>>> T1 = ['13', '17', '18', '21', '32']
>>> T3 = list(map(int, T1))
>>> T3
[13, 17, 18, 21, 32]
This does the same thing as:
>>> T3 = [int(x) for x in T1]
>>> T3
[13, 17, 18, 21, 32]
so what you are doing is
>>> T3 = [[int(letter) for letter in x] for x in T1]
>>> T3
[[1, 3], [1, 7], [1, 8], [2, 1], [3, 2]]
Hope that clears up the confusion.
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