How to convert a column or row matrix to a diagonal matrix in Python?

You can use diag method:

import numpy as np

a = np.array([1,2,3,4])
d = np.diag(a)
# or simpler: d = np.diag([1,2,3,4])

print(d)

Results in:

[[1 0 0 0]
 [0 2 0 0]
 [0 0 3 0]
 [0 0 0 4]]

If you have a row vector, you can do this:

a = np.array([[1, 2, 3, 4]])
d = np.diag(a[0])

Results in:

[[1 0 0 0]
 [0 2 0 0]
 [0 0 3 0]
 [0 0 0 4]]

For the given matrix in the question:

import numpy as np
a = np.matrix([1,2,3,4])
d = np.diag(a.A1)
print (d)

Result is again:

[[1 0 0 0]
 [0 2 0 0]
 [0 0 3 0]
 [0 0 0 4]]

I suppose you could also use diagflat:

import numpy
a = np.matrix([1,2,3,4])
d = np.diagflat(a)
print (d)

Which like the diag method results in

[[1 0 0 0]
 [0 2 0 0]
 [0 0 3 0]
 [0 0 0 4]]

but there's no need for flattening with .A1

Another solution could be:

import numpy as np
a = np.array([1,2,3,4])
d = a * np.identity(len(a))

As for performances for the various answers here, I get with timeit on 100000 repetitions:

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