declaring-loaders.rst

Declaring Item Loaders

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

from itemloaders import ItemLoader
from itemloaders.processors import TakeFirst, MapCompose, Join

class ProductLoader(ItemLoader):

    default_output_processor = TakeFirst()

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

    # using a built-in processor
    price_in = MapCompose(str.strip)

    # using a function
    def price_out(self, values):
        return float(values[0])

loader = ProductLoader()
loader.add_value('name', 'plasma TV')
loader.add_value('price', '999.98')
loader.load_item()
# {'name': 'Plasma Tv', 'price': 999.98}

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 :attr:`ItemLoader.default_input_processor` and :attr:`ItemLoader.default_output_processor` attributes.

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 keys). Check out itemadapter field metadata for more information.
3. Item Loader defaults: :meth:`ItemLoader.default_input_processor` and :meth:`ItemLoader.default_output_processor` (least precedence)

See also: :ref:`extending-loaders`.