Capitolo 5. Elaborare HTML

5.1. Immergersi

Spesso in comp.lang.python trovo domande del genere: “Come posso creare una lista di tutti gli [headers | immagini | collegamenti] presenti nel mio documento HTML?” “Come posso [analizzare | tradurre | modificare] il testo del mio documento HTML senza modificare i tag?” “Come posso [aggiungere | rimuovere | mettere tra apici] gli attributi di tutti i tag del mio HTML in una volta sola?” Questo capitolo risponderà a tutte queste domande.

Ecco un completo e funzionante programma in Python diviso in due parti. La prima parte, BaseHTMLProcessor.py, è un generico strumento che aiuta ad elaborare file HTML passando attraverso tag e blocchi di testo. La seconda parte, dialect.py, è un esempio di come usare BaseHTMLProcessor.py per tradurre il testo di un documento HTML lasciando inalterati i tag. Leggete le docstring ed i commenti per avere un'idea di cosa succede. La maggior parte sembrerà magia nera, perché non è ovvio come vengano chiamati i vari metodi delle classi. Non vi preoccupate, verrà tutto spiegato a tempo debito.

Esempio 5.1. `BaseHTMLProcessor.py`

Se non lo avete ancora fatto, potete scaricare questo ed altri esempi usati in questo libro.


from sgmllib import SGMLParser
import htmlentitydefs

class BaseHTMLProcessor(SGMLParser):
    def reset(self):                       
        # extend (called by SGMLParser.__init__)
        self.pieces = []
        SGMLParser.reset(self)

    def unknown_starttag(self, tag, attrs):
        # called for each start tag
        # attrs is a list of (attr, value) tuples
        # e.g. for <pre class="screen">, tag="pre", attrs=[("class", "screen")]
        # Ideally we would like to reconstruct original tag and attributes, but
        # we may end up quoting attribute values that weren't quoted in the source
        # document, or we may change the type of quotes around the attribute value
        # (single to double quotes).
        # Note that improperly embedded non-HTML code (like client-side Javascript)
        # may be parsed incorrectly by the ancestor, causing runtime script errors.
        # All non-HTML code must be enclosed in HTML comment tags (<!-- code -->)
        # to ensure that it will pass through this parser unaltered (in handle_comment).
        strattrs = "".join([' %s="%s"' % (key, value) for key, value in attrs])
        self.pieces.append("<%(tag)s%(strattrs)s>" % locals())

    def unknown_endtag(self, tag):         
        # called for each end tag, e.g. for </pre>, tag will be "pre"
        # Reconstruct the original end tag.
        self.pieces.append("</%(tag)s>" % locals())

    def handle_charref(self, ref):         
        # called for each character reference, e.g. for "&#160;", ref will be "160"
        # Reconstruct the original character reference.
        self.pieces.append("&#%(ref)s;" % locals())

    def handle_entityref(self, ref):       
        # called for each entity reference, e.g. for "&copy;", ref will be "copy"
        # Reconstruct the original entity reference.
        self.pieces.append("&%(ref)s" % locals())
        # standard HTML entities are closed with a semicolon; other entities are not
        if htmlentitydefs.entitydefs.has_key(ref):
            self.pieces.append(";")

    def handle_data(self, text):           
        # called for each block of plain text, i.e. outside of any tag and
        # not containing any character or entity references
        # Store the original text verbatim.
        self.pieces.append(text)

    def handle_comment(self, text):        
        # called for each HTML comment, e.g. <!-- insert Javascript code here -->
        # Reconstruct the original comment.
        # It is especially important that the source document enclose client-side
        # code (like Javascript) within comments so it can pass through this
        # processor undisturbed; see comments in unknown_starttag for details.
        self.pieces.append("<!--%(text)s-->" % locals())

    def handle_pi(self, text):             
        # called for each processing instruction, e.g. <?instruction>
        # Reconstruct original processing instruction.
        self.pieces.append("<?%(text)s>" % locals())

    def handle_decl(self, text):
        # called for the DOCTYPE, if present, e.g.
        # <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
        #     "http://www.w3.org/TR/html4/loose.dtd">
        # Reconstruct original DOCTYPE
        self.pieces.append("<!%(text)s>" % locals())

    def output(self):              
        """Return processed HTML as a single string"""
        return "".join(self.pieces)

Esempio 5.2. `dialect.py`


import re
from BaseHTMLProcessor import BaseHTMLProcessor

class Dialectizer(BaseHTMLProcessor):
    subs = ()

    def reset(self):
        # extend (called from __init__ in ancestor)
        # Reset all data attributes
        self.verbatim = 0
        BaseHTMLProcessor.reset(self)

    def start_pre(self, attrs):            
        # called for every <pre> tag in HTML source
        # Increment verbatim mode count, then handle tag like normal
        self.verbatim += 1                 
        self.unknown_starttag("pre", attrs)

    def end_pre(self):                     
        # called for every </pre> tag in HTML source
        # Decrement verbatim mode count
        self.unknown_endtag("pre")         
        self.verbatim -= 1                 

    def handle_data(self, text):                                        
        # override
        # called for every block of text in HTML source
        # If in verbatim mode, save text unaltered;
        # otherwise process the text with a series of substitutions
        self.pieces.append(self.verbatim and text or self.process(text))

    def process(self, text):
        # called from handle_data
        # Process text block by performing series of regular expression
        # substitutions (actual substitions are defined in descendant)
        for fromPattern, toPattern in self.subs:
            text = re.sub(fromPattern, toPattern, text)
        return text

