path: root/websdk/werkzeug/http.py

# -*- coding: utf-8 -*-
"""
    werkzeug.http
    ~~~~~~~~~~~~~

    Werkzeug comes with a bunch of utilities that help Werkzeug to deal with
    HTTP data.  Most of the classes and functions provided by this module are
    used by the wrappers, but they are useful on their own, too, especially if
    the response and request objects are not used.

    This covers some of the more HTTP centric features of WSGI, some other
    utilities such as cookie handling are documented in the `werkzeug.utils`
    module.


    :copyright: (c) 2011 by the Werkzeug Team, see AUTHORS for more details.
    :license: BSD, see LICENSE for more details.
"""
import re
from time import time
try:
    from email.utils import parsedate_tz
except ImportError: # pragma: no cover
    from email.Utils import parsedate_tz
from urllib2 import parse_http_list as _parse_list_header
from datetime import datetime, timedelta
try:
    from hashlib import md5
except ImportError: # pragma: no cover
    from md5 import new as md5


#: HTTP_STATUS_CODES is "exported" from this module.
#: XXX: move to werkzeug.consts or something
from werkzeug._internal import HTTP_STATUS_CODES, _dump_date, \
     _ExtendedCookie, _ExtendedMorsel, _decode_unicode


_accept_re = re.compile(r'([^\s;,]+)(?:[^,]*?;\s*q=(\d*(?:\.\d+)?))?')
_token_chars = frozenset("!#$%&'*+-.0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                         '^_`abcdefghijklmnopqrstuvwxyz|~')
_etag_re = re.compile(r'([Ww]/)?(?:"(.*?)"|(.*?))(?:\s*,\s*|$)')
_unsafe_header_chars = set('()<>@,;:\"/[]?={} \t')
_quoted_string_re = r'"[^"\\]*(?:\\.[^"\\]*)*"'
_option_header_piece_re = re.compile(r';\s*([^\s;=]+|%s)\s*(?:=\s*([^;]+|%s))?\s*' %
    (_quoted_string_re, _quoted_string_re))

_entity_headers = frozenset([
    'allow', 'content-encoding', 'content-language', 'content-length',
    'content-location', 'content-md5', 'content-range', 'content-type',
    'expires', 'last-modified'
])
_hop_by_hop_headers = frozenset([
    'connection', 'keep-alive', 'proxy-authenticate',
    'proxy-authorization', 'te', 'trailers', 'transfer-encoding',
    'upgrade'
])


def quote_header_value(value, extra_chars='', allow_token=True):
    """Quote a header value if necessary.

    .. versionadded:: 0.5

    :param value: the value to quote.
    :param extra_chars: a list of extra characters to skip quoting.
    :param allow_token: if this is enabled token values are returned
                        unchanged.
    """
    value = str(value)
    if allow_token:
        token_chars = _token_chars | set(extra_chars)
        if set(value).issubset(token_chars):
            return value
    return '"%s"' % value.replace('\\', '\\\\').replace('"', '\\"')


def unquote_header_value(value, is_filename=False):
    r"""Unquotes a header value.  (Reversal of :func:`quote_header_value`).
    This does not use the real unquoting but what browsers are actually
    using for quoting.

    .. versionadded:: 0.5

    :param value: the header value to unquote.
    """
    if value and value[0] == value[-1] == '"':
        # this is not the real unquoting, but fixing this so that the
        # RFC is met will result in bugs with internet explorer and
        # probably some other browsers as well.  IE for example is
        # uploading files with "C:\foo\bar.txt" as filename
        value = value[1:-1]

        # if this is a filename and the starting characters look like
        # a UNC path, then just return the value without quotes.  Using the
        # replace sequence below on a UNC path has the effect of turning
        # the leading double slash into a single slash and then
        # _fix_ie_filename() doesn't work correctly.  See #458.
        if not is_filename or value[:2] != '\\\\':
            return value.replace('\\\\', '\\').replace('\\"', '"')
    return value


def dump_options_header(header, options):
    """The reverse function to :func:`parse_options_header`.

    :param header: the header to dump
    :param options: a dict of options to append.
    """
    segments = []
    if header is not None:
        segments.append(header)
    for key, value in options.iteritems():
        if value is None:
            segments.append(key)
        else:
            segments.append('%s=%s' % (key, quote_header_value(value)))
    return '; '.join(segments)


def dump_header(iterable, allow_token=True):
    """Dump an HTTP header again.  This is the reversal of
    :func:`parse_list_header`, :func:`parse_set_header` and
    :func:`parse_dict_header`.  This also quotes strings that include an
    equals sign unless you pass it as dict of key, value pairs.

