apiclient/mimeparse.py

# Copyright (C) 2007 Joe Gregorio
#
# Licensed under the MIT License

"""MIME-Type Parser

This module provides basic functions for handling mime-types. It can handle
matching mime-types against a list of media-ranges. See section 14.1 of the
HTTP specification [RFC 2616] for a complete explanation.

   http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1

Contents:
 - parse_mime_type():   Parses a mime-type into its component parts.
 - parse_media_range(): Media-ranges are mime-types with wild-cards and a 'q'
                          quality parameter.
 - quality():           Determines the quality ('q') of a mime-type when
                          compared against a list of media-ranges.
 - quality_parsed():    Just like quality() except the second parameter must be
                          pre-parsed.
 - best_match():        Choose the mime-type with the highest quality ('q')
                          from a list of candidates.
"""

__version__ = '0.1.3'
__author__ = 'Joe Gregorio'
__email__ = 'joe@bitworking.org'
__license__ = 'MIT License'
__credits__ = ''


def parse_mime_type(mime_type):
    """Parses a mime-type into its component parts.

    Carves up a mime-type and returns a tuple of the (type, subtype, params)
    where 'params' is a dictionary of all the parameters for the media range.
    For example, the media range 'application/xhtml;q=0.5' would get parsed
    into:

       ('application', 'xhtml', {'q', '0.5'})
       """
    parts = mime_type.split(';')
    params = dict([tuple([s.strip() for s in param.split('=', 1)])\
            for param in parts[1:]
                  ])
    full_type = parts[0].strip()
    # Java URLConnection class sends an Accept header that includes a
    # single '*'. Turn it into a legal wildcard.
    if full_type == '*':
        full_type = '*/*'
    (type, subtype) = full_type.split('/')

    return (type.strip(), subtype.strip(), params)


def parse_media_range(range):
    """Parse a media-range into its component parts.

    Carves up a media range and returns a tuple of the (type, subtype,
    params) where 'params' is a dictionary of all the parameters for the media
    range.  For example, the media range 'application/*;q=0.5' would get parsed
    into:

       ('application', '*', {'q', '0.5'})

    In addition this function also guarantees that there is a value for 'q'
    in the params dictionary, filling it in with a proper default if
    necessary.
    """
    (type, subtype, params) = parse_mime_type(range)
    if not params.has_key('q') or not params['q'] or \
            not float(params['q']) or float(params['q']) > 1\
            or float(params['q']) < 0:
        params['q'] = '1'

    return (type, subtype, params)


def fitness_and_quality_parsed(mime_type, parsed_ranges):
    """Find the best match for a mime-type amongst parsed media-ranges.

    Find the best match for a given mime-type against a list of media_ranges
    that have already been parsed by parse_media_range(). Returns a tuple of
    the fitness value and the value of the 'q' quality parameter of the best
    match, or (-1, 0) if no match was found. Just as for quality_parsed(),
    'parsed_ranges' must be a list of parsed media ranges.
    """
    best_fitness = -1
    best_fit_q = 0
    (target_type, target_subtype, target_params) =\
            parse_media_range(mime_type)
    for (type, subtype, params) in parsed_ranges:
        type_match = (type == target_type or\
                      type == '*' or\
                      target_type == '*')
        subtype_match = (subtype == target_subtype or\
                         subtype == '*' or\
                         target_subtype == '*')
        if type_match and subtype_match:
            param_matches = reduce(lambda x, y: x + y, [1 for (key, value) in \
                    target_params.iteritems() if key != 'q' and \
                    params.has_key(key) and value == params[key]], 0)
            fitness = (type == target_type) and 100 or 0
            fitness += (subtype == target_subtype) and 10 or 0
            fitness += param_matches
            if fitness > best_fitness:
                best_fitness = fitness
                best_fit_q = params['q']

    return best_fitness, float(best_fit_q)


def quality_parsed(mime_type, parsed_ranges):
    """Find the best match for a mime-type amongst parsed media-ranges.

    Find the best match for a given mime-type against a list of media_ranges
    that have already been parsed by parse_media_range(). Returns the 'q'
    quality parameter of the best match, 0 if no match was found. This function
    bahaves the same as quality() except that 'parsed_ranges' must be a list of
    parsed media ranges.
    """

    return fitness_and_quality_parsed(mime_type, parsed_ranges)[1]


def quality(mime_type, ranges):
    """Return the quality ('q') of a mime-type against a list of media-ranges.

    Returns the quality 'q' of a mime-type when compared against the
    media-ranges in ranges. For example:

    >>> quality('text/html','text/*;q=0.3, text/html;q=0.7,
                  text/html;level=1, text/html;level=2;q=0.4, */*;q=0.5')
    0.7

    """
    parsed_ranges = [parse_media_range(r) for r in ranges.split(',')]

    return quality_parsed(mime_type, parsed_ranges)


def best_match(supported, header):
    """Return mime-type with the highest quality ('q') from list of candidates.

    Takes a list of supported mime-types and finds the best match for all the
    media-ranges listed in header. The value of header must be a string that
    conforms to the format of the HTTP Accept: header. The value of 'supported'
    is a list of mime-types. The list of supported mime-types should be sorted
    in order of increasing desirability, in case of a situation where there is
    a tie.

    >>> best_match(['application/xbel+xml', 'text/xml'],
                   'text/*;q=0.5,*/*; q=0.1')
    'text/xml'
    """
    split_header = _filter_blank(header.split(','))
    parsed_header = [parse_media_range(r) for r in split_header]
    weighted_matches = []
    pos = 0
    for mime_type in supported:
        weighted_matches.append((fitness_and_quality_parsed(mime_type,
                                 parsed_header), pos, mime_type))
        pos += 1
    weighted_matches.sort()

    return weighted_matches[-1][0][1] and weighted_matches[-1][2] or ''


def _filter_blank(i):
    for s in i:
        if s.strip():
            yield s