A matrix is a rectangular array of elements arranged in rows and columns. A scalar matrix is "a diagonal matrix in which all of the diagonal elements are equal" per
http://www.thefreedictionary.com. A diagonal matrix is "a square matrix with all elements not on the main diagonal equal to zero". Its effect on a vector is scalar multiplication (from Wikipedia).
 >>> m = Matrix(3,3)

>>> m

Matrix(3, 3)

>>> m.show()

0 0 0

0 0 0

0 0 0

>>> for i in range(3):

... m[(i,i)]=1

...

>>> m.show()

1 0 0

0 1 0

0 0 1

>>>