    >>> dump_header({'foo': 'bar baz'})
    'foo="bar baz"'
    >>> dump_header(('foo', 'bar baz'))
    'foo, "bar baz"'

    :param iterable: the iterable or dict of values to quote.
    :param allow_token: if set to `False` tokens as values are disallowed.
                        See :func:`quote_header_value` for more details.
    """
    if isinstance(iterable, dict):
        items = []
        for key, value in iterable.iteritems():
            if value is None:
                items.append(key)
            else:
                items.append('%s=%s' % (
                    key,
                    quote_header_value(value, allow_token=allow_token)
                ))
    else:
        items = [quote_header_value(x, allow_token=allow_token)
                 for x in iterable]
    return ', '.join(items)


def parse_list_header(value):
    """Parse lists as described by RFC 2068 Section 2.

    In particular, parse comma-separated lists where the elements of
    the list may include quoted-strings.  A quoted-string could
    contain a comma.  A non-quoted string could have quotes in the
    middle.  Quotes are removed automatically after parsing.

    It basically works like :func:`parse_set_header` just that items
    may appear multiple times and case sensitivity is preserved.

    The return value is a standard :class:`list`:

    >>> parse_list_header('token, "quoted value"')
    ['token', 'quoted value']

    To create a header from the :class:`list` again, use the
    :func:`dump_header` function.

    :param value: a string with a list header.
    :return: :class:`list`
    """
    result = []
    for item in _parse_list_header(value):
        if item[:1] == item[-1:] == '"':
            item = unquote_header_value(item[1:-1])
        result.append(item)
    return result


def parse_dict_header(value):
    """Parse lists of key, value pairs as described by RFC 2068 Section 2 and
    convert them into a python dict:

    >>> d = parse_dict_header('foo="is a fish", bar="as well"')
    >>> type(d) is dict
    True
    >>> sorted(d.items())
    [('bar', 'as well'), ('foo', 'is a fish')]

    If there is no value for a key it will be `None`:

    >>> parse_dict_header('key_without_value')
    {'key_without_value': None}

    To create a header from the :class:`dict` again, use the
    :func:`dump_header` function.

    :param value: a string with a dict header.
    :return: :class:`dict`
    """
    result = {}
    for item in _parse_list_header(value):
        if '=' not in item:
            result[item] = None
            continue
        name, value = item.split('=', 1)
        if value[:1] == value[-1:] == '"':
            value = unquote_header_value(value[1:-1])
        result[name] = value
    return result


def parse_options_header(value):
    """Parse a ``Content-Type`` like header into a tuple with the content
    type and the options:

    >>> parse_options_header('Content-Type: text/html; mimetype=text/html')
    ('Content-Type:', {'mimetype': 'text/html'})

    This should not be used to parse ``Cache-Control`` like headers that use
    a slightly different format.  For these headers use the
    :func:`parse_dict_header` function.

    .. versionadded:: 0.5

    :param value: the header to parse.
    :return: (str, options)
    """
    def _tokenize(string):
        for match in _option_header_piece_re.finditer(string):
            key, value = match.groups()
            key = unquote_header_value(key)
            if value is not None:
                value = unquote_header_value(value, key == 'filename')
            yield key, value

    if not value:
        return '', {}

    parts = _tokenize(';' + value)
    name = parts.next()[0]
    extra = dict(parts)
    return name, extra


def parse_accept_header(value, cls=None):
    """Parses an HTTP Accept-* header.  This does not implement a complete
    valid algorithm but one that supports at least value and quality
    extraction.

    Returns a new :class:`Accept` object (basically a list of ``(value, quality)``
    tuples sorted by the quality with some additional accessor methods).

    The second parameter can be a subclass of :class:`Accept` that is created
    with the parsed values and returned.

