j=10 _ = [print(''.join([' ' for o in range(j-it-1)]) + '_'.join(['#' for _ in range(1+it)]) + ''.join([' ' for o in range(j-it-1)])) for it in range(j)]

_ is the list comprehension (or generator) in this case. The value of _ is not important as long as it is not used.
print(''.join([' ' for o in range(j-it-1)]) + '_'.join(['#' for _ in range(1+it)]) + ''.join([' ' for o in range(j-it-1)])) is the print statement. The first and third parts are creating spaces and the second part is creating the #. The other parts are controlling the number of spaces and #.
for it in range(j) is the for loop.