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sirMackk
100 lines
3.7 KiB
ReStructuredText
100 lines
3.7 KiB
ReStructuredText
HTML Scraping
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=============
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Web Scraping
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------------
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Web sites are written using HTML, which means that each web page is a
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structured document. Sometimes it would be great to obtain some data from
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them and preserve the structure while we're at it, but this isn't always easy.
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It's not often that web sites provide their data in comfortable formats
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such as ``.csv``.
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This is where web scraping comes in. Web scraping is the practice of using
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computer program to sift through a web page and gather the data that you need
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in a format most useful to you.
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lxml and Requests
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-----------------
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`lxml <http://lxml.de/>`_ is a pretty extensive library written for parsing
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XML and HTML documents really fast. It even handles messed up tags. We will
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also be using the `Requests <http://docs.python-requests.org/en/latest/>`_ module instead of the already built-in urlib2
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due to improvements in speed and readability. You can easily install both
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using ``pip install lxml`` and ``pip install requests``.
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Lets start with the imports:
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.. code-block:: python
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from lxml import html
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import requests
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Next we will use ``requests.get`` to retrieve the web page with our data
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and parse it using the ``html`` module and save the results in ``tree``:
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.. code-block:: python
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page = requests.get('http://econpy.pythonanywhere.com/ex/001.html')
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tree = html.fromstring(page.text)
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``tree`` now contains the whole HTML file in a nice tree structure which
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we can go over two different ways: XPath and CSSSelect. In this example, I
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will focus on the former.
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XPath is a way of locating information in structured documents such as
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HTML or XML pages. A good introduction to XPath is `here <http://www.w3schools.com/xpath/default.asp>`_ .
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One can also use various tools for obtaining the XPath of elements such as
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FireBug for Firefox or in Chrome you can right click an element, choose
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'Inspect element', highlight the code and the right click again and choose
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'Copy XPath'.
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After a quick analysis, we see that in our page the data is contained in
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two elements - one is a div with title 'buyer-name' and the other is a
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span with class 'item-price':
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::
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<div title="buyer-name">Carson Busses</div>
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<span class="item-price">$29.95</span>
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Knowing this we can create the correct XPath query and use the lxml
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``xpath`` function like this:
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.. code-block:: python
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#This will create a list of buyers:
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buyers = tree.xpath('//div[@title="buyer-name"]/text()')
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#This will create a list of prices
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prices = tree.xpath('//span[@class="item-price"]/text()')
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Lets see what we got exactly:
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.. code-block:: python
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print 'Buyers: ', buyers
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print 'Prices: ', prices
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::
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Buyers: ['Carson Busses', 'Earl E. Byrd', 'Patty Cakes',
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'Derri Anne Connecticut', 'Moe Dess', 'Leda Doggslife', 'Dan Druff',
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'Al Fresco', 'Ido Hoe', 'Howie Kisses', 'Len Lease', 'Phil Meup',
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'Ira Pent', 'Ben D. Rules', 'Ave Sectomy', 'Gary Shattire',
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'Bobbi Soks', 'Sheila Takya', 'Rose Tattoo', 'Moe Tell']
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Prices: ['$29.95', '$8.37', '$15.26', '$19.25', '$19.25',
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'$13.99', '$31.57', '$8.49', '$14.47', '$15.86', '$11.11',
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'$15.98', '$16.27', '$7.50', '$50.85', '$14.26', '$5.68',
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'$15.00', '$114.07', '$10.09']
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Congratulations! We have successfully scraped all the data we wanted from
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a web page using lxml and we have it stored in memory as two lists. Now we
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can either continue our work on it, analyzing it using python or we can
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export it to a file and share it with friends.
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A cool idea to think about is writing a script to iterate through the rest
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of the pages of this example data set or making this application use
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threads to improve its speed.
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