    :param value: the accept header string to be parsed.
    :param cls: the wrapper class for the return value (can be
                         :class:`Accept` or a subclass thereof)
    :return: an instance of `cls`.
    """
    if cls is None:
        cls = Accept

    if not value:
        return cls(None)

    result = []
    for match in _accept_re.finditer(value):
        quality = match.group(2)
        if not quality:
            quality = 1
        else:
            quality = max(min(float(quality), 1), 0)
        result.append((match.group(1), quality))
    return cls(result)


def parse_cache_control_header(value, on_update=None, cls=None):
    """Parse a cache control header.  The RFC differs between response and
    request cache control, this method does not.  It's your responsibility
    to not use the wrong control statements.

    .. versionadded:: 0.5
       The `cls` was added.  If not specified an immutable
       :class:`~werkzeug.datastructures.RequestCacheControl` is returned.

    :param value: a cache control header to be parsed.
    :param on_update: an optional callable that is called every time a value
                      on the :class:`~werkzeug.datastructures.CacheControl`
                      object is changed.
    :param cls: the class for the returned object.  By default
                :class:`~werkzeug.datastructures.RequestCacheControl` is used.
    :return: a `cls` object.
    """
    if cls is None:
        cls = RequestCacheControl
    if not value:
        return cls(None, on_update)
    return cls(parse_dict_header(value), on_update)


def parse_set_header(value, on_update=None):
    """Parse a set-like header and return a
    :class:`~werkzeug.datastructures.HeaderSet` object:

    >>> hs = parse_set_header('token, "quoted value"')

    The return value is an object that treats the items case-insensitively
    and keeps the order of the items:

    >>> 'TOKEN' in hs
    True
    >>> hs.index('quoted value')
    1
    >>> hs
    HeaderSet(['token', 'quoted value'])

    To create a header from the :class:`HeaderSet` again, use the
    :func:`dump_header` function.

    :param value: a set header to be parsed.
    :param on_update: an optional callable that is called every time a
                      value on the :class:`~werkzeug.datastructures.HeaderSet`
                      object is changed.
    :return: a :class:`~werkzeug.datastructures.HeaderSet`
    """
    if not value:
        return HeaderSet(None, on_update)
    return HeaderSet(parse_list_header(value), on_update)


def parse_authorization_header(value):
    """Parse an HTTP basic/digest authorization header transmitted by the web
    browser.  The return value is either `None` if the header was invalid or
    not given, otherwise an :class:`~werkzeug.datastructures.Authorization`
    object.

    :param value: the authorization header to parse.
    :return: a :class:`~werkzeug.datastructures.Authorization` object or `None`.
    """
    if not value:
        return
    try:
        auth_type, auth_info = value.split(None, 1)
        auth_type = auth_type.lower()
    except ValueError:
        return
    if auth_type == 'basic':
        try:
            username, password = auth_info.decode('base64').split(':', 1)
        except Exception, e:
            return
        return Authorization('basic', {'username': username,
                                       'password': password})
    elif auth_type == 'digest':
        auth_map = parse_dict_header(auth_info)
        for key in 'username', 'realm', 'nonce', 'uri', 'response':
            if not key in auth_map:
                return
        if 'qop' in auth_map:
            if not auth_map.get('nc') or not auth_map.get('cnonce'):
                return
        return Authorization('digest', auth_map)


def parse_www_authenticate_header(value, on_update=None):
    """Parse an HTTP WWW-Authenticate header into a
    :class:`~werkzeug.datastructures.WWWAuthenticate` object.

    :param value: a WWW-Authenticate header to parse.
    :param on_update: an optional callable that is called every time a value
                      on the :class:`~werkzeug.datastructures.WWWAuthenticate`
                      object is changed.
    :return: a :class:`~werkzeug.datastructures.WWWAuthenticate` object.
    """
    if not value:
        return WWWAuthenticate(on_update=on_update)
    try:
        auth_type, auth_info = value.split(None, 1)
        auth_type = auth_type.lower()
    except (ValueError, AttributeError):
        return WWWAuthenticate(value.strip().lower(), on_update=on_update)
    return WWWAuthenticate(auth_type, parse_dict_header(auth_info),
                           on_update)


def parse_if_range_header(value):
    """Parses an if-range header which can be an etag or a date.  Returns
    a :class:`~werkzeug.datastructures.IfRange` object.

    .. versionadded:: 0.7
    """
    if not value:
        return IfRange()
    date = parse_date(value)
    if date is not None:
        return IfRange(date=date)
    # drop weakness information
    return IfRange(unquote_etag(value)[0])


def parse_range_header(value, make_inclusive=True):
    """Parses a range header into a :class:`~werkzeug.datastructures.Range`
    object.  If the header is missing or malformed `None` is returned.
    `ranges` is a list of ``(start, stop)`` tuples where the ranges are
    non-inclusive.

