# Components

A Scrapy component is any class whose objects are built using `build_from_crawler()`.

That includes the classes that you may assign to the following settings:

- `DNS_RESOLVER`

- `DOWNLOAD_HANDLERS`

- `DOWNLOADER_CLIENTCONTEXTFACTORY`

- `DOWNLOADER_MIDDLEWARES`

- `DUPEFILTER_CLASS`

- `EXTENSIONS`

- `FEED_EXPORTERS`

- `FEED_STORAGES`

- `ITEM_PIPELINES`

- `SCHEDULER`

- `SCHEDULER_DISK_QUEUE`

- `SCHEDULER_MEMORY_QUEUE`

- `SCHEDULER_PRIORITY_QUEUE`

- `SPIDER_MIDDLEWARES`

Third-party Scrapy components may also let you define additional Scrapy components, usually configurable through settings, to modify their behavior.

## Enforcing component requirements

Sometimes, your components may only be intended to work under certain conditions. For example, they may require a minimum version of Scrapy to work as intended, or they may require certain settings to have specific values.

In addition to describing those conditions in the documentation of your component, it is a good practice to raise an exception from the `__init__` method of your component if those conditions are not met at run time.

In the case of downloader middlewares, extensions, item pipelines, and spider middlewares, you should raise `scrapy.exceptions.NotConfigured`, passing a description of the issue as a parameter to the exception so that it is printed in the logs, for the user to see. For other components, feel free to raise whatever other exception feels right to you; for example, `RuntimeError` would make sense for a Scrapy version mismatch, while `ValueError` may be better if the issue is the value of a setting.

If your requirement is a minimum Scrapy version, you may use `scrapy.__version__` to enforce your requirement. For example:

    from packaging.version import parse as parse_version

    import scrapy

    class MyComponent:
        def __init__(self):
            if parse_version(scrapy.__version__) < parse_version("2.7"):
                raise RuntimeError(
                    f"{MyComponent.__qualname__} requires Scrapy 2.7 or "
                    f"later, which allow defining the process_spider_output "
                    f"method of spider middlewares as an asynchronous "
                    f"generator."
                )

## API reference

The following function can be used to create an instance of a component class:

scrapy.utils.misc.build_from_crawler(objcls: type[T], crawler: Crawler, /, *args: Any, **kwargs: Any) → T

  Construct a class instance using its `from_crawler` or `from_settings` constructor.

  Added in version 2.12.

  `*args` and `**kwargs` are forwarded to the constructor.

  Raises `TypeError` if the resulting instance is `None`.

The following function can also be useful when implementing a component, to report the import path of the component class, e.g. when reporting problems:

scrapy.utils.python.global_object_name(obj: Any) → str

  Return the full import path of the given class.

      >>> from scrapy import Request
      >>> global_object_name(Request)
      'scrapy.http.request.Request'