text: #!/usr/bin/env python
  #
  # Copyright 2009 Facebook
  #
  # Licensed under the Apache License, Version 2.0 (the "License"); you may
  # not use this file except in compliance with the License. You may obtain
  # a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
  # License for the specific language governing permissions and limitations
  # under the License.
  
  """Escaping/unescaping methods for HTML, JSON, URLs, and others.
  
  Also includes a few other miscellaneous string manipulation functions that
  have crept in over time.
  """
  
  import htmlentitydefs
  import re
  import sys
  import urllib
  
  # Python3 compatibility:  On python2.5, introduce the bytes alias from 2.6
  try: bytes
  except Exception: bytes = str
  
  try:
      from urlparse import parse_qs  # Python 2.6+
  except ImportError:
      from cgi import parse_qs
  
  # json module is in the standard library as of python 2.6; fall back to
  # simplejson if present for older versions.
  try:
      import json
      assert hasattr(json, "loads") and hasattr(json, "dumps")
      _json_decode = json.loads
      _json_encode = json.dumps
  except Exception:
      try:
          import simplejson
          _json_decode = lambda s: simplejson.loads(_unicode(s))
          _json_encode = lambda v: simplejson.dumps(v)
      except ImportError:
          try:
              # For Google AppEngine
              from django.utils import simplejson
              _json_decode = lambda s: simplejson.loads(_unicode(s))
              _json_encode = lambda v: simplejson.dumps(v)
          except ImportError:
              def _json_decode(s):
                  raise NotImplementedError(
                      "A JSON parser is required, e.g., simplejson at "
                      "http://pypi.python.org/pypi/simplejson/")
              _json_encode = _json_decode
  
  
  _XHTML_ESCAPE_RE = re.compile('[&<>"]')
  _XHTML_ESCAPE_DICT = {'&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;'}
  def xhtml_escape(value):
      """Escapes a string so it is valid within XML or XHTML."""
      return _XHTML_ESCAPE_RE.sub(lambda match: _XHTML_ESCAPE_DICT[match.group(0)],
                                  to_basestring(value))
  
  
  def xhtml_unescape(value):
      """Un-escapes an XML-escaped string."""
      return re.sub(r"&(#?)(\w+?);", _convert_entity, _unicode(value))
  
  
  def json_encode(value):
      """JSON-encodes the given Python object."""
      # JSON permits but does not require forward slashes to be escaped.
      # This is useful when json data is emitted in a <script> tag
      # in HTML, as it prevents </script> tags from prematurely terminating
      # the javscript.  Some json libraries do this escaping by default,
      # although python's standard library does not, so we do it here.
      # http://stackoverflow.com/questions/1580647/json-why-are-forward-slashes-escaped
      return _json_encode(recursive_unicode(value)).replace("</", "<\\/")
  
  
  def json_decode(value):
      """Returns Python objects for the given JSON string."""
      return _json_decode(to_basestring(value))
  
  
  def squeeze(value):
      """Replace all sequences of whitespace chars with a single space."""
      return re.sub(r"[\x00-\x20]+", " ", value).strip()
  
  
  def url_escape(value):
      """Returns a valid URL-encoded version of the given value."""
      return urllib.quote_plus(utf8(value))
  
  # python 3 changed things around enough that we need two separate
  # implementations of url_unescape.  We also need our own implementation
  # of parse_qs since python 3's version insists on decoding everything.
  if sys.version_info[0] < 3:
      def url_unescape(value, encoding='utf-8'):
          """Decodes the given value from a URL.
  
          The argument may be either a byte or unicode string.
  
          If encoding is None, the result will be a byte string.  Otherwise,
          the result is a unicode string in the specified encoding.
          """
          if encoding is None:
              return urllib.unquote_plus(utf8(value))
          else:
              return unicode(urllib.unquote_plus(utf8(value)), encoding)
  
      parse_qs_bytes = parse_qs
  else:
      def url_unescape(value, encoding='utf-8'):
          """Decodes the given value from a URL.
  
          The argument may be either a byte or unicode string.
  
          If encoding is None, the result will be a byte string.  Otherwise,
          the result is a unicode string in the specified encoding.
          """
          if encoding is None:
              return urllib.parse.unquote_to_bytes(value)
          else:
              return urllib.unquote_plus(to_basestring(value), encoding=encoding)
  
      def parse_qs_bytes(qs, keep_blank_values=False, strict_parsing=False):
          """Parses a query string like urlparse.parse_qs, but returns the
          values as byte strings.
  
          Keys still become type str (interpreted as latin1 in python3!)
          because it's too painful to keep them as byte strings in
          python3 and in practice they're nearly always ascii anyway.
          """
          # This is gross, but python3 doesn't give us another way.
          # Latin1 is the universal donor of character encodings.
          result = parse_qs(qs, keep_blank_values, strict_parsing,
                            encoding='latin1', errors='strict')
          encoded = {}
          for k,v in result.iteritems():
              encoded[k] = [i.encode('latin1') for i in v]
          return encoded
          
  
  
  _UTF8_TYPES = (bytes, type(None))
  def utf8(value):
      """Converts a string argument to a byte string.
  
      If the argument is already a byte string or None, it is returned unchanged.
      Otherwise it must be a unicode string and is encoded as utf8.
      """
      if isinstance(value, _UTF8_TYPES):
          return value
      assert isinstance(value, unicode)
      return value.encode("utf-8")
  
  _TO_UNICODE_TYPES = (unicode, type(None))
  def to_unicode(value):
      """Converts a string argument to a unicode string.
  
