# pattern.web

The pattern.web module has tools for online data mining: asynchronous requests, a uniform API for web services (Google, Bing, Twitter, Facebook, Wikipedia, Wiktionary, Flickr, RSS), a HTML DOM parser, HTML tag stripping functions, a web crawler, webmail, caching, Unicode support.

It can be used by itself or with other pattern modules: web | db | en | search | vector | graph.

## Documentation

- URLs
- Asynchronous requests
- Search engine + web services (google, bing, twitter, facebook, wikipedia, flickr)
- Web sort
- HTML to plaintext
- HTML DOM parser
- PDF parser
- Crawler
- E-mail
- Locale
- Cache

## URLs

The URL object is a subclass of Python's urllib2.Request that can be used to connect to a web address. The URL.download() method can be used to retrieve the content (e.g., HTML source code). The constructor's method parameter defines how query data is encoded:

- GET: query data is encoded in the URL string (usually for retrieving data).
- POST: query data is encoded in the message body (for posting data).

    url = URL(string='', method=GET, query={})

    url.string                  # u'http://user:pw@domain.com:30/path/page?p=1#anchor'
    url.parts                   # Dictionary of attributes:

    url.protocol                # u'http'
    url.username                # u'user'
    url.password                # u'pw'
    url.domain                  # u'domain.com'
    url.port                    # 30
    url.path                    # [u'path']
    url.page                    # u'page'
    url.query                   # {u'p': 1}
    url.querystring             # u'p=1'
    url.anchor                  # u'anchor'

    url.exists                  # False if URL.open() raises a HTTP404NotFound.
    url.redirect                # Actual URL after redirection, or None.
    url.headers                 # Dictionary of HTTP response headers.
    url.mimetype                # Document MIME-type.

    url.open(timeout=10, proxy=None)
    url.download(timeout=10, cached=True, throttle=0, proxy=None, unicode=False)
    url.copy()

- URL() expects a string that starts with a valid protocol (e.g. http://).
- URL.open() returns a connection from which data can be retrieved with connection.read().
- URL.download() caches and returns the retrieved data.
  It raises a URLTimeout if the download time exceeds the given timeout.
  It sleeps for throttle seconds after the download is complete.
  A proxy server can be given as a (host, protocol)-tuple, e.g., ('proxy.com', 'https').
  With unicode=True, returns the data as a Unicode string. By default it is False because the data can be binary (e.g., JPEG, ZIP) but unicode=True is advised for HTML.

The example below downloads an image.
The extension() helper function parses the file extension from a file name:

    >>> from pattern.web import URL, extension
    >>>
    >>> url = URL('http://www.clips.ua.ac.be/media/pattern_schema.gif')
    >>> f = open('test' + extension(url.page), 'wb') # save as test.gif
    >>> f.write(url.download())
    >>> f.close()

### URL downloads

The download() function takes a URL string, calls URL.download() and returns the retrieved data. It takes the same optional parameters as URL.download(). This saves you a line of code.

    >>> from pattern.web import download
    >>> html = download('http://www.clips.ua.ac.be/', unicode=True)

### URL mime-type

The URL.mimetype can be used to check the type of document at the given URL. This is more reliable than sniffing the filename extension (which may be omitted).

    >>> from pattern import URL, MIMETYPE_IMAGE
    >>>
    >>> url = URL('http://www.clips.ua.ac.be/media/pattern_schema.gif')
    >>> print url.mimetype in MIMETYPE_IMAGE

    True

Global | Value
MIMETYPE_WEBPAGE | ['text/html']
MIMETYPE_STYLESHEET | ['text/css']
MIMETYPE_PLAINTEXT | ['text/plain']
MIMETYPE_PDF | ['application/pdf']
MIMETYPE_NEWSFEED | ['application/rss+xml', 'application/atom+xml']
MIMETYPE_IMAGE | ['image/gif', 'image/jpeg', 'image/png']
MIMETYPE_AUDIO | ['audio/mpeg', 'audio/mp4', 'audio/x-wav']
MIMETYPE_VIDEO | ['video/mpeg', 'video/mp4', 'video/avi', 'video/quicktime']
MIMETYPE_ARCHIVE | ['application/x-tar', 'application/zip']
MIMETYPE_SCRIPT | ['application/javascript']

### URL exceptions

The URL.open() and URL.download() methods raise a URLError if an error occurs (e.g., no internet connection, server is down). URLError has a number of subclasses:

Exception | Description
URLError | URL has errors (e.g. a missing t in htp://)
URLTimeout | URL takes too long to load.
HTTPError | URL causes an error on the contacted server.
HTTP301Redirect | URL causes too many redirects.
HTTP400BadRequest | URL contains an invalid request.
HTTP401Authentication | URL requires a login and a password.
HTTP403Forbidden | URL is not accessible (check user-agent).
HTTP404NotFound | URL doesn't exist.
HTTP500InternalServerError | URL causes an error (bug?) on the server.