    .. versionadded:: 0.7
    """
    if not value or '=' not in value:
        return None

    ranges = []
    last_end = 0
    units, rng = value.split('=', 1)
    units = units.strip().lower()

    for item in rng.split(','):
        item = item.strip()
        if '-' not in item:
            return None
        if item.startswith('-'):
            if last_end < 0:
                return None
            begin = int(item)
            end = None
            last_end = -1
        elif '-' in item:
            begin, end = item.split('-', 1)
            begin = int(begin)
            if begin < last_end or last_end < 0:
                return None
            if end:
                end = int(end) + 1
                if begin >= end:
                    return None
            else:
                end = None
            last_end = end
        ranges.append((begin, end))

    return Range(units, ranges)


def parse_content_range_header(value, on_update=None):
    """Parses a range header into a
    :class:`~werkzeug.datastructures.ContentRange` object or `None` if
    parsing is not possible.

    .. versionadded:: 0.7

    :param value: a content range header to be parsed.
    :param on_update: an optional callable that is called every time a value
                      on the :class:`~werkzeug.datastructures.ContentRange`
                      object is changed.
    """
    if value is None:
        return None
    try:
        units, rangedef = (value or '').strip().split(None, 1)
    except ValueError:
        return None

    if '/' not in rangedef:
        return None
    rng, length = rangedef.split('/', 1)
    if length == '*':
        length = None
    elif length.isdigit():
        length = int(length)
    else:
        return None

    if rng == '*':
        return ContentRange(units, None, None, length, on_update=on_update)
    elif '-' not in rng:
        return None

    start, stop = rng.split('-', 1)
    try:
        start = int(start)
        stop = int(stop) + 1
    except ValueError:
        return None

    if is_byte_range_valid(start, stop, length):
        return ContentRange(units, start, stop, length, on_update=on_update)


def quote_etag(etag, weak=False):
    """Quote an etag.

    :param etag: the etag to quote.
    :param weak: set to `True` to tag it "weak".
    """
    if '"' in etag:
        raise ValueError('invalid etag')
    etag = '"%s"' % etag
    if weak:
        etag = 'w/' + etag
    return etag


def unquote_etag(etag):
    """Unquote a single etag:

    >>> unquote_etag('w/"bar"')
    ('bar', True)
    >>> unquote_etag('"bar"')
    ('bar', False)

    :param etag: the etag identifier to unquote.
    :return: a ``(etag, weak)`` tuple.
    """
    if not etag:
        return None, None
    etag = etag.strip()
    weak = False
    if etag[:2] in ('w/', 'W/'):
        weak = True
        etag = etag[2:]
    if etag[:1] == etag[-1:] == '"':
        etag = etag[1:-1]
    return etag, weak


def parse_etags(value):
    """Parse an etag header.

    :param value: the tag header to parse
    :return: an :class:`~werkzeug.datastructures.ETags` object.
    """
    if not value:
        return ETags()
    strong = []
    weak = []
    end = len(value)
    pos = 0
    while pos < end:
        match = _etag_re.match(value, pos)
        if match is None:
            break
        is_weak, quoted, raw = match.groups()
        if raw == '*':
            return ETags(star_tag=True)
        elif quoted:
            raw = quoted
        if is_weak:
            weak.append(raw)
        else:
            strong.append(raw)
        pos = match.end()
    return ETags(strong, weak)


def generate_etag(data):
    """Generate an etag for some data."""
    return md5(data).hexdigest()


def parse_date(value):
    """Parse one of the following date formats into a datetime object:

    .. sourcecode:: text

        Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
        Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
        Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

    If parsing fails the return value is `None`.

    :param value: a string with a supported date format.
    :return: a :class:`datetime.datetime` object.
    """
    if value:
        t = parsedate_tz(value.strip())
        if t is not None:
            try:
                year = t[0]
                # unfortunately that function does not tell us if two digit
                # years were part of the string, or if they were prefixed
                # with two zeroes.  So what we do is to assume that 69-99
                # refer to 1900, and everything below to 2000
                if year >= 0 and year <= 68:
                    year += 2000
                elif year >= 69 and year <= 99:
                    year += 1900
                return datetime(*((year,) + t[1:7])) - \
                       timedelta(seconds=t[-1] or 0)
            except (ValueError, OverflowError):
                return None


def cookie_date(expires=None):
    """Formats the time to ensure compatibility with Netscape's cookie
    standard.