class ChefDialectizer(Dialectizer):
    """convert HTML to Swedish Chef-speak
    
    based on the classic chef.x, copyright (c) 1992, 1993 John Hagerman
    """
    subs = ((r'a([nu])', r'u\1'),
            (r'A([nu])', r'U\1'),
            (r'a\B', r'e'),
            (r'A\B', r'E'),
            (r'en\b', r'ee'),
            (r'\Bew', r'oo'),
            (r'\Be\b', r'e-a'),
            (r'\be', r'i'),
            (r'\bE', r'I'),
            (r'\Bf', r'ff'),
            (r'\Bir', r'ur'),
            (r'(\w*?)i(\w*?)$', r'\1ee\2'),
            (r'\bow', r'oo'),
            (r'\bo', r'oo'),
            (r'\bO', r'Oo'),
            (r'the', r'zee'),
            (r'The', r'Zee'),
            (r'th\b', r't'),
            (r'\Btion', r'shun'),
            (r'\Bu', r'oo'),
            (r'\BU', r'Oo'),
            (r'v', r'f'),
            (r'V', r'F'),
            (r'w', r'w'),
            (r'W', r'W'),
            (r'([a-z])[.]', r'\1.  Bork Bork Bork!'))

class FuddDialectizer(Dialectizer):
    """convert HTML to Elmer Fudd-speak"""
    subs = ((r'[rl]', r'w'),
            (r'qu', r'qw'),
            (r'th\b', r'f'),
            (r'th', r'd'),
            (r'n[.]', r'n, uh-hah-hah-hah.'))

class OldeDialectizer(Dialectizer):
    """convert HTML to mock Middle English"""
    subs = ((r'i([bcdfghjklmnpqrstvwxyz])e\b', r'y\1'),
            (r'i([bcdfghjklmnpqrstvwxyz])e', r'y\1\1e'),
            (r'ick\b', r'yk'),
            (r'ia([bcdfghjklmnpqrstvwxyz])', r'e\1e'),
            (r'e[ea]([bcdfghjklmnpqrstvwxyz])', r'e\1e'),
            (r'([bcdfghjklmnpqrstvwxyz])y', r'\1ee'),
            (r'([bcdfghjklmnpqrstvwxyz])er', r'\1re'),
            (r'([aeiou])re\b', r'\1r'),
            (r'ia([bcdfghjklmnpqrstvwxyz])', r'i\1e'),
            (r'tion\b', r'cioun'),
            (r'ion\b', r'ioun'),
            (r'aid', r'ayde'),
            (r'ai', r'ey'),
            (r'ay\b', r'y'),
            (r'ay', r'ey'),
            (r'ant', r'aunt'),
            (r'ea', r'ee'),
            (r'oa', r'oo'),
            (r'ue', r'e'),
            (r'oe', r'o'),
            (r'ou', r'ow'),
            (r'ow', r'ou'),
            (r'\bhe', r'hi'),
            (r've\b', r'veth'),
            (r'se\b', r'e'),
            (r"'s\b", r'es'),
            (r'ic\b', r'ick'),
            (r'ics\b', r'icc'),
            (r'ical\b', r'ick'),
            (r'tle\b', r'til'),
            (r'll\b', r'l'),
            (r'ould\b', r'olde'),
            (r'own\b', r'oune'),
            (r'un\b', r'onne'),
            (r'rry\b', r'rye'),
            (r'est\b', r'este'),
            (r'pt\b', r'pte'),
            (r'th\b', r'the'),
            (r'ch\b', r'che'),
            (r'ss\b', r'sse'),
            (r'([wybdp])\b', r'\1e'),
            (r'([rnt])\b', r'\1\1e'),
            (r'from', r'fro'),
            (r'when', r'whan'))

def translate(url, dialectName="chef"):
    """fetch URL and translate using dialect
    
    dialect in ("chef", "fudd", "olde")"""
    import urllib                      
    sock = urllib.urlopen(url)         
    htmlSource = sock.read()           
    sock.close()                       
    parserName = "%sDialectizer" % dialectName.capitalize()
    parserClass = globals()[parserName]                    
    parser = parserClass()                                 
    parser.feed(htmlSource)
    parser.close()         
    return parser.output() 

def test(url):
    """test all dialects against URL"""
    for dialect in ("chef", "fudd", "olde"):
        outfile = "%s.html" % dialect
        fsock = open(outfile, "wb")
        fsock.write(translate(url, dialect))
        fsock.close()
        import webbrowser
        webbrowser.open_new(outfile)

if __name__ == "__main__":
    test("https://book.diveintopython.org/odbchelper_list.html")

Esempio 5.3. Output di `dialect.py`

L'avvio di questo script tradurrà il paragrafo Introduzione alle liste in pseudo Swedish Chef-speak (dai Muppets), in pseudo Elmer Fudd-speak (dal cartone di Bugs Bunny) e pseudo Middle English (malamente basato su Chaucer's The Canterbury Tales). Se guardate nel codice HTML delle pagine in output, noterete che tutti i tag HTML ed i loro attributi non sono stati modificati, mentre il testo tra i tags è stato “tradotto” nello pseudo linguaggio. Se guardate più attentamente vedrete che, di fatto, solo i titoli ed i paragrafi sono stati tradotti; il codice e gli esempi sono rimasti invariati.

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