### User-agent and referrer

The URL.open() and URL.download() methods have two optional parameters user_agent and referrer, which can be used to identify the application accessing the web. Some websites include code to block out any application except browsers. By setting a user_agent you can make the application appear as a browser. This is called spoofing and it is not encouraged, but sometimes necessary.

For example, to pose as a Firefox browser:

    >>> URL('http://www.clips.ua.ac.be').download(user_agent='Mozilla/5.0')

### Find URLs

The find_urls() function can be used to parse URLs from a text string. It will retrieve a list of links starting with http://, https://, www. and domain names ending with .com, .org. .net. It will detect and strip leading punctuation (open parens) and trailing punctuation (period, comma, close parens). Similarly, the find_email() function can be used to parse e-mail addresses from a string.

    >>> from pattern.web import find_urls
    >>> print find_urls('Visit our website (www․clips.ua.ac.be)', unique=True)

    ['www.clips.ua.ac.be']

## Asynchronous requests

The asynchronous() function can be used to execute a function "in the background" (i.e., threaded). It takes the function, its arguments and optional keyword arguments. It returns an AsynchronousRequest object that contains the function's return value (when done). The main program does not halt in the meantime.

    request = asynchronous(function, *args, **kwargs)

    request.done                # True when the function is done.
    request.elapsed             # Running time, in seconds.
    request.value               # Function return value when done (or None).
    request.error               # Function Exception (or None).

    request.now()               # Waits for function and returns its value.

The example below executes a Google query without halting the main program. Instead, it displays a "busy" message (e.g., a progress bar updated in the application's event loop) until request.done.

    >>> from pattern.web import asynchronous, time, Google
    >>>
    >>> request = asynchronous(Google().search, 'holy grail', timeout=4)
    >>> while not request.done:
    >>>    time.sleep(0.1)
    >>>    print 'busy...'
    >>> print request.value

There is no way to stop a thread. You are responsible for ensuring that the given function doesn't hang.

## Search engine + web services

The SearchEngine object has a number of subclasses that can be used to query different web services (e.g., Google, Wikipedia). SearchEngine.search() returns a list of Result objects for a given query string – similar to a search field and a results page in a browser.

    engine = SearchEngine(license=None, throttle=1.0, language=None)

    engine.license              # Service license key.
    engine.throttle             # Time between requests (being nice to server).
    engine.language             # Restriction for Result.language (e.g., 'en').

    engine.search(query,
        type = SEARCH,          # SEARCH | IMAGE | NEWS
       start = 1,               # Starting page.
       count = 10,              # Results per page.
        size = None             # Image size: TINY | SMALL | MEDIUM | LARGE
      cached = True)            # Cache locally?

Note: SearchEngine.search() takes the same optional parameters as URL.download().

### Google, Bing, Twitter, Facebook, Wikipedia, Flickr

SearchEngine is subclassed by Google, Yahoo, Bing, DuckDuckGo, Twitter, Facebook, Wikipedia, Wiktionary, Wikia, DBPedia, Flickr and Newsfeed. The constructors take the same parameters:

    engine = Google(license=None, throttle=0.5, language=None)

    engine = Bing(license=None, throttle=0.5, language=None)

    engine = Twitter(license=None, throttle=0.5, language=None)

    engine = Facebook(license=None, throttle=1.0, language='en')

    engine = Wikipedia(license=None, throttle=5.0, language=None)

    engine = Flickr(license=None, throttle=5.0, language=None)

Each search engine has different settings for the search() method. For example, Twitter.search() returns up to 3000 results for a given query (30 queries with 100 results each, or 300 queries with 10 results each). It has a limit of 150 queries per 15 minutes. Each call to search() counts as one query.

Engine | type | start | count | limit | throttle
Google | SEARCH1 | 1-100/count | 1-10 | paid | 0.5
Bing | SEARCH | NEWS | IMAGE12 | 1-1000/count | 1-50 | paid | 0.5
Yahoo | SEARCH | NEWS | IMAGE13 | 1-1000/count | 1-50 | paid | 0.5
DuckDuckGo | SEARCH | 1 | - | - | 0.5
Twitter | SEARCH | 1-3000/count | 1-100 | 600/hour | 0.5
Facebook | SEARCH | NEWS | 1 | 1-100 | 500/hour | 1.0
Wikipedia | SEARCH | 1 | 1 | - | 5.0
Wiktionary | SEARCH | 1 | 1 | - | 5.0
Wikia | SEARCH | 1 | 1 | - | 5.0
DBPedia | SPARQL | 1+ | 1-1000 | 10/sec | 1.0
Flickr | IMAGE | 1+ | 1-500 | - | 5.0
Newsfeed | NEWS | 1 | 1+ | ? | 1.0

1 Google, Bing and Yahoo are paid services – see further how to obtain a license key.
2 Bing.search(type=NEWS) has a count of 1-15.
3 Yahoo.search(type=IMAGES) has a count of 1-35.
Web service license key

Some services require a license key. They may work without one, but this implies that you share a public license key (and query limit) with other users of the pattern.web module. If the query limit is exceeded, SearchEngine.search() raises a SearchEngineLimitError.

