TypeError: ‘_io.TextIOWrapper’ object is not subscriptable

You can’t index (__getitem__) a _io.TextIOWrapper object. What you can do is work with a list of lines. Try this in your code:

lst = open(input("Input file name: "), "r").readlines()

Also, you aren’t closing the file object, this would be better:

with open(input("Input file name: ", "r") as lst:
    print(medianStrat(lst.readlines()))

with ensures that file get closed, see docs