Get the highest and lowest possible number if you could only swap one number at a time.

swap.py

def swap(num):
    # convert the number to a string so that we can split into a list
    # and get the index for each
    old_arr = list(str(num)) 
    # empty list where we'll push the newly swapped numbers for comparison
    new_arr = []
    # we then loop through the length of the list of numbers and get the 
    # index, pairing it up with the next index in line
    # for instance: for index 0, we pair it with 1, then 0 with 2 etc.
    for i, val in enumerate(old_arr):
        for j, val in enumerate(old_arr):
            if i != j:
                # as long as i and j aren't the same e.g 0,0 we copy the 
                # initial list (so that our swaps don't affect it) 
                new_list = list(old_arr)
                # pythonic method for swapping list positions using indexes
                new_list[i], new_list[j] = new_list[j], new_list[i]

                # get rid of any number-string whose first index is 0
                if new_list[0] != '0':
                    # We then join the split arrays and convert them back to integers
                    new_arr.append(int(''.join(new_list)))
    
    # now we check that the highest number in the list is not the original
    if max(new_arr) == num:
        print "The highest possible swap is already {} but the lowest swap is {}".format(num, min(new_arr))
    elif min(new_arr) == num:
        print "The lowest possible swap is already {} but the highest swap is {}".format(num, max(new_arr))
    # if the input is not the highest or the lowest we go ahead to print:
    else: 
        print "The lowest possible swap is {} and the highest swap is {}".format(min(new_arr), max(new_arr))

# example for running the file directly
swap(10900)