- Google is a paid service ($1 for 200 queries), with a 100 free queries per day. When you obtain a license key (follow the link below), activate "Custom Search API" and "Translate API" under "Services" and look up the key under "API Access".
- Bing is a paid service ($1 for 500 queries), with a 5,000 free queries per month.
- Yahoo is a paid service ($1 for 1250 queries) that requires an OAuth key + secret, which can be passed as a tuple: Yahoo(license=(key, secret)).

Obtain a license key: Google, Bing, Yahoo, Twitter, Facebook, Flickr.
Web service request throttle

A SearchEngine.search() request takes a minimum amount of time to complete, as outlined in the table above. This is intended as etiquette towards the server providing the service. Raise the throttle value if you plan to run multiple queries in batch. Wikipedia requests are especially intensive. If you plan to mine a lot of data from Wikipedia, download the Wikipedia database instead.

## Search Engine results

SearchEngine.search() returns a list of Result objects. It has an additional total property, which is the total number of results available for the given query. Each Result is a dictionary with extra properties:

    result = Result(url)

    result.url                  # URL of content associated with the given query.
    result.title                # Content title.
    result.text                 # Content summary.
    result.language             # Content language.
    result.author               # For news items and images.
    result.date                 # For news items.

    result.download(timeout=10, cached=True, proxy=None)

- Result.download() takes the same optional parameters as URL.download().
- The attributes (e.g., result.text) are Unicode strings.

For example:

    >>> from pattern.web import Bing, SEARCH, plaintext
    >>>
    >>> engine = Bing(license=None) # Enter your license key.
    >>> for i in range(1,5):
    >>>    for result in engine.search('holy handgrenade', type=SEARCH, start=i):
    >>>        print repr(plaintext(result.text))
    >>>        print

    u"The Holy Hand Grenade of Antioch is a fictional weapon from ..."
    u'Once the number three, being the third number, be reached, then ...'

Since SearchEngine.search() takes the same optional parameters as URL.download() it is easy to disable local caching, set a proxy server, a throttle (minimum time) or a timeout (maximum time).

    >>> from pattern.web import Google
    >>>
    >>> engine = Google(license=None) # Enter your license key.
    >>> for result in engine.search('tim', cached=False, proxy=('proxy.com', 'https'))
    >>>     print result.url
    >>>     print result.text

Image search

For Flickr, Bing and Yahoo, image URLs retrieved with search(type=IMAGE) can be filtered by setting the size to TINY, SMALL, MEDIUM, LARGE or None (any size). Images may be subject to copyright.

For Flickr, use search(copyright=False) to retrieve results with no copyright restrictions (either public domain or Creative Commons by-sa).

For Twitter, each result has a Result.profile property with the URL to the user's profile picture.

## Google translate

Google.translate() returns the translated string in the given language.
Google.identify() returns a (language code, confidence)-tuple for a given string.

    >>> from pattern.web import Google
    >>>
    >>> s = "C'est un lapin, lapin de bois. Quoi? Un cadeau."
    >>> g = Google()
    >>> print g.translate(s, input='fr', output='en', cached=False)
    >>> print g.identify(s)

    u"It's a rabbit, wood. What? A gift."
    (u'fr', 0.76)

Remember to activate the Translate API in the Google API Console. Max. 1,000 characters per request.

## Twitter search

The start parameter of Twitter.search() takes an int (= the starting page, cfr. other search engines) or a tweet.id. If you create two Twitter objects, their result pages for a given query may not correspond, since new tweets become available more quickly than we can query pages. The best way is to pass the last seen tweet id:

    >>> from pattern.web import Twitter
    >>>
    >>> t = Twitter()
    >>> i = None
    >>> for j in range(3):
    >>>     for tweet in t.search('win', start=i, count=10):
    >>>         print tweet.text
    >>>         print
    >>>         i = tweet.id

## Twitter streams

Twitter.stream() returns an endless, live stream of Result objects. A Stream is a Python list that accumulates each time Stream.update() is called:

    >>> from pattern.web import Twitter
    >>>
    >>> s = Twitter().stream('#fail')
    >>> for i in range(10):
    >>>     time.sleep(1)
    >>>     s.update(bytes=1024)
    >>>     print s[-1].text if s else ''

To clear the accumulated list, call Stream.clear().

## Twitter trends

Twitter.trends() returns a list of 10 "trending topics":

    >>> from pattern.web import Twitter
    >>> print Twitter().trends(cached=False)

    [u'#neverunderstood', u'Not Top 10', ...]

