可调用对象，调用时传入需要被分析的数据， 处理后返回分析得到的值。因此你可以使用任意函数作为输入、输出处理器。 唯一需注意的是它们必须接收一个（并且只是一个）迭代器性质的positional参数。

注解

Both input and output processors must receive an iterator as their first argument. The output of those functions can be anything. The result of input processors will be appended to an internal list (in the Loader) containing the collected VALUES (NULL, but not least, Scrapy comes with some commonly used processors built-in for convenience.

Declaring Item Loaders

Item Loaders are declared like Items, by using a class definition syntax. Here is an example:

from scrapy.contrib.loader import ItemLoader
from scrapy.contrib.loader.processor import TakeFirst, MapCompose, Join

class ProductLoader(ItemLoader):

    default_output_processor = TakeFirst()

    name_in = MapCompose(unicode.title)
    name_out = Join()

    price_in = MapCompose(unicode.strip)

    # ...

As you can see, input processors are declared using the _in suffix while output processors are declared using the _out suffix. And you can also declare a default input/output processors using the ItemLoader.default_input_processor and ItemLoader.default_output_processor attributes.

Declaring Input and Output Processors

As seen in the previous section, input and output processors can be declared in the Item Loader definition, and it’s very common to declare input processors this way. However, there is one more place where you can specify the input and output processors to use: in the Item Field metadata. Here is an example:

import scrapy
from scrapy.contrib.loader.processor import Join, MapCompose, TakeFirst
from w3lib.html import remove_tags

def filter_price(value):
    if value.isdigit():
        return value

class Product(scrapy.Item):
    name = scrapy.Field(
        input_processor=MapCompose(remove_tags),
        output_processor=Join(),
    )
    price = scrapy.Field(
        input_processor=MapCompose(remove_tags, filter_price),
        output_processor=TakeFirst(),
    )

>>> from scrapy.contrib.loader import ItemLoader
>>> il = ItemLoader(item=Product())
>>> il.add_value('name', [u'Welcome to my', u'<strong>website</strong>'])
>>> il.add_value('price', [u'&euro;', u'<span>1000</span>'])
>>> il.load_item()
{'name': u'Welcome to my website', 'price': u'1000'}

The precedence order, for both input and output processors, is as follows:

1. Item Loader field-specific attributes: field_in and field_out (most precedence)
2. Field metadata (input_processor and output_processor key)
3. Item Loader defaults: ItemLoader.default_input_processor() and ItemLoader.default_output_processor() (least precedence)

See also: Reusing and extending Item Loaders.

Item Loader Context

The Item Loader Context is a dict of arbitrary key/values which is shared among all input and output processors in the Item Loader. It can be passed when declaring, instantiating or using Item Loader. They are used to modify the behaviour of the input/output processors.

For example, suppose you have a function parse_length which receives a text value and extracts a length from it:

def parse_length(text, loader_context):
    unit = loader_context.get('unit', 'm')
    # ... length parsing code goes here ...
    return parsed_length

By accepting a loader_context argument the function is explicitly telling the Item Loader that it’s able to receive an Item Loader context, so the Item Loader passes the currently active context when calling it, and the processor function (parse_length in this case) can thus use them.

There are several ways to modify Item Loader context values:

1. By modifying the currently active Item Loader context (context attribute):

   loader = ItemLoader(product)
   loader.context['unit'] = 'cm'

2. On Item Loader instantiation (the keyword arguments of Item Loader constructor are stored in the Item Loader context):

   loader = ItemLoader(product, unit='cm')

3. On Item Loader declaration, for those input/output processors that support instantiating them with an Item Loader context. MapCompose is one of them:

   class ProductLoader(ItemLoader):
       length_out = MapCompose(parse_length, unit='cm')

ItemLoader objects

class scrapy.contrib.loader.ItemLoader([item, selector, response, ]**kwargs)

Return a new Item Loader for populating the given Item. If no item is given, one is instantiated automatically using the class in default_item_class.

When instantiated with a selector or a response parameters the ItemLoader class provides convenient mechanisms for extracting data from web pages using selectors.

参数:

item (Item object) – The item instance to populate using subsequent calls to add_xpath(), add_css(), or add_value(). selector (Selector object) – The selector to extract data from, when using the add_xpath() (resp. add_css()) or replace_xpath() (resp. replace_css()) method. response (Response object) – The response used to construct the selector using the default_selector_class, unless the selector argument is given, in which case this argument is ignored.

The item, selector, response and the remaining keyword arguments are assigned to the Loader context (accessible through the context attribute).

ItemLoader instances have the following methods:

get_value(value, *processors, **kwargs)

Process the given value by the given processors and keyword arguments.

Available keyword arguments:

参数:	re (str or compiled regex) – a regular expression to use for extracting data from the given value using extract_regex() method, applied before processors

Examples:

>>> from scrapy.contrib.loader.processor import TakeFirst
>>> loader.get_value(u'name: foo', TakeFirst(), unicode.upper, re='name: (.+)')
'FOO`

add_value(field_name, value, *processors, **kwargs)

Process and then add the given value for the given field.