    Accepts a floating point number expressed in seconds since the epoch in, a
    datetime object or a timetuple.  All times in UTC.  The :func:`parse_date`
    function can be used to parse such a date.

    Outputs a string in the format ``Wdy, DD-Mon-YYYY HH:MM:SS GMT``.

    :param expires: If provided that date is used, otherwise the current.
    """
    return _dump_date(expires, '-')


def http_date(timestamp=None):
    """Formats the time to match the RFC1123 date format.

    Accepts a floating point number expressed in seconds since the epoch in, a
    datetime object or a timetuple.  All times in UTC.  The :func:`parse_date`
    function can be used to parse such a date.

    Outputs a string in the format ``Wdy, DD Mon YYYY HH:MM:SS GMT``.

    :param timestamp: If provided that date is used, otherwise the current.
    """
    return _dump_date(timestamp, ' ')


def is_resource_modified(environ, etag=None, data=None, last_modified=None):
    """Convenience method for conditional requests.

    :param environ: the WSGI environment of the request to be checked.
    :param etag: the etag for the response for comparison.
    :param data: or alternatively the data of the response to automatically
                 generate an etag using :func:`generate_etag`.
    :param last_modified: an optional date of the last modification.
    :return: `True` if the resource was modified, otherwise `False`.
    """
    if etag is None and data is not None:
        etag = generate_etag(data)
    elif data is not None:
        raise TypeError('both data and etag given')
    if environ['REQUEST_METHOD'] not in ('GET', 'HEAD'):
        return False

    unmodified = False
    if isinstance(last_modified, basestring):
        last_modified = parse_date(last_modified)

    # ensure that microsecond is zero because the HTTP spec does not transmit
    # that either and we might have some false positives.  See issue #39
    if last_modified is not None:
        last_modified = last_modified.replace(microsecond=0)

    modified_since = parse_date(environ.get('HTTP_IF_MODIFIED_SINCE'))

    if modified_since and last_modified and last_modified <= modified_since:
        unmodified = True
    if etag:
        if_none_match = parse_etags(environ.get('HTTP_IF_NONE_MATCH'))
        if if_none_match:
            unmodified = if_none_match.contains_raw(etag)

    return not unmodified


def remove_entity_headers(headers, allowed=('expires', 'content-location')):
    """Remove all entity headers from a list or :class:`Headers` object.  This
    operation works in-place.  `Expires` and `Content-Location` headers are
    by default not removed.  The reason for this is :rfc:`2616` section
    10.3.5 which specifies some entity headers that should be sent.

    .. versionchanged:: 0.5
       added `allowed` parameter.

    :param headers: a list or :class:`Headers` object.
    :param allowed: a list of headers that should still be allowed even though
                    they are entity headers.
    """
    allowed = set(x.lower() for x in allowed)
    headers[:] = [(key, value) for key, value in headers if
                  not is_entity_header(key) or key.lower() in allowed]


def remove_hop_by_hop_headers(headers):
    """Remove all HTTP/1.1 "Hop-by-Hop" headers from a list or
    :class:`Headers` object.  This operation works in-place.

    .. versionadded:: 0.5

    :param headers: a list or :class:`Headers` object.
    """
    headers[:] = [(key, value) for key, value in headers if
                  not is_hop_by_hop_header(key)]


def is_entity_header(header):
    """Check if a header is an entity header.

    .. versionadded:: 0.5

    :param header: the header to test.
    :return: `True` if it's an entity header, `False` otherwise.
    """
    return header.lower() in _entity_headers


def is_hop_by_hop_header(header):
    """Check if a header is an HTTP/1.1 "Hop-by-Hop" header.

    .. versionadded:: 0.5

    :param header: the header to test.
    :return: `True` if it's an entity header, `False` otherwise.
    """
    return header.lower() in _hop_by_hop_headers


def parse_cookie(header, charset='utf-8', errors='replace',
                 cls=None):
    """Parse a cookie.  Either from a string or WSGI environ.

    Per default encoding errors are ignored.  If you want a different behavior
    you can set `errors` to ``'replace'`` or ``'strict'``.  In strict mode a
    :exc:`HTTPUnicodeError` is raised.

