tokenize.py 文件源码

def _all_string_prefixes():
    # The valid string prefixes. Only contain the lower case versions,
    #  and don't contain any permuations (include 'fr', but not
    #  'rf'). The various permutations will be generated.
    _valid_string_prefixes = ['b', 'r', 'u', 'br']
    if py_version >= 36:
        _valid_string_prefixes += ['f', 'fr']
    if py_version <= 27:
        # TODO this is actually not 100% valid. ur is valid in Python 2.7,
        # while ru is not.
        _valid_string_prefixes.append('ur')

    # if we add binary f-strings, add: ['fb', 'fbr']
    result = set([''])
    for prefix in _valid_string_prefixes:
        for t in _itertools.permutations(prefix):
            # create a list with upper and lower versions of each
            #  character
            for u in _itertools.product(*[(c, c.upper()) for c in t]):
                result.add(''.join(u))
    return result