__init__.py

# Use of this source code is governed by the MIT license.
__license__ = "MIT"

from collections import defaultdict
import itertools
import sys
from bs4.element import (
    CharsetMetaAttributeValue,
    ContentMetaAttributeValue,
    Stylesheet,
    Script,
    TemplateString,
    nonwhitespace_re
)

__all__ = [
    'HTMLTreeBuilder',
    'SAXTreeBuilder',
    'TreeBuilder',
    'TreeBuilderRegistry',
    ]

# Some useful features for a TreeBuilder to have.
FAST = 'fast'
PERMISSIVE = 'permissive'
STRICT = 'strict'
XML = 'xml'
HTML = 'html'
HTML_5 = 'html5'


class TreeBuilderRegistry(object):
    """A way of looking up TreeBuilder subclasses by their name or by desired
    features.
    """
    
    def __init__(self):
        self.builders_for_feature = defaultdict(list)
        self.builders = []

    def register(self, treebuilder_class):
        """Register a treebuilder based on its advertised features.

        :param treebuilder_class: A subclass of Treebuilder. its .features
           attribute should list its features.
        """
        for feature in treebuilder_class.features:
            self.builders_for_feature[feature].insert(0, treebuilder_class)
        self.builders.insert(0, treebuilder_class)

    def lookup(self, *features):
        """Look up a TreeBuilder subclass with the desired features.

        :param features: A list of features to look for. If none are
            provided, the most recently registered TreeBuilder subclass
            will be used.
        :return: A TreeBuilder subclass, or None if there's no
            registered subclass with all the requested features.
        """
        if len(self.builders) == 0:
            # There are no builders at all.
            return None

        if len(features) == 0:
            # They didn't ask for any features. Give them the most
            # recently registered builder.
            return self.builders[0]

        # Go down the list of features in order, and eliminate any builders
        # that don't match every feature.
        features = list(features)
        features.reverse()
        candidates = None
        candidate_set = None
        while len(features) > 0:
            feature = features.pop()
            we_have_the_feature = self.builders_for_feature.get(feature, [])
            if len(we_have_the_feature) > 0:
                if candidates is None:
                    candidates = we_have_the_feature
                    candidate_set = set(candidates)
                else:
                    # Eliminate any candidates that don't have this feature.
                    candidate_set = candidate_set.intersection(
                        set(we_have_the_feature))

        # The only valid candidates are the ones in candidate_set.
        # Go through the original list of candidates and pick the first one
        # that's in candidate_set.
        if candidate_set is None:
            return None
        for candidate in candidates:
            if candidate in candidate_set:
                return candidate
        return None

# The BeautifulSoup class will take feature lists from developers and use them
# to look up builders in this registry.
builder_registry = TreeBuilderRegistry()

class TreeBuilder(object):
    """Turn a textual document into a Beautiful Soup object tree."""

    NAME = "[Unknown tree builder]"
    ALTERNATE_NAMES = []
    features = []

    is_xml = False
    picklable = False
    empty_element_tags = None # A tag will be considered an empty-element
                              # tag when and only when it has no contents.
    
    # A value for these tag/attribute combinations is a space- or
    # comma-separated list of CDATA, rather than a single CDATA.
    DEFAULT_CDATA_LIST_ATTRIBUTES = {}

    # Whitespace should be preserved inside these tags.
    DEFAULT_PRESERVE_WHITESPACE_TAGS = set()

    # The textual contents of tags with these names should be
    # instantiated with some class other than NavigableString.
    DEFAULT_STRING_CONTAINERS = {}
    
    USE_DEFAULT = object()

    # Most parsers don't keep track of line numbers.
    TRACKS_LINE_NUMBERS = False
    
    def __init__(self, multi_valued_attributes=USE_DEFAULT,
                 preserve_whitespace_tags=USE_DEFAULT,
                 store_line_numbers=USE_DEFAULT,
                 string_containers=USE_DEFAULT,
    ):
        """Constructor.

        :param multi_valued_attributes: If this is set to None, the
         TreeBuilder will not turn any values for attributes like
         'class' into lists. Setting this to a dictionary will
         customize this behavior; look at DEFAULT_CDATA_LIST_ATTRIBUTES
         for an example.

         Internally, these are called "CDATA list attributes", but that
         probably doesn't make sense to an end-user, so the argument name
         is `multi_valued_attributes`.

