Document how to select dynamically-loaded content

docs/index.rst

@@ -158,6 +158,7 @@ Solving specific problems
    topics/practices
    topics/broad-crawls
    topics/developer-tools
+   topics/dynamic-content
    topics/leaks
    topics/media-pipeline
    topics/deploy
@@ -183,6 +184,9 @@ Solving specific problems
 :doc:`topics/developer-tools`
     Learn how to scrape with your browser's developer tools.
 
+:doc:`topics/dynamic-content`
+    Read webpage data that is loaded dynamically.
+
 :doc:`topics/leaks`
     Learn how to find and get rid of memory leaks in your crawler.
 

docs/topics/dynamic-content.rst

@@ -0,0 +1,246 @@
+.. _topics-dynamic-content:
+
+====================================
+Selecting dynamically-loaded content
+====================================
+
+Some webpages show the desired data when you load them in a web browser.
+However, when you download them using Scrapy, you cannot reach the desired data
+using :ref:`selectors <topics-selectors>`.
+
+When this happens, the recommended approach is to
+:ref:`find the data source <topics-finding-data-source>` and extract the data
+from it.
+
+If you fail to do that, and you can nonetheless access the desired data through
+the :ref:`DOM <topics-livedom>` from your web browser, see
+:ref:`topics-javascript-rendering`.
+
+.. _topics-finding-data-source:
+
+Finding the data source
+=======================
+
+To extract the desired data, you must first find its source location.
+
+If the data is in a non-text-based format, such as an image or a PDF document,
+use the :ref:`network tool <topics-network-tool>` of your web browser to find
+the corresponding request, and :ref:`reproduce it
+<topics-reproducing-requests>`.
+
+If your web browser lets you select the desired data as text, the data may be
+defined in embedded JavaScript code, or loaded from an external resource in a
+text-based format.
+
+In that case, you can use a tool like wgrep_ to find the URL of that resource.
+
+If the data turns out to come from the original URL itself, you must
+:ref:`inspect the source code of the webpage <topics-inspecting-source>` to
+determine where the data is located.
+
+If the data comes from a different URL, you will need to :ref:`reproduce the
+corresponding request <topics-reproducing-requests>`.
+
+.. _topics-inspecting-source:
+
+Inspecting the source code of a webpage
+=======================================
+
+Sometimes you need to inspect the source code of a webpage (not the
+:ref:`DOM <topics-livedom>`) to determine where some desired data is located.
+
+Use Scrapy’s :command:`fetch` command to download the webpage contents as seen
+by Scrapy::
+
+    scrapy fetch --nolog https://example.com > response.html
+
+If the desired data is in embedded JavaScript code within a ``<script/>``
+element, see :ref:`topics-parsing-javascript`.
+
+If you cannot find the desired data, first make sure it’s not just Scrapy:
+download the webpage with an HTTP client like curl_ or wget_ and see if the
+information can be found in the response they get.
+
+If they get a response with the desired data, modify your Scrapy
+:class:`~scrapy.http.Request` to match that of the other HTTP client. For
+example, try using the same user-agent string (:setting:`USER_AGENT`) or the
+same :attr:`~scrapy.http.Request.headers`.
+
+If they also get a response without the desired data, you’ll need to take
+steps to make your request more similar to that of the web browser. See
+:ref:`topics-reproducing-requests`.
+
+.. _topics-reproducing-requests:
+
+Reproducing requests
+====================
+
+Sometimes we need to reproduce a request the way our web browser performs it.
+
+Use the :ref:`network tool <topics-network-tool>` of your web browser to see
+how your web browser performs the desired request, and try to reproduce that
+request with Scrapy.
+
+It might be enough to yield a :class:`~scrapy.http.Request` with the same HTTP
+method and URL. However, you may also need to reproduce the body, headers and
+form parameters (see :class:`~scrapy.http.FormRequest`) of that request.
+
+Once you get the expected response, you can :ref:`extract the desired data from
+it <topics-handling-response-formats>`.
+
+You can reproduce any request with Scrapy. However, some times reproducing all
+necessary requests may not seem efficient in developer time. If that is your
+case, and crawling speed is not a major concern for you, you can alternatively
+consider :ref:`JavaScript pre-rendering <topics-javascript-rendering>`.
+
+If you get the expected response `sometimes`, but not always, the issue is
+probably not your request, but the target server. The target server might be
+buggy, overloaded, or :ref:`banning <bans>` some of your requests.
+
+.. _topics-handling-response-formats:
+
+Handling different response formats
+===================================
+
+Once you have a response with the desired data, how you extract the desired
+data from it depends on the type of response:
+
+-   If the response is HTML or XML, use :ref:`selectors
+    <topics-selectors>` as usual.
+
+-   If the response is JSON, use `json.loads`_ to load the desired data from
+    :attr:`response.text <scrapy.http.TextResponse.text>`::
+
+        data = json.loads(response.text)
+
+    If the desired data is inside HTML or XML code embedded within JSON data,
+    you can load that HTML or XML code into a
+    :class:`~scrapy.selector.Selector` and then
