.. _topics-link-extractors:

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

A link extractor is an object that extracts links from responses.

The ``__init__`` method of
:class:`~scrapy.linkextractors.lxmlhtml.LxmlLinkExtractor` takes settings that
determine which links may be extracted. :class:`LxmlLinkExtractor.extract_links
<scrapy.linkextractors.lxmlhtml.LxmlLinkExtractor.extract_links>` returns a
list of matching :class:`~scrapy.link.Link` objects from a
:class:`~scrapy.http.Response` object.

Link extractors are used in :class:`~scrapy.spiders.CrawlSpider` spiders
through a set of :class:`~scrapy.spiders.Rule` objects.

You can also use link extractors in regular spiders. For example, you can instantiate
:class:`LinkExtractor <scrapy.linkextractors.lxmlhtml.LxmlLinkExtractor>` into a class
variable in your spider, and use it from your spider callbacks:

.. code-block:: python

    def parse(self, response):
        for link in self.link_extractor.extract_links(response):
            yield Request(link.url, callback=self.parse)

.. _topics-link-extractors-ref:

Link extractor reference
========================

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

The link extractor class is
:class:`scrapy.linkextractors.lxmlhtml.LxmlLinkExtractor`. For convenience it
can also be imported as ``scrapy.linkextractors.LinkExtractor``::

    from scrapy.linkextractors import LinkExtractor

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: str or list

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

    :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
        :data:`scrapy.linkextractors.IGNORED_EXTENSIONS`.

    :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.
    :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 restrict_text: a single regular expression (or list of regular expressions)
        that the link's text must match in order to be extracted. If not
        given (or empty), it will match all links. If a list of regular expressions is
        given, the link will be extracted if it matches at least one.
    :type restrict_text: 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: bool

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

    :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``:

        .. code-block:: python

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

    :type process_value: collections.abc.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: bool

    .. automethod:: extract_links

Link
----

.. module:: scrapy.link
   :synopsis: Link from link extractors

.. autoclass:: Link