## Wikipedia articles

Wikipedia.search() returns a single WikipediaArticle for the given (case-sensitive) query, which is the title of an article. Wikipedia.index() returns an iterator over all article titles on Wikipedia. The language parameter of the Wikipedia()defines the language of the returned articles (by default it is "en", which corresponds to en.wikipedia.org).

    article = WikipediaArticle(title='', source='', links=[])

    article.source              # Article HTML source.
    article.string              # Article plaintext unicode string.

    article.title               # Article title.
    article.sections            # Article sections.
    article.links               # List of titles of linked articles.
    article.external            # List of external links.
    article.categories          # List of categories.
    article.media               # List of linked media (images, sounds, ...)
    article.languages           # Dictionary of (language, article)-items.
    article.language            # Article language (i.e., 'en').
    article.disambiguation      # True if it is a disambiguation page

    article.plaintext(**kwargs) # See plaintext() for parameters overview.
    article.download(media, **kwargs)

WikipediaArticle.plaintext() is similar to plaintext(), with special attention for MediaWiki markup. It strips metadata, infoboxes, table of contents, annotations, thumbnails and disambiguation links.

### Wikipedia article sections

WikipediaArticle.sections is a list of WikipediaSection objects. Each section has a title and a number of paragraphs that belong together.

    section = WikipediaSection(article, title='', start=0, stop=0, level=1)

    section.article             # WikipediaArticle parent.
    section.parent              # WikipediaSection this section is part of.
    section.children            # WikipediaSections belonging to this section.

    section.title               # Section title.
    section.source              # Section HTML source.
    section.string              # Section plaintext unicode string.
    section.content             # Section string minus title.
    section.level               # Section nested depth (from 0).
    section.links               # List of titles of linked articles.
    section.tables              # List of WikipediaTable objects.

The following example downloads a Wikipedia article and prints the title of each section, indented according to the section level:

    >>> from pattern.web import Wikipedia
    >>>
    >>> article = Wikipedia().search('cat')
    >>> for section in article.sections:
    >>>    print repr(' ' * section.level + section.title)

    u'Cat'
    u' Nomenclature and etymology'
    u' Taxonomy and evolution'
    u' Genetics'
    u' Anatomy'
    u' Behavior'
    u'  Sociability'
    u'  Grooming'
    u'  Fighting'
    ...

### Wikipedia article tables

WikipediaSection.tables is a list of WikipediaTable objects. Each table has a title, headers and rows.

    table = WikipediaTable(section, title='', headers=[], rows=[], source='')

    table.section               # WikipediaSection parent.
    table.source                # Table HTML source.
    table.title                 # Table title.
    table.headers               # List of table column headers.
    table.rows                  # List of table rows, each a list of column values.

## Wikia

Wikia is a free hosting service for thousands of wikis. Wikipedia, Wiktionary and Wikia all inherit the MediaWiki base class, so Wikia has the same methods and properties as Wikipedia. Its constructor takes the name of a domain on Wikia. Note the use of Wikia.index(), which returns an iterator over all available article titles:

    >>> from pattern.web import Wikia
    >>>
    >>> w = Wikia(domain='montypython')
    >>> for i, title in enumerate(w.index(start='a', throttle=1.0, cached=True)):
    >>>     if i >= 3:
    >>>         break
    >>>     article = w.search(title)
    >>>     print repr(article.title)

    u'Albatross'
    u'Always Look on the Bright Side of Life'
    u'And Now for Something Completely Different'

## DBPedia

DBPedia is a database of structured information mined from Wikipedia and stored as (subject, predicate, object)-triples (e.g., cat is-a animal). DBPedia can be queried with SPARQL, where subject, predicate and/or object can be given as ?variables. The Result objects in the list returned from DBPedia.search() have the variables as additional properties:

    >>> from pattern.web import DBPedia
    >>>
    >>> sparql = '\n'.join((
    >>>     'prefix dbo: <http://dbpedia.org/ontology/>',
    >>>     'select ?person ?place where {',
    >>>     '    ?person a dbo:President.',
    >>>     '    ?person dbo:birthPlace ?place.',
    >>>     '}'
    >>> ))
    >>> for r in DBPedia().search(sparql, start=1, count=10):
    >>>     print '%s (%s)' % (r.person.name, r.place.name)

    Álvaro Arzú (Guatemala City)
    Árpád Göncz (Budapest)
    ...

## Facebook posts, comments & likes

Facebook.search(query, type=SEARCH) returns a list of Result objects, where each result is a (publicly available) post that contains (or which comments contain) the given query.

Facebook.search(id, type=NEWS) returns posts from a given user profile. You need to supply a personal license key. You can get a key when you authorize Pattern to search Facebook in your name.

Facebook.search(id, type=COMMENTS) retrieves comments for a given post's Result.id. You can also pass the id of a post or a comment to Facebook.search(id, type=LIKES) to retrieve users that liked it.

