NumPy array/matrix of mixed types

Question

I'm trying to create a NumPy array/matrix (Nx3) with mixed data types (string, integer, integer). But when I'm appending this matrix by adding some data, I get an error: TypeError: invalid type promotion. Please, can anybody help me to solve this problem?

When I create an array with the sample data, NumPy casts all columns in the matrix to the one 'S' data type. And I can't specify data type for an array, because when i do this res = np.array(["TEXT", 1, 1], dtype='S, i4, i4') - I get an error: TypeError: expected a readable buffer object

templates.py

import numpy as np
from pprint import pprint

test_array = np.zeros((0, 3), dtype='S, i4, i4')
pprint(test_array)

test_array = np.append(test_array, [["TEXT", 1, 1]], axis=0)
pprint(test_array)

print("Array example:")
res = np.array(["TEXT", 1, 1])
pprint(res)

Output:

array([], shape=(0L, 3L), 
  dtype=[('f0', 'S'), ('f1', '<i4'), ('f2', '<i4')])

 Array example:
 array(['TEXT', '1', '1'], dtype='|S4')

Error:

Traceback (most recent call last):

File "templates.py", line 5, in <module>
test_array = np.append(test_array, [["TEXT", 1, 1]], axis=0)

File "lib\site-packages\numpy\lib\function_base.py", line 3543, in append
return concatenate((arr, values), axis=axis)

TypeError: invalid type promotion

Answer 1

Your problem is in the data. Try this:

res = np.array(("TEXT", 1, 1), dtype='|S4, i4, i4')

or

res = np.array([("TEXT", 1, 1), ("XXX", 2, 2)], dtype='|S4, i4, i4')

The data has to be a tuple or a list of tuples. Not quite evident form the error message, is it?

Also, please note that the length of the text field has to be specified for the text data to really be saved. If you want to save the text as objects (only references in the array, then:

res = np.array([("TEXT", 1, 1), ("XXX", 2, 2)], dtype='object, i4, i4')

This is often quite useful, as well.