    .. versionchanged:: 0.5
       This function now returns a :class:`TypeConversionDict` instead of a
       regular dict.  The `cls` parameter was added.

    :param header: the header to be used to parse the cookie.  Alternatively
                   this can be a WSGI environment.
    :param charset: the charset for the cookie values.
    :param errors: the error behavior for the charset decoding.
    :param cls: an optional dict class to use.  If this is not specified
                       or `None` the default :class:`TypeConversionDict` is
                       used.
    """
    if isinstance(header, dict):
        header = header.get('HTTP_COOKIE', '')
    if cls is None:
        cls = TypeConversionDict
    cookie = _ExtendedCookie()
    cookie.load(header)
    result = {}

    # decode to unicode and skip broken items.  Our extended morsel
    # and extended cookie will catch CookieErrors and convert them to
    # `None` items which we have to skip here.
    for key, value in cookie.iteritems():
        if value.value is not None:
            result[key] = _decode_unicode(unquote_header_value(value.value),
                                          charset, errors)

    return cls(result)


def dump_cookie(key, value='', max_age=None, expires=None, path='/',
                domain=None, secure=None, httponly=False, charset='utf-8',
                sync_expires=True):
    """Creates a new Set-Cookie header without the ``Set-Cookie`` prefix
    The parameters are the same as in the cookie Morsel object in the
    Python standard library but it accepts unicode data, too.

    :param max_age: should be a number of seconds, or `None` (default) if
                    the cookie should last only as long as the client's
                    browser session.  Additionally `timedelta` objects
                    are accepted, too.
    :param expires: should be a `datetime` object or unix timestamp.
    :param path: limits the cookie to a given path, per default it will
                 span the whole domain.
    :param domain: Use this if you want to set a cross-domain cookie. For
                   example, ``domain=".example.com"`` will set a cookie
                   that is readable by the domain ``www.example.com``,
                   ``foo.example.com`` etc. Otherwise, a cookie will only
                   be readable by the domain that set it.
    :param secure: The cookie will only be available via HTTPS
    :param httponly: disallow JavaScript to access the cookie.  This is an
                     extension to the cookie standard and probably not
                     supported by all browsers.
    :param charset: the encoding for unicode values.
    :param sync_expires: automatically set expires if max_age is defined
                         but expires not.
    """
    try:
        key = str(key)
    except UnicodeError:
        raise TypeError('invalid key %r' % key)
    if isinstance(value, unicode):
        value = value.encode(charset)
    value = quote_header_value(value)
    morsel = _ExtendedMorsel(key, value)
    if isinstance(max_age, timedelta):
        max_age = (max_age.days * 60 * 60 * 24) + max_age.seconds
    if expires is not None:
        if not isinstance(expires, basestring):
            expires = cookie_date(expires)
        morsel['expires'] = expires
    elif max_age is not None and sync_expires:
        morsel['expires'] = cookie_date(time() + max_age)
    if domain and ':' in domain:
        # The port part of the domain should NOT be used. Strip it
        domain = domain.split(':', 1)[0]
    if domain:
        assert '.' in domain, (
            "Setting \"domain\" for a cookie on a server running localy (ex: "
            "localhost) is not supportted by complying browsers. You should "
            "have something like: \"127.0.0.1 localhost dev.localhost\" on "
            "your hosts file and then point your server to run on "
            "\"dev.localhost\" and also set \"domain\" for \"dev.localhost\""
        )
    for k, v in (('path', path), ('domain', domain), ('secure', secure),
                 ('max-age', max_age), ('httponly', httponly)):
        if v is not None and v is not False:
            morsel[k] = str(v)
    return morsel.output(header='').lstrip()


def is_byte_range_valid(start, stop, length):
    """Checks if a given byte content range is valid for the given length.

    .. versionadded:: 0.7
    """
    if (start is None) != (stop is None):
        return False
    elif start is None:
        return length is None or length >= 0
    elif length is None:
        return 0 <= start < stop
    elif start >= stop:
        return False
    return 0 <= start < length


# circular dependency fun
from werkzeug.datastructures import Accept, HeaderSet, ETags, Authorization, \
     WWWAuthenticate, TypeConversionDict, IfRange, Range, ContentRange, \
     RequestCacheControl


# DEPRECATED
# backwards compatible imports
from werkzeug.datastructures import MIMEAccept, CharsetAccept, \
     LanguageAccept, Headers