        :param preserve_whitespace_tags: A list of tags to treat
         the way <pre> tags are treated in HTML. Tags in this list
         are immune from pretty-printing; their contents will always be
         output as-is.

        :param string_containers: A dictionary mapping tag names to
        the classes that should be instantiated to contain the textual
        contents of those tags. The default is to use NavigableString
        for every tag, no matter what the name. You can override the
        default by changing DEFAULT_STRING_CONTAINERS.

        :param store_line_numbers: If the parser keeps track of the
         line numbers and positions of the original markup, that
         information will, by default, be stored in each corresponding
         `Tag` object. You can turn this off by passing
         store_line_numbers=False. If the parser you're using doesn't 
         keep track of this information, then setting store_line_numbers=True
         will do nothing.
        """
        self.soup = None
        if multi_valued_attributes is self.USE_DEFAULT:
            multi_valued_attributes = self.DEFAULT_CDATA_LIST_ATTRIBUTES
        self.cdata_list_attributes = multi_valued_attributes
        if preserve_whitespace_tags is self.USE_DEFAULT:
            preserve_whitespace_tags = self.DEFAULT_PRESERVE_WHITESPACE_TAGS
        self.preserve_whitespace_tags = preserve_whitespace_tags
        if store_line_numbers == self.USE_DEFAULT:
            store_line_numbers = self.TRACKS_LINE_NUMBERS
        self.store_line_numbers = store_line_numbers 
        if string_containers == self.USE_DEFAULT:
            string_containers = self.DEFAULT_STRING_CONTAINERS
        self.string_containers = string_containers
        
    def initialize_soup(self, soup):
        """The BeautifulSoup object has been initialized and is now
        being associated with the TreeBuilder.

        :param soup: A BeautifulSoup object.
        """
        self.soup = soup
        
    def reset(self):
        """Do any work necessary to reset the underlying parser
        for a new document.

        By default, this does nothing.
        """
        pass

    def can_be_empty_element(self, tag_name):
        """Might a tag with this name be an empty-element tag?

        The final markup may or may not actually present this tag as
        self-closing.

        For instance: an HTMLBuilder does not consider a <p> tag to be
        an empty-element tag (it's not in
        HTMLBuilder.empty_element_tags). This means an empty <p> tag
        will be presented as "<p></p>", not "<p/>" or "<p>".

        The default implementation has no opinion about which tags are
        empty-element tags, so a tag will be presented as an
        empty-element tag if and only if it has no children.
        "<foo></foo>" will become "<foo/>", and "<foo>bar</foo>" will
        be left alone.

        :param tag_name: The name of a markup tag.
        """
        if self.empty_element_tags is None:
            return True
        return tag_name in self.empty_element_tags
    
    def feed(self, markup):
        """Run some incoming markup through some parsing process,
        populating the `BeautifulSoup` object in self.soup.

        This method is not implemented in TreeBuilder; it must be
        implemented in subclasses.

        :return: None.
        """
        raise NotImplementedError()

    def prepare_markup(self, markup, user_specified_encoding=None,
                       document_declared_encoding=None, exclude_encodings=None):
        """Run any preliminary steps necessary to make incoming markup
        acceptable to the parser.

        :param markup: Some markup -- probably a bytestring.
        :param user_specified_encoding: The user asked to try this encoding.
        :param document_declared_encoding: The markup itself claims to be
            in this encoding.
        :param exclude_encodings: The user asked _not_ to try any of
            these encodings.

        :yield: A series of 4-tuples:
         (markup, encoding, declared encoding,
          has undergone character replacement)

         Each 4-tuple represents a strategy for converting the
         document to Unicode and parsing it. Each strategy will be tried 
         in turn.

         By default, the only strategy is to parse the markup
         as-is. See `LXMLTreeBuilderForXML` and
         `HTMLParserTreeBuilder` for implementations that take into
         account the quirks of particular parsers.
        """
        yield markup, None, None, False

    def test_fragment_to_document(self, fragment):
        """Wrap an HTML fragment to make it look like a document.

        Different parsers do this differently. For instance, lxml
        introduces an empty <head> tag, and html5lib
        doesn't. Abstracting this away lets us write simple tests
        which run HTML fragments through the parser and compare the
        results against other HTML fragments.

        This method should not be used outside of tests.

