Q Loaders

[Ariane Delbrune]

modulerules allows you to specify several loaders within your webpack configuration.This is a concise way to display loaders, and helps to maintain clean code. It also offers you a full overview of each respective loader.

Loaders are evaluated/executed from right to left (or from bottom to top). In the example below execution starts with sass-loader, continues with css-loader and finally ends with style-loader. See "Loader Features" for more information about loaders order.

Loaders provide a way to customize the output through their preprocessing functions. Users now have more flexibility to include fine-grained logic such as compression, packaging, language translations and more.

A loader module is expected to export a function and be written in Node.js compatible JavaScript. They are most commonly managed with npm, but you can also have custom loaders as files within your application. By convention, loaders are usually named xxx-loader (e.g. json-loader). See "Writing a Loader" for more information.

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Item Loaders provide a convenient mechanism for populating scraped items. Even though items can be populated directly, Item Loaders provide amuch more convenient API for populating them from a scraping process, by automatingsome common tasks like parsing the raw extracted data before assigning it.

Item Loaders are designed to provide a flexible, efficient and easy mechanismfor extending and overriding different field parsing rules, either by spider,or by source format (HTML, XML, etc) without becoming a nightmare to maintain.

To use an Item Loader, you must first instantiate it. You can eitherinstantiate it with an item object or without one, in whichcase an item object is automatically created in theItem Loader __init__ method using the item classspecified in the ItemLoader.default_item_class attribute.

Afterwards, similar calls are used for price and stock fields(the latter using a CSS selector with the add_css() method),and finally the last_update field is populated directly with a literal value(today) using a different method: add_value().

This case is also similar to the previous ones, except that the value to becollected is assigned directly, instead of being extracted from a XPathexpression or a CSS selector.However, the value is still passed through the input processors. In thiscase, since the value is not iterable it is converted to an iterable of asingle element before passing it to the input processor, because inputprocessor always receive iterables.

Both input and output processors must receive an iterable as theirfirst argument. The output of those functions can be anything. The result ofinput processors will be appended to an internal list (in the Loader)containing the collected values (for that field). The result of the outputprocessors is the value that will be finally assigned to the item.

As you can see, input processors are declared using the _in suffix whileoutput processors are declared using the _out suffix. And you can alsodeclare a default input/output processors using theItemLoader.default_input_processor andItemLoader.default_output_processor attributes.

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

A user-friendly abstraction to populate an item with databy applying field processors to scraped data.When instantiated with a selector or a response it supportsdata extraction from web pages using selectors.

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

The value is first passed through get_value() by giving theprocessors and kwargs, and then passed through thefield input processor and its resultappended to the data collected for that field. If the field alreadycontains collected data, the new data is added.

Create a nested loader with a css selector.The supplied selector is applied relative to selector associatedwith this ItemLoader. The nested loader shares the itemwith the parent ItemLoader so calls to add_xpath(),add_value(), replace_value(), etc. will behave as expected.

Create a nested loader with an xpath selector.The supplied selector is applied relative to selector associatedwith this ItemLoader. The nested loader shares the itemwith the parent ItemLoader so calls to add_xpath(),add_value(), replace_value(), etc. will behave as expected.

You can nest loaders arbitrarily and they work with either xpath or css selectors.As a general guideline, use nested loaders when they make your code simpler but donot go overboard with nesting or your parser can become difficult to read.

As your project grows bigger and acquires more and more spiders, maintenancebecomes a fundamental problem, especially when you have to deal with manydifferent parsing rules for each spider, having a lot of exceptions, but alsowanting 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 convenientmechanism for extending and overriding them. For this reason Item Loaderssupport traditional Python class inheritance for dealing with differences ofspecific spiders (or groups of spiders).

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 parsingrule (as input processors do). See also:Declaring Input and Output Processors.

Without React Router, the browser would have made a Request to your server, but React Router prevented it! Instead of the browser sending the request to your server, React Router sends the request to your loaders.

If you are Server-Side Rendering and leveraging the future.v7_partialHydration flag for Partial Hydration, then you may wish to opt-into running a route loader on initial hydration even though it has hydration data (for example, to let a user prime a cache with the hydration data). To force a loader to run on hydration in a partial hydration scenario, you can set a hydrate property on the loader function:

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