When you using the -1 (or any other negative integer numbers, i made this test kkk) in

b = numpy.reshape(a, -1)

you are only saying for the numpy.reshape to automatically calculate the size of the vector (rows x columns) and relocate it into a 1-D vector with that dimension. This command is interesting because it does it automatically for you. If you wanted to reshape the vector to 1-D by putting a positive integer value, the reshape command would only work if you correctly entered the value "rows x columns". So being able to enter a negative integer makes the process easier, you know.