Item Loader Context¶
The Item Loader Context is a mechanism that allows to change the input/ouput processors behavior.
It’s just a dict
of arbitrary key/values which is shared among all processors.
By default, the context contains the selector
and any other keyword arguments
sent to the Loaders’s __init__
.
The context can be passed when declaring, instantiating or using Item Loader.
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:
By modifying the currently active Item Loader context (
context
attribute):loader = ItemLoader(product) loader.context['unit'] = 'cm'
On Item Loader instantiation (the keyword arguments of Item Loader
__init__
method are stored in the Item Loader context):loader = ItemLoader(product, unit='cm')
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')