The value is first passed through get_value() by giving the processors and kwargs, and then passed through the field input processor and its result appended to the data collected for that field. If the field already contains collected data, the new data is added.

The given field_name can be None, in which case values for multiple fields may be added. And the processed value should be a dict with field_name mapped to values.

Examples:

loader.add_value('name', u'Color TV')
loader.add_value('colours', [u'white', u'blue'])
loader.add_value('length', u'100')
loader.add_value('name', u'name: foo', TakeFirst(), re='name: (.+)')
loader.add_value(None, {'name': u'foo', 'sex': u'male'})

replace_value(field_name, value, *processors, **kwargs)

Similar to add_value() but replaces the collected data with the new value instead of adding it.

get_xpath(xpath, *processors, **kwargs)

Similar to ItemLoader.get_value() but receives an XPath instead of a value, which is used to extract a list of unicode strings from the selector associated with this ItemLoader.

参数:	xpath (str) – the XPath to extract data from re (str or compiled regex) – a regular expression to use for extracting data from the selected XPath region

Examples:

# HTML snippet: <p class="product-name">Color TV</p>
loader.get_xpath('//p[@class="product-name"]')
# HTML snippet: <p id="price">the price is $1200</p>
loader.get_xpath('//p[@id="price"]', TakeFirst(), re='the price is (.*)')

add_xpath(field_name, xpath, *processors, **kwargs)

Similar to ItemLoader.add_value() but receives an XPath instead of a value, which is used to extract a list of unicode strings from the selector associated with this ItemLoader.

See get_xpath() for kwargs.

参数:	xpath (str) – the XPath to extract data from

Examples:

# HTML snippet: <p class="product-name">Color TV</p>
loader.add_xpath('name', '//p[@class="product-name"]')
# HTML snippet: <p id="price">the price is $1200</p>
loader.add_xpath('price', '//p[@id="price"]', re='the price is (.*)')

replace_xpath(field_name, xpath, *processors, **kwargs)

Similar to add_xpath() but replaces collected data instead of adding it.

get_css(css, *processors, **kwargs)

Similar to ItemLoader.get_value() but receives a CSS selector instead of a value, which is used to extract a list of unicode strings from the selector associated with this ItemLoader.

参数:	css (str) – the CSS selector to extract data from re (str or compiled regex) – a regular expression to use for extracting data from the selected CSS region

Examples:

# HTML snippet: <p class="product-name">Color TV</p>
loader.get_css('p.product-name')
# HTML snippet: <p id="price">the price is $1200</p>
loader.get_css('p#price', TakeFirst(), re='the price is (.*)')

add_css(field_name, css, *processors, **kwargs)

Similar to ItemLoader.add_value() but receives a CSS selector instead of a value, which is used to extract a list of unicode strings from the selector associated with this ItemLoader.

See get_css() for kwargs.

参数:	css (str) – the CSS selector to extract data from

Examples:

# HTML snippet: <p class="product-name">Color TV</p>
loader.add_css('name', 'p.product-name')
# HTML snippet: <p id="price">the price is $1200</p>
loader.add_css('price', 'p#price', re='the price is (.*)')

replace_css(field_name, css, *processors, **kwargs)

Similar to add_css() but replaces collected data instead of adding it.

load_item()

Populate the item with the data collected so far, and return it. The data collected is first passed through the output processors to get the final value to assign to each item field.

get_collected_values(field_name)

Return the collected values for the given field.

get_output_value(field_name)

Return the collected values parsed using the output processor, for the given field. This method doesn’t populate or modify the item at all.

get_input_processor(field_name)

Return the input processor for the given field.

get_output_processor(field_name)

Return the output processor for the given field.

ItemLoader instances have the following attributes:

item

The Item object being parsed by this Item Loader.

context

The currently active Context of this Item Loader.

default_item_class

An Item class (or factory), used to instantiate items when not given in the constructor.

default_input_processor

The default input processor to use for those fields which don’t specify one.

default_output_processor

The default output processor to use for those fields which don’t specify one.

default_selector_class

The class used to construct the selector of this ItemLoader, if only a response is given in the constructor. If a selector is given in the constructor this attribute is ignored. This attribute is sometimes overridden in subclasses.

selector

The Selector object to extract data from. It’s either the selector given in the constructor or one created from the response given in the constructor using the default_selector_class. This attribute is meant to be read-only.

Reusing and extending Item Loaders

As your project grows bigger and acquires more and more spiders, maintenance becomes a fundamental problem, especially when you have to deal with many different parsing rules for each spider, having a lot of exceptions, but also wanting to reuse the common processors.

Item Loaders are designed to ease the maintenance burden of parsing rules, without losing flexibility and, at the same time, providing a convenient mechanism for extending and overriding them. For this reason Item Loaders support traditional Python class inheritance for dealing with differences of specific spiders (or groups of spiders).

Suppose, for example, that some particular site encloses their product names in three dashes (e.g. ---Plasma TV---) and you don’t want to end up scraping those dashes in the final product names.

