.. _topics-link-extractors:

===============
Link Extractors
===============

Link extractors are objects whose only purpose is to extract links from web
pages (:class:`scrapy.http.Response` objects) which will be eventually
followed.

There is ``scrapy.linkextractors.LinkExtractor`` available
in Scrapy, but you can create your own custom Link Extractors to suit your
needs by implementing a simple interface.

The only public method that every link extractor has is ``extract_links``,
which receives a :class:`~scrapy.http.Response` object and returns a list
of :class:`scrapy.link.Link` objects. Link extractors are meant to be
instantiated once and their ``extract_links`` method called several times
with different responses to extract links to follow.

Link extractors are used in the :class:`~scrapy.spiders.CrawlSpider`
class (available in Scrapy), through a set of rules, but you can also use it in
your spiders, even if you don't subclass from
:class:`~scrapy.spiders.CrawlSpider`, as its purpose is very simple: to
extract links.


.. _topics-link-extractors-ref:

Built-in link extractors reference
==================================

.. module:: scrapy.linkextractors
   :synopsis: Link extractors classes

Link extractors classes bundled with Scrapy are provided in the
:mod:`scrapy.linkextractors` module.

The default link extractor is ``LinkExtractor``, which is the same as
:class:`~.LxmlLinkExtractor`::

    from scrapy.linkextractors import LinkExtractor

There used to be other link extractor classes in previous Scrapy versions,
but they are deprecated now.

LxmlLinkExtractor
-----------------

.. module:: scrapy.linkextractors.lxmlhtml
   :synopsis: lxml's HTMLParser-based link extractors


.. class:: LxmlLinkExtractor(allow=(), deny=(), allow_domains=(), deny_domains=(), deny_extensions=None, restrict_xpaths=(), restrict_css=(), tags=('a', 'area'), attrs=('href',), canonicalize=False, unique=True, process_value=None, strip=True)

    LxmlLinkExtractor is the recommended link extractor with handy filtering
    options. It is implemented using lxml's robust HTMLParser.

    :param allow: a single regular expression (or list of regular expressions)
        that the (absolute) urls must match in order to be extracted. If not
        given (or empty), it will match all links.
    :type allow: a regular expression (or list of)

    :param deny: a single regular expression (or list of regular expressions)
        that the (absolute) urls must match in order to be excluded (ie. not
        extracted). It has precedence over the ``allow`` parameter. If not
        given (or empty) it won't exclude any links.
    :type deny: a regular expression (or list of)

    :param allow_domains: a single value or a list of string containing
        domains which will be considered for extracting the links
    :type allow_domains: str or list

    :param deny_domains: a single value or a list of strings containing
        domains which won't be considered for extracting the links
    :type deny_domains: str or list

    :param deny_extensions: a single value or list of strings containing
        extensions that should be ignored when extracting links.
        If not given, it will default to the
        ``IGNORED_EXTENSIONS`` list defined in the
        `scrapy.linkextractors`_ package.
    :type deny_extensions: list

    :param restrict_xpaths: is an XPath (or list of XPath's) which defines
        regions inside the response where links should be extracted from.
        If given, only the text selected by those XPath will be scanned for
        links. See examples below.
    :type restrict_xpaths: str or list

    :param restrict_css: a CSS selector (or list of selectors) which defines
        regions inside the response where links should be extracted from.
        Has the same behaviour as ``restrict_xpaths``.
    :type restrict_css: str or list

    :param tags: a tag or a list of tags to consider when extracting links.
        Defaults to ``('a', 'area')``.
    :type tags: str or list

    :param attrs: an attribute or list of attributes which should be considered when looking
        for links to extract (only for those tags specified in the ``tags``
        parameter). Defaults to ``('href',)``
    :type attrs: list

    :param canonicalize: canonicalize each extracted url (using
        w3lib.url.canonicalize_url). Defaults to ``False``.
        Note that canonicalize_url is meant for duplicate checking;
        it can change the URL visible at server side, so the response can be
        different for requests with canonicalized and raw URLs. If you're
        using LinkExtractor to follow links it is more robust to
        keep the default ``canonicalize=False``.
    :type canonicalize: boolean

    :param unique: whether duplicate filtering should be applied to extracted
        links.
    :type unique: boolean

    :param process_value: a function which receives each value extracted from
        the tag and attributes scanned and can modify the value and return a
        new one, or return ``None`` to ignore the link altogether. If not
        given, ``process_value`` defaults to ``lambda x: x``.

        .. highlight:: html

        For example, to extract links from this code::

            <a href="javascript:goToPage('../other/page.html'); return false">Link text</a>

        .. highlight:: python

        You can use the following function in ``process_value``::

            def process_value(value):
                m = re.search("javascript:goToPage\('(.*?)'", value)
                if m:
                    return m.group(1)

    :type process_value: callable

    :param strip: whether to strip whitespaces from extracted attributes.
        According to HTML5 standard, leading and trailing whitespaces
        must be stripped from ``href`` attributes of ``<a>``, ``<area>``
        and many other elements, ``src`` attribute of ``<img>``, ``<iframe>``
        elements, etc., so LinkExtractor strips space chars by default.
        Set ``strip=False`` to turn it off (e.g. if you're extracting urls
        from elements or attributes which allow leading/trailing whitespaces).
    :type strip: boolean

.. _scrapy.linkextractors: https://github.com/scrapy/scrapy/blob/master/scrapy/linkextractors/__init__.py