def roll(d_str):
    import random

    # the following line can be broken into several lines: 
    #    num, size = map(int, d_str.split('d'))[:2]

    # suppose `d_str` has the value "4d20". After the following line, 
    # `roll_fragments` will have the value ["4", "20"]. That is, 
    # `roll_fragments` is a list of strings. 
    roll_fragments = d_str.split('d')

    # We'd like to try to prevent bad input to this function from crashing the
    # program. Assume the function is called with errant input of 
    # d_str="4d20d10". We can Choose to use only the first two values from the 
    # `split` function in the previous line by using Python's list slicing 
    # syntax. The following line, translated to English would read: "set the 
    # variable `num_size` equal to the first two elements of the 
    # `roll_fragments` list":
    num_size = roll_fragments[:2]

    # We ultimately need to determine the integer (int) representation of both
    # of these variables. We can do that with the following: 
    num, size = map(int, num_size)
    # In English, the previous line reads: set `num` and `size` to the first
    # and second values (respectively) returned by calling the built-in 
    # function `int` for each element in the list `num_size`. Recall that the 
    # list num_size will have at most two elements in it. 

    # To reiterate, the lines: 

    #   roll_fragments = d_str.split('d')
    #   num_size = roll_fragments[:2]
    #   num, size = map(int, num_size)

    # are together equivalent to the single line: 

    #   num, size = map(int, d_str.split('d'))[:2] 

    # For more information about list slicing see: 
    #    http://stackoverflow.com/a/509295/60366
    # For more information about the `map` and `int` Python built-in functions
    # see: 
    #   http://docs.python.org/2/library/functions.html#map 
    #   http://docs.python.org/2/library/functions.html#int

    face_values = range(1, size+1)
    rolls = [str(random.choice(face_values)) for i in xrange(num)]
    print(' '.join(rolls))