    >>> from pattern.web import Facebook, NEWS, COMMENTS, LIKES
    >>>
    >>> fb = Facebook(license='your key')
    >>> me = fb.profile(id=None) # (id, name, date, gender, locale, likes)-tuple
    >>>
    >>> for post in fb.search(me[0], type=NEWS, count=100):
    >>>     print repr(post.id)
    >>>     print repr(post.text)
    >>>     print repr(post.url)
    >>>     if post.comments > 0:
    >>>         print '%i comments' % post.comments
    >>>         print [(r.text, r.author) for r in fb.search(post.id, type=COMMENTS)]
    >>>     if post.likes > 0:
    >>>         print '%i likes' % post.likes
    >>>         print [r.author for r in fb.search(post.id, type=LIKES)]

    u'530415277_10151455896030278'
    u'Tom De Smedt likes CLiPS Research Center'
    u'http://www.facebook.com/CLiPS.UA'
    1 likes
    [(u'485942414773810', u'CLiPS Research Center')]
    ....

The maximum count for COMMENTS and LIKES is 1000 (by default, 10).

## RSS + Atom newsfeeds

The Newsfeed object is a wrapper for Mark Pilgrim's Universal Feed Parser. Newsfeed.search() takes the URL of an RSS or Atom news feed and returns a list of Result objects.

    >>> from pattern.web import Newsfeed
    >>>
    >>> NATURE = 'http://www.nature.com/nature/current_issue/rss/index.html'
    >>> for result in Newsfeed().search(NATURE)[:5]:
    >>>     print repr(result.title)

    u'Biopiracy rules should not block biological control'
    u'Animal behaviour: Same-shaped shoals'
    u'Genetics: Fast disease factor'
    u'Biomimetics: Material monitors mugginess'
    u'Cell biology: Lung lipid hurts breathing'

Newsfeed.search() has an optional parameter tags, which is a list of custom tags to parse:

    >>> for result in Newsfeed().search(NATURE, tags=['dc:identifier']):
    >>>     print result.dc_identifier

## Web sort

The return value of SearchEngine.search() has a total property which can be used to sort queries by "crowdvoting". The sort() function sorts a given list of terms according to their total result count, and returns a list of (percentage, term)-tuples.

    sort(
        terms = [],             # List of search terms.
      context = '',             # Term used for sorting.
      service = GOOGLE,         # GOOGLE | BING | YAHOO | FLICKR
      license = None,           # Service license key.
       strict = True,           # Wrap query in quotes?
       prefix = False,          # context + term or term + context?
       cached = True)

When a context is defined, the function sorts by relevance to the context, e.g., sort(["black", "white"], context="Darth Vader") yields black as the best candidate, because "black Darth Vader" is more common in search results.

Now let's see who is more dangerous:

    >>> from pattern.web import sort
    >>>
    >>> results = sort(terms=[
    >>>   'arnold schwarzenegger',
    >>>   'chuck norris',
    >>>   'dolph lundgren',
    >>>   'steven seagal',
    >>>   'sylvester stallone',
    >>>   'mickey mouse'], context='dangerous', prefix=True)
    >>>
    >>> for weight, term in results:
    >>>     print "%.2f" % (weight * 100) + '%', term

    84.34% 'dangerous mickey mouse'
     9.24% 'dangerous chuck norris'
     2.41% 'dangerous sylvester stallone'
     2.01% 'dangerous arnold schwarzenegger'
     1.61% 'dangerous steven seagal'
     0.40% 'dangerous dolph lundgren'

## HTML to plaintext

The HTML source code of a web page can be retrieved with URL.download(). HTML is a markup language that uses tags to define text formatting. For example, <b>hello</b> displays hello in bold. For many tasks we may want to strip the formatting so we can analyze (e.g., parse or count) the plain text.

The plaintext() function removes HTML formatting from a string.

    plaintext(html, keep=[], replace=blocks, linebreaks=2, indentation=False)

It performs the following steps to clean up the given string:

- Strip javascript: remove all <script> elements.
- Strip CSS: remove all <style> elements.
- Strip comments: remove all <!-- --> elements.
- Strip forms: remove all <form> elements.
- Strip tags: remove all HTML tags.
- Decode entities: replace &lt; with < (for example).
- Collapse spaces: replace consecutive spaces with a single space.
- Collapse linebreaks: replace consecutive linebreaks with a single linebreak.
- Collapse tabs: replace consecutive tabs with a single space, optionally indentation (i.e., tabs at the start of a line) can be preserved.

plaintext parameters

The keep parameter is a list of tags to retain. By default, attributes are stripped, e.g., <table border="0"> becomes <table>. To preserve specific attributes, a dictionary can be given: {"a": ["href"]}.

The replace parameter defines how HTML elements are replaced with other characters to improve plain text layout. It is a dictionary of tag → (before, after) items. By default, it replaces block elements (i.e., <h1>, <h2>, <p>, <div>, <table>, ...) with two linebreaks, <th> and <tr> with one linebreak, <td> with one tab, and <li> with an asterisk (*) before and a linebreak after.

The linebreaks parameter defines the maximum number of consecutive linebreaks to retain.

The indentation parameter defines whether or not to retain tab indentation.

The following example downloads a HTML document and keeps a minimal amount of formatting (headings, bold, links).

