49 

    50 def _rapply(f, xs):

    51     if xs is None:

    52         # assume None means non-value of optional data

    53         return xs

    54     if isinstance(xs, (list, set, tuple)):

    55         return type(xs)(_rapply(f, x) for x in xs)

    56     if isinstance(xs, dict):

    57         return type(xs)((_rapply(f, k), _rapply(f, v)) for k, v in xs.items())

    58     return f(xs)

    59 

    60 def rapply(f, xs):

    61     """Apply function recursively to every item preserving the data structure

    62 

    63     >>> def f(x):

    64     ...     return 'f(%s)' % x

    65     >>> rapply(f, None) is None

    66     True

    67     >>> rapply(f, 'a')

    68     'f(a)'

    69     >>> rapply(f, {'a'}) == {'f(a)'}

    70     True

    71     >>> rapply(f, ['a', 'b', None, {'c': 'd'}, []])

    72     ['f(a)', 'f(b)', None, {'f(c)': 'f(d)'}, []]

    73 

    74     >>> xs = [object()]

    75     >>> rapply(identity, xs) is xs

    76     True

    77     """

    78     if f is identity:

    79         # fast path mainly for py2

    80         return xs

    81     return _rapply(f, xs)