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      1 # Copyright 2014 Joe Gregorio
      2 #
      3 # Licensed under the MIT License
      4 
      5 """MIME-Type Parser
      6 
      7 This module provides basic functions for handling mime-types. It can handle
      8 matching mime-types against a list of media-ranges. See section 14.1 of the
      9 HTTP specification [RFC 2616] for a complete explanation.
     10 
     11    http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1
     12 
     13 Contents:
     14  - parse_mime_type():   Parses a mime-type into its component parts.
     15  - parse_media_range(): Media-ranges are mime-types with wild-cards and a 'q'
     16                           quality parameter.
     17  - quality():           Determines the quality ('q') of a mime-type when
     18                           compared against a list of media-ranges.
     19  - quality_parsed():    Just like quality() except the second parameter must be
     20                           pre-parsed.
     21  - best_match():        Choose the mime-type with the highest quality ('q')
     22                           from a list of candidates.
     23 """
     24 from __future__ import absolute_import
     25 from functools import reduce
     26 import six
     27 
     28 __version__ = '0.1.3'
     29 __author__ = 'Joe Gregorio'
     30 __email__ = 'joe (at] bitworking.org'
     31 __license__ = 'MIT License'
     32 __credits__ = ''
     33 
     34 
     35 def parse_mime_type(mime_type):
     36     """Parses a mime-type into its component parts.
     37 
     38     Carves up a mime-type and returns a tuple of the (type, subtype, params)
     39     where 'params' is a dictionary of all the parameters for the media range.
     40     For example, the media range 'application/xhtml;q=0.5' would get parsed
     41     into:
     42 
     43        ('application', 'xhtml', {'q', '0.5'})
     44        """
     45     parts = mime_type.split(';')
     46     params = dict([tuple([s.strip() for s in param.split('=', 1)])\
     47             for param in parts[1:]
     48                   ])
     49     full_type = parts[0].strip()
     50     # Java URLConnection class sends an Accept header that includes a
     51     # single '*'. Turn it into a legal wildcard.
     52     if full_type == '*':
     53         full_type = '*/*'
     54     (type, subtype) = full_type.split('/')
     55 
     56     return (type.strip(), subtype.strip(), params)
     57 
     58 
     59 def parse_media_range(range):
     60     """Parse a media-range into its component parts.
     61 
     62     Carves up a media range and returns a tuple of the (type, subtype,
     63     params) where 'params' is a dictionary of all the parameters for the media
     64     range.  For example, the media range 'application/*;q=0.5' would get parsed
     65     into:
     66 
     67        ('application', '*', {'q', '0.5'})
     68 
     69     In addition this function also guarantees that there is a value for 'q'
     70     in the params dictionary, filling it in with a proper default if
     71     necessary.
     72     """
     73     (type, subtype, params) = parse_mime_type(range)
     74     if 'q' not in params or not params['q'] or \
     75             not float(params['q']) or float(params['q']) > 1\
     76             or float(params['q']) < 0:
     77         params['q'] = '1'
     78 
     79     return (type, subtype, params)
     80 
     81 
     82 def fitness_and_quality_parsed(mime_type, parsed_ranges):
     83     """Find the best match for a mime-type amongst parsed media-ranges.
     84 
     85     Find the best match for a given mime-type against a list of media_ranges
     86     that have already been parsed by parse_media_range(). Returns a tuple of
     87     the fitness value and the value of the 'q' quality parameter of the best
     88     match, or (-1, 0) if no match was found. Just as for quality_parsed(),
     89     'parsed_ranges' must be a list of parsed media ranges.
     90     """
     91     best_fitness = -1
     92     best_fit_q = 0
     93     (target_type, target_subtype, target_params) =\
     94             parse_media_range(mime_type)
     95     for (type, subtype, params) in parsed_ranges:
     96         type_match = (type == target_type or\
     97                       type == '*' or\
     98                       target_type == '*')
     99         subtype_match = (subtype == target_subtype or\
    100                          subtype == '*' or\
    101                          target_subtype == '*')
    102         if type_match and subtype_match:
    103             param_matches = reduce(lambda x, y: x + y, [1 for (key, value) in \
    104                     six.iteritems(target_params) if key != 'q' and \
    105                     key in params and value == params[key]], 0)
    106             fitness = (type == target_type) and 100 or 0
    107             fitness += (subtype == target_subtype) and 10 or 0
    108             fitness += param_matches
    109             if fitness > best_fitness:
    110                 best_fitness = fitness
    111                 best_fit_q = params['q']
    112 
    113     return best_fitness, float(best_fit_q)
    114 
    115 
    116 def quality_parsed(mime_type, parsed_ranges):
    117     """Find the best match for a mime-type amongst parsed media-ranges.
    118 
    119     Find the best match for a given mime-type against a list of media_ranges
    120     that have already been parsed by parse_media_range(). Returns the 'q'
    121     quality parameter of the best match, 0 if no match was found. This function
    122     bahaves the same as quality() except that 'parsed_ranges' must be a list of
    123     parsed media ranges.
    124     """
    125 
    126     return fitness_and_quality_parsed(mime_type, parsed_ranges)[1]
    127 
    128 
    129 def quality(mime_type, ranges):
    130     """Return the quality ('q') of a mime-type against a list of media-ranges.
    131 
    132     Returns the quality 'q' of a mime-type when compared against the
    133     media-ranges in ranges. For example:
    134 
    135     >>> quality('text/html','text/*;q=0.3, text/html;q=0.7,
    136                   text/html;level=1, text/html;level=2;q=0.4, */*;q=0.5')
    137     0.7
    138 
    139     """
    140     parsed_ranges = [parse_media_range(r) for r in ranges.split(',')]
    141 
    142     return quality_parsed(mime_type, parsed_ranges)
    143 
    144 
    145 def best_match(supported, header):
    146     """Return mime-type with the highest quality ('q') from list of candidates.
    147 
    148     Takes a list of supported mime-types and finds the best match for all the
    149     media-ranges listed in header. The value of header must be a string that
    150     conforms to the format of the HTTP Accept: header. The value of 'supported'
    151     is a list of mime-types. The list of supported mime-types should be sorted
    152     in order of increasing desirability, in case of a situation where there is
    153     a tie.
    154 
    155     >>> best_match(['application/xbel+xml', 'text/xml'],
    156                    'text/*;q=0.5,*/*; q=0.1')
    157     'text/xml'
    158     """
    159     split_header = _filter_blank(header.split(','))
    160     parsed_header = [parse_media_range(r) for r in split_header]
    161     weighted_matches = []
    162     pos = 0
    163     for mime_type in supported:
    164         weighted_matches.append((fitness_and_quality_parsed(mime_type,
    165                                  parsed_header), pos, mime_type))
    166         pos += 1
    167     weighted_matches.sort()
    168 
    169     return weighted_matches[-1][0][1] and weighted_matches[-1][2] or ''
    170 
    171 
    172 def _filter_blank(i):
    173     for s in i:
    174         if s.strip():
    175             yield s
    176