    >>> from pattern.web import URL, plaintext
    >>>
    >>> s = URL('http://www.clips.ua.ac.be').download()
    >>> s = plaintext(s, keep={'h1':[], 'h2':[], 'strong':[], 'a':['href']})
    >>> print s

plaintext = strip + decode + collapse

The different steps in plaintext() are available as separate functions:

    decode_utf8(string)         # Byte string to Unicode string.

    encode_utf8(string)         # Unicode string to byte string.

    strip_tags(html, keep=[], replace=blocks) # Non-trivial, using SGML parser.

    strip_between(a, b, string) # Remove anything between (and including) a and b.

    strip_javascript(html)      # Strips between '<script*>' and '</script'.

    strip_inline_css(html)      # Strips between '<style*>' and '</style>'.

    strip_comments(html)        # Strips between '<!--' and '-->'.

    strip_forms(html)           # Strips between '<form*>' and '</form>'.

    decode_entities(string)     # '&lt;' => '<'

    encode_entities(string)     # '<' => '&lt;'

    decode_url(string)          # 'and%2For' => 'and/or'

    encode_url(string)          # 'and/or' => 'and%2For'

    collapse_spaces(string, indentation=False, replace=' ')

    collapse_tabs(string, indentation=False, replace=' ')

    collapse_linebreaks(string, threshold=1)

## HTML DOM parser

The Document Object Model (DOM) is a language-independent convention for representing HTML, XHTML and XML documents. The pattern.web module includes a HTML DOM parser (based on Leonard Richardson's BeautifulSoup) that can be used to traverse a HTML document as a tree of linked Python objects. This is useful to extract specific portions from a HTML string retrieved with URL.download().

### Node

The DOM consists of a DOM object that contains Text, Comment and Element objects.
All of these are subclasses of Node.

    node = Node(html, type=NODE)

    node.type                   # NODE | TEXT | COMMENT | ELEMENT | DOCUMENT
    node.source                 # HTML source.
    node.parent                 # Parent node.
    node.children               # List of child nodes.
    node.next                   # Next child in node.parent (or None).
    node.previous               # Previous child in node.parent (or None).

    node.traverse(visit=lambda node: None)

### Element

Text, Comment and Element are subclasses of Node. For example, 'the <b>cat</b>' is parsed to Text('the') + Element('cat', tag='b'). The Element object has a number of additional properties:

    element = Element(html)

    element.tag                 # Tag name.
    element.attrs               # Dictionary of attributes, e.g. {'class':'comment'}.
    element.id                  # Value for id attribute (or None).

    element.source              # HTML source.
    element.content             # HTML source minus open and close tag.

    element.by_id(str)          # First nested Element with given id.
    element.by_tag(str)         # List of nested Elements with given tag name.
    element.by_class(str)       # List of nested Elements with given class.
    element.by_attr(**kwargs)   # List of nested Elements with given attribute.
    element(selector)           # List of nested Elements matching a CSS selector.

- Element.by_tag() can include a class (e.g., "div.header") or an id (e.g., "div#content").
  A wildcard can be used to match any tag. (e.g. "*.even").
  The element is searched recursively (children in children, etc.)
- Element.by_attr() takes one or more keyword arguments (e.g., name="keywords").
- Element(selector) returns a list of nested elements that match the given CSS selector:

Overview of CSS selectors:

CSS Selector | Description
element('*') | all nested elements
element('*#x') | all nested elements with id="x"
element('div#x') | all nested <div> elements with id="x"
element('div.x') | all nested <div> elements with class="x"
element('div[class="x"]') | all nested <div> elements with attribute "class" = "x"
element('div:first-child') | the first child in a <div>
element('div a') | all nested <a>'s inside a nested <div>
element('div, a') | all nested <a>'s and <div> elements
element('div + a') | all nested <a>'s directly preceded by a <div>
element('div > a') | all nested <a>'s directly inside a nested <div>
element('div < a') | all nested <div>'s directly containing an <a>

    >>> from pattern.web import Element
    >>>
    >>> div = Element('<div> <a>1st</a> <a>2nd<a> </div>')
    >>> print div('a:first-child')
    >>> print div('a:first-child')[0].source

    [Element(tag='a')]
    <a>1st</a>

### DOM

The top-level element in the Document Object Model.

    dom = DOM(html)

    dom.declaration             # <!doctype> TEXT Node.
    dom.head                    # <head> Element.
    dom.body                    # <body> Element.

The following example retrieves the most recent reddit entries. The pattern.web module does not include a reddit search engine, but we can parse entries directly from the HTML source. This is called screen scraping, and many websites will strongly dislike it.

    >>> from pattern.web import URL, DOM, plaintext
    >>>
    >>> url = URL('http://www.reddit.com/top/')
    >>> dom = DOM(url.download(cached=True))
    >>> for e in dom('div.entry')[:3]: # Top 3 reddit entries.
    >>>     for a in e('a.title')[:1]: # First <a class="title">.
    >>>         print repr(plaintext(a.content))

    u'Invisible Kitty'
    u'Naturally, he said yes.'
    u"I'd just like to remind everyone that /r/minecraft exists and not everyone wants"
     "to have 10 Minecraft posts a day on their front page."