+    :ref:`use it <topics-selectors>` as usual::
+
+        selector = Selector(data['html'])
+
+-   If the response is JavaScript, or HTML with a ``<script/>`` element
+    containing the desired data, see :ref:`topics-parsing-javascript`.
+
+-   If the response is CSS, use a `regular expression`_ to extract the desired
+    data from :attr:`response.text <scrapy.http.TextResponse.text>`.
+
+.. _topics-parsing-images:
+
+-   If the response is an image or another format based on images (e.g. PDF),
+    read the response as bytes from
+    :attr:`response.body <scrapy.http.TextResponse.body>` and use an OCR
+    solution to extract the desired data as text.
+
+    For example, you can use pytesseract_. To read a table from a PDF,
+    `tabula-py`_ may be a better choice.
+
+-   If the response is SVG, or HTML with embedded SVG containing the desired
+    data, you may be able to extract the desired data using
+    :ref:`selectors <topics-selectors>`, since SVG is based on XML.
+
+    Otherwise, you might need to convert the SVG code into a raster image, and
+    :ref:`handle that raster image <topics-parsing-images>`.
+
+.. _topics-parsing-javascript:
+
+Parsing JavaScript code
+=======================
+
+If the desired data is hardcoded in JavaScript, you first need to get the
+JavaScript code:
+
+-   If the JavaScript code is in a JavaScript file, simply read
+    :attr:`response.text <scrapy.http.TextResponse.text>`.
+
+-   If the JavaScript code is within a ``<script/>`` element of an HTML page,
+    use :ref:`selectors <topics-selectors>` to extract the text within that
+    ``<script/>`` element.
+
+Once you have a string with the JavaScript code, you can extract the desired
+data from it:
+
+-   You might be able to use a `regular expression`_ to extract the desired
+    data in JSON format, which you can then parse with `json.loads`_.
+
+    For example, if the JavaScript code contains a separate line like
+    ``var data = {"field": "value"};`` you can extract that data as follows::
+
+        >>> pattern = r'\bvar\s+data\s*=\s*(\{.*?\})\s*;\s*\n'
+        >>> json_data = response.css('script::text').re_first(pattern)
+        >>> json.loads(json_data)
+        {'field': 'value'}
+
+-   Otherwise, use js2xml_ to convert the JavaScript code into an XML document
+    that you can parse using :ref:`selectors <topics-selectors>`.
+
+    For example, if the JavaScript code contains
+    ``var data = {field: "value"};`` you can extract that data as follows::
+
+        >>> import js2xml
+        >>> import lxml.etree
+        >>> from parsel import Selector
+        >>> javascript = response.css('script::text').get()
+        >>> xml = lxml.etree.tostring(js2xml.parse(javascript), encoding='unicode')
+        >>> selector = Selector(text=xml)
+        >>> selector.css('var[name="data"]').get()
+        '<var name="data"><object><property name="field"><string>value</string></property></object></var>'
+
+.. _topics-javascript-rendering:
+
+Pre-rendering JavaScript
+========================
+
+On webpages that fetch data from additional requests, reproducing those
+requests that contain the desired data is the preferred approach. The effort is
+often worth the result: structured, complete data with minimum parsing time and
+network transfer.
+
+However, sometimes it can be really hard to reproduce certain requests. Or you
+may need something that no request can give you, such as a screenshot of a
+webpage as seen in a web browser.
+
+In these cases use the Splash_ JavaScript-rendering service, along with
+`scrapy-splash`_ for seamless integration.
+
+Splash returns as HTML the :ref:`DOM <topics-livedom>` of a webpage, so that
+you can parse it with :ref:`selectors <topics-selectors>`. It provides great
+flexibility through configuration_ or scripting_.
+
+If you need something beyond what Splash offers, such as interacting with the
+DOM on-the-fly from Python code instead of using a previously-written script,
+or handling multiple web browser windows, you might need to
+:ref:`use a headless browser <topics-headless-browsing>` instead.
+
+.. _configuration: https://splash.readthedocs.io/en/stable/api.html
+.. _scripting: https://splash.readthedocs.io/en/stable/scripting-tutorial.html
+
+.. _topics-headless-browsing:
+
+Using a headless browser
+========================
+
+A `headless browser`_ is a special web browser that provides an API for
+automation.
+
+The easiest way to use a headless browser with Scrapy is to use Selenium_,
+along with `scrapy-selenium`_ for seamless integration.
+
+
+.. _AJAX: https://en.wikipedia.org/wiki/Ajax_%28programming%29
+.. _CSS: https://en.wikipedia.org/wiki/Cascading_Style_Sheets
+.. _curl: https://curl.haxx.se/
+.. _headless browser: https://en.wikipedia.org/wiki/Headless_browser
+.. _JavaScript: https://en.wikipedia.org/wiki/JavaScript
+.. _js2xml: https://github.com/scrapinghub/js2xml
+.. _json.loads: https://docs.python.org/library/json.html#json.loads
+.. _pytesseract: https://github.com/madmaze/pytesseract
+.. _regular expression: https://docs.python.org/library/re.html
+.. _scrapy-selenium: https://github.com/clemfromspace/scrapy-selenium
+.. _scrapy-splash: https://github.com/scrapy-plugins/scrapy-splash
+.. _Selenium: https://www.seleniumhq.org/
+.. _Splash: https://github.com/scrapinghub/splash
+.. _tabula-py: https://github.com/chezou/tabula-py
+.. _wget: https://www.gnu.org/software/wget/
+.. _wgrep: https://github.com/stav/wgrep