      If the argument is already a unicode string or None, it is returned
      unchanged.  Otherwise it must be a byte string and is decoded as utf8.
      """
      if isinstance(value, _TO_UNICODE_TYPES):
          return value
      assert isinstance(value, bytes)
      return value.decode("utf-8")
  
  # to_unicode was previously named _unicode not because it was private,
  # but to avoid conflicts with the built-in unicode() function/type
  _unicode = to_unicode
  
  # When dealing with the standard library across python 2 and 3 it is
  # sometimes useful to have a direct conversion to the native string type
  if str is unicode:
      native_str = to_unicode
  else:
      native_str = utf8
  
  _BASESTRING_TYPES = (basestring, type(None))
  def to_basestring(value):
      """Converts a string argument to a subclass of basestring.
  
      In python2, byte and unicode strings are mostly interchangeable,
      so functions that deal with a user-supplied argument in combination
      with ascii string constants can use either and should return the type
      the user supplied.  In python3, the two types are not interchangeable,
      so this method is needed to convert byte strings to unicode.
      """
      if isinstance(value, _BASESTRING_TYPES):
          return value
      assert isinstance(value, bytes)
      return value.decode("utf-8")
  
  def recursive_unicode(obj):
      """Walks a simple data structure, converting byte strings to unicode.
  
      Supports lists, tuples, and dictionaries.
      """
      if isinstance(obj, dict):
          return dict((recursive_unicode(k), recursive_unicode(v)) for (k,v) in obj.iteritems())
      elif isinstance(obj, list):
          return list(recursive_unicode(i) for i in obj)
      elif isinstance(obj, tuple):
          return tuple(recursive_unicode(i) for i in obj)
      elif isinstance(obj, bytes):
          return to_unicode(obj)
      else:
          return obj
  
  # I originally used the regex from 
  # http://daringfireball.net/2010/07/improved_regex_for_matching_urls
  # but it gets all exponential on certain patterns (such as too many trailing
  # dots), causing the regex matcher to never return.
  # This regex should avoid those problems.
  _URL_RE = re.compile(ur"""\b((?:([\w-]+):(/{1,3})|www[.])(?:(?:(?:[^\s&()]|&amp;|&quot;)*(?:[^!"#$%&'()*+,.:;<=>?@\[\]^`{|}~\s]))|(?:\((?:[^\s&()]|&amp;|&quot;)*\)))+)""")
  
  
  def linkify(text, shorten=False, extra_params="",
              require_protocol=False, permitted_protocols=["http", "https"]):
      """Converts plain text into HTML with links.
  
      For example: ``linkify("Hello http://tornadoweb.org!")`` would return
      ``Hello <a href="http://tornadoweb.org">http://tornadoweb.org</a>!``
  
      Parameters:
  
      shorten: Long urls will be shortened for display.
  
      extra_params: Extra text to include in the link tag,
          e.g. linkify(text, extra_params='rel="nofollow" class="external"')
  
      require_protocol: Only linkify urls which include a protocol. If this is
          False, urls such as www.facebook.com will also be linkified.
  
      permitted_protocols: List (or set) of protocols which should be linkified,
          e.g. linkify(text, permitted_protocols=["http", "ftp", "mailto"]).
          It is very unsafe to include protocols such as "javascript".
      """
      if extra_params:
          extra_params = " " + extra_params.strip()
  
      def make_link(m):
          url = m.group(1)
          proto = m.group(2)
          if require_protocol and not proto:
              return url  # not protocol, no linkify
  
          if proto and proto not in permitted_protocols:
              return url  # bad protocol, no linkify
  
          href = m.group(1)
          if not proto:
              href = "http://" + href   # no proto specified, use http
  
          params = extra_params
  
          # clip long urls. max_len is just an approximation
          max_len = 30
          if shorten and len(url) > max_len:
              before_clip = url
              if proto:
                  proto_len = len(proto) + 1 + len(m.group(3) or "")  # +1 for :
              else:
                  proto_len = 0
  
              parts = url[proto_len:].split("/")
              if len(parts) > 1:
                  # Grab the whole host part plus the first bit of the path
                  # The path is usually not that interesting once shortened
                  # (no more slug, etc), so it really just provides a little
                  # extra indication of shortening.
                  url = url[:proto_len] + parts[0] + "/" + \
                          parts[1][:8].split('?')[0].split('.')[0]
  
              if len(url) > max_len * 1.5:  # still too long
                  url = url[:max_len]
  
              if url != before_clip:
                  amp = url.rfind('&')
                  # avoid splitting html char entities
                  if amp > max_len - 5:
                      url = url[:amp]
                  url += "..."
  
                  if len(url) >= len(before_clip):
                      url = before_clip
                  else:
                      # full url is visible on mouse-over (for those who don't
                      # have a status bar, such as Safari by default)
                      params += ' title="%s"' % href
  
          return u'<a href="%s"%s>%s</a>' % (href, params, url)
  
      # First HTML-escape so that our strings are all safe.
      # The regex is modified to avoid character entites other than &amp; so
      # that we won't pick up &quot;, etc.
      text = _unicode(xhtml_escape(text))
      return _URL_RE.sub(make_link, text)
  
  
  def _convert_entity(m):
      if m.group(1) == "#":
          try:
              return unichr(int(m.group(2)))
          except ValueError:
              return "&#%s;" % m.group(2)
      try:
          return _HTML_UNICODE_MAP[m.group(2)]
      except KeyError:
          return "&%s;" % m.group(2)
  
  
  def _build_unicode_map():
      unicode_map = {}
      for name, value in htmlentitydefs.name2codepoint.iteritems():
          unicode_map[name] = unichr(value)
      return unicode_map
  
  _HTML_UNICODE_MAP = _build_unicode_map()
  