        :param fragment: A string -- fragment of HTML.
        :return: A string -- a full HTML document.
        """
        return fragment

    def set_up_substitutions(self, tag):
        """Set up any substitutions that will need to be performed on 
        a `Tag` when it's output as a string.

        By default, this does nothing. See `HTMLTreeBuilder` for a
        case where this is used.

        :param tag: A `Tag`
        :return: Whether or not a substitution was performed.
        """
        return False

    def _replace_cdata_list_attribute_values(self, tag_name, attrs):
        """When an attribute value is associated with a tag that can
        have multiple values for that attribute, convert the string
        value to a list of strings.

        Basically, replaces class="foo bar" with class=["foo", "bar"]

        NOTE: This method modifies its input in place.

        :param tag_name: The name of a tag.
        :param attrs: A dictionary containing the tag's attributes.
           Any appropriate attribute values will be modified in place.
        """
        if not attrs:
            return attrs
        if self.cdata_list_attributes:
            universal = self.cdata_list_attributes.get('*', [])
            tag_specific = self.cdata_list_attributes.get(
                tag_name.lower(), None)
            for attr in list(attrs.keys()):
                if attr in universal or (tag_specific and attr in tag_specific):
                    # We have a "class"-type attribute whose string
                    # value is a whitespace-separated list of
                    # values. Split it into a list.
                    value = attrs[attr]
                    if isinstance(value, str):
                        values = nonwhitespace_re.findall(value)
                    else:
                        # html5lib sometimes calls setAttributes twice
                        # for the same tag when rearranging the parse
                        # tree. On the second call the attribute value
                        # here is already a list.  If this happens,
                        # leave the value alone rather than trying to
                        # split it again.
                        values = value
                    attrs[attr] = values
        return attrs

class SAXTreeBuilder(TreeBuilder):
    """A Beautiful Soup treebuilder that listens for SAX events.

    This is not currently used for anything, but it demonstrates
    how a simple TreeBuilder would work.
    """

    def feed(self, markup):
        raise NotImplementedError()

    def close(self):
        pass

    def startElement(self, name, attrs):
        attrs = dict((key[1], value) for key, value in list(attrs.items()))
        #print("Start %s, %r" % (name, attrs))
        self.soup.handle_starttag(name, attrs)

    def endElement(self, name):
        #print("End %s" % name)
        self.soup.handle_endtag(name)

    def startElementNS(self, nsTuple, nodeName, attrs):
        # Throw away (ns, nodeName) for now.
        self.startElement(nodeName, attrs)

    def endElementNS(self, nsTuple, nodeName):
        # Throw away (ns, nodeName) for now.
        self.endElement(nodeName)
        #handler.endElementNS((ns, node.nodeName), node.nodeName)

    def startPrefixMapping(self, prefix, nodeValue):
        # Ignore the prefix for now.
        pass

    def endPrefixMapping(self, prefix):
        # Ignore the prefix for now.
        # handler.endPrefixMapping(prefix)
        pass

    def characters(self, content):
        self.soup.handle_data(content)

    def startDocument(self):
        pass

    def endDocument(self):
        pass


class HTMLTreeBuilder(TreeBuilder):
    """This TreeBuilder knows facts about HTML.

    Such as which tags are empty-element tags.
    """

    empty_element_tags = set([
        # These are from HTML5.
        'area', 'base', 'br', 'col', 'embed', 'hr', 'img', 'input', 'keygen', 'link', 'menuitem', 'meta', 'param', 'source', 'track', 'wbr',
        
        # These are from earlier versions of HTML and are removed in HTML5.
        'basefont', 'bgsound', 'command', 'frame', 'image', 'isindex', 'nextid', 'spacer'
    ])

    # The HTML standard defines these as block-level elements. Beautiful
    # Soup does not treat these elements differently from other elements,
    # but it may do so eventually, and this information is available if
    # you need to use it.
    block_elements = set(["address", "article", "aside", "blockquote", "canvas", "dd", "div", "dl", "dt", "fieldset", "figcaption", "figure", "footer", "form", "h1", "h2", "h3", "h4", "h5", "h6", "header", "hr", "li", "main", "nav", "noscript", "ol", "output", "p", "pre", "section", "table", "tfoot", "ul", "video"])