Here’s how you can remove those dashes by reusing and extending the default Product Item Loader (ProductLoader):

from scrapy.contrib.loader.processor import MapCompose
from myproject.ItemLoaders import ProductLoader

def strip_dashes(x):
    return x.strip('-')

class SiteSpecificLoader(ProductLoader):
    name_in = MapCompose(strip_dashes, ProductLoader.name_in)

Another case where extending Item Loaders can be very helpful is when you have multiple source formats, for example XML and HTML. In the XML version you may want to remove CDATA occurrences. Here’s an example of how to do it:

from scrapy.contrib.loader.processor import MapCompose
from myproject.ItemLoaders import ProductLoader
from myproject.utils.xml import remove_cdata

class XmlProductLoader(ProductLoader):
    name_in = MapCompose(remove_cdata, ProductLoader.name_in)

And that’s how you typically extend input processors.

As for output processors, it is more common to declare them in the field metadata, as they usually depend only on the field and not on each specific site parsing rule (as input processors do). See also: Declaring Input and Output Processors.

There are many other possible ways to extend, inherit and override your Item Loaders, and different Item Loaders hierarchies may fit better for different projects. Scrapy only provides the mechanism; it doesn’t impose any specific organization of your Loaders collection - that’s up to you and your project’s needs.

Available built-in processors

Even though you can use any callable function as input and output processors, Scrapy provides some commonly used processors, which are described below. Some of them, like the MapCompose (which is typically used as input processor) compose the output of several functions executed in order, to produce the final parsed value.

Here is a list of all built-in processors:

class scrapy.contrib.loader.processor.Identity

The simplest processor, which doesn’t do anything. It returns the original values unchanged. It doesn’t receive any constructor arguments nor accepts Loader contexts.

Example:

>>> from scrapy.contrib.loader.processor import Identity
>>> proc = Identity()
>>> proc(['one', 'two', 'three'])
['one', 'two', 'three']

class scrapy.contrib.loader.processor.TakeFirst

Returns the first non-null/non-empty value from the values received, so it’s typically used as an output processor to single-valued fields. It doesn’t receive any constructor arguments, nor accept Loader contexts.

Example:

>>> from scrapy.contrib.loader.processor import TakeFirst
>>> proc = TakeFirst()
>>> proc(['', 'one', 'two', 'three'])
'one'

class scrapy.contrib.loader.processor.Join(separator=u' ')

Returns the values joined with the separator given in the constructor, which defaults to u' '. It doesn’t accept Loader contexts.

When using the default separator, this processor is equivalent to the function: u' '.join

Examples:

>>> from scrapy.contrib.loader.processor import Join
>>> proc = Join()
>>> proc(['one', 'two', 'three'])
u'one two three'
>>> proc = Join('<br>')
>>> proc(['one', 'two', 'three'])
u'one<br>two<br>three'

class scrapy.contrib.loader.processor.Compose(*functions, **default_loader_context)

A processor which is constructed from the composition of the given functions. This means that each input value of this processor is passed to the first function, and the result of that function is passed to the second function, and so on, until the last function returns the output value of this processor.

By default, stop process on None value. This behaviour can be changed by passing keyword argument stop_on_none=False.

Example:

>>> from scrapy.contrib.loader.processor import Compose
>>> proc = Compose(lambda v: v[0], str.upper)
>>> proc(['hello', 'world'])
'HELLO'

Each function can optionally receive a loader_context parameter. For those which do, this processor will pass the currently active Loader context through that parameter.

The keyword arguments passed in the constructor are used as the default Loader context values passed to each function call. However, the final Loader context values passed to functions are overridden with the currently active Loader context accessible through the ItemLoader.context() attribute.

class scrapy.contrib.loader.processor.MapCompose(*functions, **default_loader_context)

A processor which is constructed from the composition of the given functions, similar to the Compose processor. The difference with this processor is the way internal results are passed among functions, which is as follows:

The input value of this processor is iterated and the first function is applied to each element. The results of these function calls (one for each element) are concatenated to construct a new iterable, which is then used to apply the second function, and so on, until the last function is applied to each value of the list of values collected so far. The output values of the last function are concatenated together to produce the output of this processor.

Each particular function can return a value or a list of values, which is flattened with the list of values returned by the same function applied to the other input values. The functions can also return None in which case the output of that function is ignored for further processing over the chain.

This processor provides a convenient way to compose functions that only work with single VALUES (NULL, since data is often extracted using the extract() method of selectors, which returns a list of unicode strings.

The example below should clarify how it works:

>>> def filter_world(x):
...     return None if x == 'world' else x
...
>>> from scrapy.contrib.loader.processor import MapCompose
>>> proc = MapCompose(filter_world, unicode.upper)
>>> proc([u'hello', u'world', u'this', u'is', u'scrapy'])
[u'HELLO, u'THIS', u'IS', u'SCRAPY']

As with the Compose processor, functions can receive Loader contexts, and constructor keyword arguments are used as default context values. See Compose processor for more info.