Absolute URLs

Links parsed from the DOM can be relative (e.g., starting with "../" instead of "http://").
To get the absolute URL, you can use the abs() function in combination with URL.redirect:

    >>> from pattern.web import URL, DOM, abs
    >>>
    >>> url = URL('http://www.clips.ua.ac.be')
    >>> dom = DOM(url.download())
    >>> for link in dom('a'):
    >>>     print abs(link.attributes.get('href',''), base=url.redirect or url.string)

## PDF Parser

Portable Document Format (PDF) is a popular open standard, where text, fonts, images and layout are contained in a single document that displays the same across systems. However, extracting the source text from a PDF can be difficult.

The PDF object (based on PDFMiner) parses the source text from a PDF file.

    >>> from pattern.web import URL, PDF
    >>>
    >>> url = URL('http://www.clips.ua.ac.be/sites/default/files/ctrs-002_0.pdf')
    >>> pdf = PDF(url.download())
    >>> print pdf.string

    CLiPS Technical Report series 002 September 7, 2010
    Tom De Smedt, Vincent Van Asch, Walter Daelemans
    Computational Linguistics & Psycholinguistics Research Center
    ...

URLs linking to a PDF document can be identified with: URL.mimetype in MIMETYPE_PDF.

## Crawler

A web crawler or web spider can be used to traverse the web automatically. The Crawler object takes a list of URLs. These are then visited by the crawler. If they lead to a web page, the HTML content is parsed for new links. These are added to the list of links scheduled for a visit.

The given domains is a list of allowed domain names. An empty list means the crawler can visit the entire web. The given delay defines the number of seconds to wait before revisiting the same (sub)domain – continually hammering one server with a robot disrupts requests from the website's regular visitors (this is called a denial-of-service attack).

    crawler = Crawler(links=[], domains=[], delay=20.0, sort=FIFO)

    crawler.domains              # Domains allowed to visit (e.g., ['clips.ua.ac.be']).
    crawler.delay                # Delay between visits to the same (sub)domain.
    crawler.history              # Dictionary of (domain, time last visited)-items.
    crawler.visited              # Dictionary of URLs visited.
    crawler.sort                 # FIFO | LIFO (how new links are queued).
    crawler.done                 # True when all links have been visited.

    crawler.push(link, priority=1.0, sort=LIFO)
    crawler.pop(remove=True)
    crawler.next                 # Yields the next scheduled link = Crawler.pop(False)

    crawler.crawl(method=DEPTH)  # DEPTH | BREADTH | None.

    crawler.priority(link, method=DEPTH)
    crawler.follow(link)
    crawler.visit(link, source=None)
    crawler.fail(link)

### Crawling process

- Crawler.crawl() is meant to be called continuously in a loop. It selects a link to visit and parses the HTML content for new links. The method parameter defines whether the crawler prefers internal links (DEPTH) or external links to other domains (BREADTH). If the link leads to a recently visited domain (i.e., elapsed time < Crawler.delay) it is temporarily skipped. To disable this behaviour, use an optional throttle parameter >= Crawler.delay.

- Crawler.priority() is called from Crawler.crawl() to determine the priority (0.0-1.0) of a new Link, where links with highest priority are visited first. It can be overridden in a subclass.

- Crawler.follow() is called from Crawler.crawl() to determine if it should schedule the given Link for a visit. By default it yields True. It can be overridden to disallow selected links.

- Crawler.visit() is called from Crawler.crawl() when a Link is visited. The given source is a HTML string with the page content. By default, this method does nothing (it should be overridden).

- Crawler.fail() is called from Crawler.crawl() for links whose MIME-type could not be determined, or which raise a URLError while downloading.

The crawler uses Link objects internally, which contain additional information besides the URL string:

    link = Link(url, text='', relation='')

    link.url                    # Parsed from <a href=''> attribute.
    link.text                   # Parsed from <a title=''> attribute.
    link.relation               # Parsed from <a rel=''> attribute.
    link.referrer               # Parent web page URL.

The following example shows a subclass of Crawler that prints each link it visits. Since it uses DEPTH for crawling, it will prefer internal links.

    >>> from pattern.web import Crawler
    >>>
    >>> class Polly(Crawler):
    >>>     def visit(self, link, source=None):
    >>>         print 'visited:', repr(link.url), 'from:', link.referrer
    >>>     def fail(self, link):
    >>>         print 'failed:', repr(link.url)
    >>>
    >>> p = Polly(links=['http://www.clips.ua.ac.be/'], delay=3)
    >>> while not p.done:
    >>>     p.crawl(method=DEPTH, cached=False, throttle=3)

    visited: u'http://www.clips.ua.ac.be/'
    visited: u'http://www.clips.ua.ac.be/#navigation'
    visited: u'http://www.clips.ua.ac.be/colloquia'
    visited: u'http://www.clips.ua.ac.be/computational-linguistics'
    visited: u'http://www.clips.ua.ac.be/contact'

Note: Crawler.crawl() takes the same parameters as URL.download(), e.g., cached=False or throttle=10.

