Python: Map a function over recursive iterables

Question

I have an arbitrarily nested iterable like so:

numbers = (1, 2, (3, (4, 5)), 7)

and I'd like to map a function over it without changing the structure. For example, I might want to convert all the numbers to strings to get

strings = recursive_map(str, numbers)
assert strings == ('1', '2', ('3', ('4', '5')), '7')

Is there a nice way to do this? I can imaging writing my own method to manually traverse numbers, but I'd like to know if there's a general way to map over recursive iterables.

Also, in my example, it's okay if strings gives me nested lists (or some iterable) rather nested tuples.

Answer 1

We scan every element in the sequence, and proceeds into deeper recursion if the current item is a sub-sequence, or yields it's mapping if we reached a non-sequence data type (could be int, str, or any complex classes).

We use collections.Sequence to generalize the idea for every sequence, and not only tuples or lists, and type(item) upon yield to ensure that the sub-sequences we get back remains of the same type they were.

from collections import Sequence

def recursive_map (seq, func):
    for item in seq:
        if isinstance(item, Sequence):
            yield type(item)(recursive_map(item, func))
        else:
            yield func(item)

Demo:

>>> numbers = (1, 2, (3, (4, 5)), 7)
>>> mapped = recursive_map(numbers, str)
>>> tuple(mapped)
('1', '2', ('3', ('4', '5')), '7')

Or a more complex example:

>>> complex_list = (1, 2, [3, (complex('4+2j'), 5)], map(str, (range(7, 10))))
>>> tuple(recursive_map(complex_list, lambda x: x.__class__.__name__))
('int', 'int', ['int', ('complex', 'int')], 'map')