    # The HTML standard defines an unusual content model for these tags.
    # We represent this by using a string class other than NavigableString
    # inside these tags.
    #
    # I made this list by going through the HTML spec
    # (https://html.spec.whatwg.org/#metadata-content) and looking for
    # "metadata content" elements that can contain strings.
    #
    # TODO: Arguably <noscript> could go here but it seems
    # qualitatively different from the other tags.
    DEFAULT_STRING_CONTAINERS = {
        'style': Stylesheet,
        'script': Script,
        'template': TemplateString,
    }    
    
    # The HTML standard defines these attributes as containing a
    # space-separated list of values, not a single value. That is,
    # class="foo bar" means that the 'class' attribute has two values,
    # 'foo' and 'bar', not the single value 'foo bar'.  When we
    # encounter one of these attributes, we will parse its value into
    # a list of values if possible. Upon output, the list will be
    # converted back into a string.
    DEFAULT_CDATA_LIST_ATTRIBUTES = {
        "*" : ['class', 'accesskey', 'dropzone'],
        "a" : ['rel', 'rev'],
        "link" :  ['rel', 'rev'],
        "td" : ["headers"],
        "th" : ["headers"],
        "td" : ["headers"],
        "form" : ["accept-charset"],
        "object" : ["archive"],

        # These are HTML5 specific, as are *.accesskey and *.dropzone above.
        "area" : ["rel"],
        "icon" : ["sizes"],
        "iframe" : ["sandbox"],
        "output" : ["for"],
        }

    DEFAULT_PRESERVE_WHITESPACE_TAGS = set(['pre', 'textarea'])
    
    def set_up_substitutions(self, tag):
        """Replace the declared encoding in a <meta> tag with a placeholder,
        to be substituted when the tag is output to a string.

        An HTML document may come in to Beautiful Soup as one
        encoding, but exit in a different encoding, and the <meta> tag
        needs to be changed to reflect this.

        :param tag: A `Tag`
        :return: Whether or not a substitution was performed.
        """
        # We are only interested in <meta> tags
        if tag.name != 'meta':
            return False

        http_equiv = tag.get('http-equiv')
        content = tag.get('content')
        charset = tag.get('charset')

        # We are interested in <meta> tags that say what encoding the
        # document was originally in. This means HTML 5-style <meta>
        # tags that provide the "charset" attribute. It also means
        # HTML 4-style <meta> tags that provide the "content"
        # attribute and have "http-equiv" set to "content-type".
        #
        # In both cases we will replace the value of the appropriate
        # attribute with a standin object that can take on any
        # encoding.
        meta_encoding = None
        if charset is not None:
            # HTML 5 style:
            # <meta charset="utf8">
            meta_encoding = charset
            tag['charset'] = CharsetMetaAttributeValue(charset)

        elif (content is not None and http_equiv is not None
              and http_equiv.lower() == 'content-type'):
            # HTML 4 style:
            # <meta http-equiv="content-type" content="text/html; charset=utf8">
            tag['content'] = ContentMetaAttributeValue(content)

        return (meta_encoding is not None)

def register_treebuilders_from(module):
    """Copy TreeBuilders from the given module into this module."""
    # I'm fairly sure this is not the best way to do this.
    this_module = sys.modules['bs4.builder']
    for name in module.__all__:
        obj = getattr(module, name)

        if issubclass(obj, TreeBuilder):
            setattr(this_module, name, obj)
            this_module.__all__.append(name)
            # Register the builder while we're at it.
            this_module.builder_registry.register(obj)

class ParserRejectedMarkup(Exception):
    """An Exception to be raised when the underlying parser simply
    refuses to parse the given markup.
    """
    def __init__(self, message_or_exception):
        """Explain why the parser rejected the given markup, either
        with a textual explanation or another exception.
        """
        if isinstance(message_or_exception, Exception):
            e = message_or_exception
            message_or_exception = "%s: %s" % (e.__class__.__name__, str(e))
        super(ParserRejectedMarkup, self).__init__(message_or_exception)
            
# Builders are registered in reverse order of priority, so that custom
# builder registrations will take precedence. In general, we want lxml
# to take precedence over html5lib, because it's faster. And we only
# want to use HTMLParser as a last resort.
from . import _htmlparser
register_treebuilders_from(_htmlparser)
try:
    from . import _html5lib
    register_treebuilders_from(_html5lib)
except ImportError:
    # They don't have html5lib installed.
    pass
try:
    from . import _lxml
    register_treebuilders_from(_lxml)
except ImportError:
    # They don't have lxml installed.
    pass