### Crawl function

The crawl() function returns an iterator that yields (Link, source)-tuples. When it is idle (e.g., waiting for the delay on a domain) it yields (None, None).

    crawl(
        links = [],
      domains = [],
        delay = 20.0,
         sort = FIFO,
       method = DEPTH, **kwargs)

    >>> from pattern.web import crawl
    >>>
    >>> for link, source in crawl('http://www.clips.ua.ac.be/', delay=3, throttle=3):
    >>>     print link

    Link(url=u'http://www.clips.ua.ac.be/')
    Link(url=u'http://www.clips.ua.ac.be/#navigation')
    Link(url=u'http://www.clips.ua.ac.be/computational-linguistics')
    ...

## E-mail

The Mail object can be used to retrieve e-mail messages from Gmail, provided that IMAP is enabled. It may also work with other services, by passing the server address to the service parameter (e.g., service="imap.gmail.com"). With secure=False (no SSL) the default port is 143.

    mail = Mail(username, password, service=GMAIL, port=993, secure=True)

    mail.folders               # Dictionary of (name, MailFolder)-items.
    mail.[folder]              # E.g., Mail.inbox.read(id)
    mail.[folder].count        # Number of messages in folder.

    mail.[folder].search(query, field=FROM) # FROM | SUBJECT | DATE
    mail.[folder].read(id, attachments=False, cached=True)

- Mail.folders is a name → MailFolder dictionary. Common names include inbox, spam and trash.
- MailFolder.search() returns a list of e-mail id's, most recent first.
- MailFolder.read() retrieves the e-mail with given id as a Message.

A Message has the following properties:

    message = Mail.[folder].read(i)

    message.author              # Unicode string, sender name + e-mail address.
    message.email_address       # Unicode string, sender e-mail address.
    message.date                # Unicode string, date received.
    message.subject             # Unicode string, message subject.
    message.body                # Unicode string, message body.
    message.attachments         # List of (MIME-type, str)-tuples.

The following example retrieves spam e-mails containing the word "wish":

    >>> from pattern.web import Mail, GMAIL, SUBJECT
    >>>
    >>> gmail = Mail(username='...', password='...', service=GMAIL)
    >>> print gmail.folders.keys()

    ['drafts', 'spam', 'personal', 'work', 'inbox', 'mail', 'starred', 'trash']

    >>> i = gmail.spam.search('wish', field=SUBJECT)[0] # What riches await...
    >>> m = gmail.spam.read(i)
    >>> print '   From:', m.author
    >>> print 'Subject:', m.subject
    >>> print 'Message:'
    >>> print m.body

       From: u'Vegas VIP Clib <amllhbmjb@acciongeoda.org>'
    Subject: u'Your wish has been granted'
    Message: u'No one has claimed our jackpot! This is your chance to try!'

## Locale

The pattern.web.locale module contains functions for region and language codes, based on the ISO-639 language code (e.g., en), the ISO-3166 region code (e.g., US) and the IETF BCP 47 language-region specification (en-US):

    encode_language(name)       # 'English' => 'en'

    decode_language(code)       # 'en' => 'English'

    encode_region(name)         # 'United States' => 'US'

    decode_region(code)         # 'US' => 'United States'

    languages(region)           # 'US' => ['en']

    regions(language)           # 'en' => ['AU', 'BZ', 'CA', ...]

    regionalize(language)       # 'en' => ['en-US', 'en-AU', ...]

    market(language)            # 'en' => 'en-US'

The geocode() function recognizes a number of world capital cities and returns a tuple (latitude, longitude, ISO-639, region).

    geocode(location)           # 'Brussels' => (50.83, 4.33, u'nl', u'Belgium')

This is useful in combination with the geo parameter for Twitter.search() to obtain regional tweets:

    >>> from pattern.web import Twitter
    >>> from pattern.web.locale import geocode
    >>>
    >>> twitter = Twitter(language='en')
    >>> for tweet in twitter.search('restaurant', geo=geocode('Brussels')[:2]):
    >>>      print tweet.text

    u'Did you know: every McDonalds restaurant has free internet in Belgium...'

## Cache

By, default, URL.download() and SearchEngine.search() will cache results locally. Once the results of a query have been cached, there is no need to connect to the internet (i.e., the query runs faster). Over time the cache can grow quite large, filling up with whatever was downloaded – from tweets to zip archives.

To empty the cache:

    >>> from pattern.web import cache
    >>> cache.clear()

## See also

- BeautifulSoup (BSD): robust HTML parser for Python.
- Scrapy (BSD): s creen scraping and web crawling with Python.