Numpy object array of numerical arrays

Question

I want to create an array with dtype=np.object, where each element is an array with a numerical type, e.g int or float. For example:

>>> a = np.array([1,2,3])
>>> b = np.empty(3,dtype=np.object)
>>> b[0] = a
>>> b[1] = a
>>> b[2] = a

Creates what I want:

>>> print b.dtype
object

>>> print b.shape
(3,)

>>> print b[0].dtype
int64

but I am wondering whether there isn't a way to write lines 3 to 6 in one line (especially since I might want to concatenate 100 arrays). I tried

>>> b = np.array([a,a,a],dtype=np.object)

but this actually converts all the elements to np.object:

>>> print b.dtype
object

>>> print b.shape
(3,)

>>> print b[0].dtype
object

Does anyone have any ideas how to avoid this?

Answer 1

I think anyarray is what you need here:

b = np.asanyarray([a,a,a])
>>> b[0].dtype
dtype('int32')

not sure what happened to the other 32bits of the ints though.

Not sure if it helps but if you add another array of a different shape, it converts back to the types you want:

import numpy as np
a = np.array([1,2,3])
b = np.array([1,2,3,4])
b = np.asarray([a,b,a], dtype=np.object)
print(b.dtype)
>>> object
print(b[0].dtype)
>>> int32

Answer 2

I can't find any elegant solution, but at least a more general solution to doing everything by hand is to declare a function of the form:

def object_array(*args):
    array = np.empty(len(args), dtype=np.object)
    for i in range(len(args)):
        array[i] = args[i]
    return array

I can then do:

a = np.array([1,2,3])
b = object_array(a,a,a)

I then get:

>>> a = np.array([1,2,3])
>>> b = object_array(a,a,a)
>>> print b.dtype
object
>>> print b.shape
(3,)
>>> print b[0].dtype
int64

Answer 3

It's not exactly pretty, but...

import numpy as np

a = np.array([1,2,3])
b = np.array([None, a, a, a])[1:]

print b.dtype, b[0].dtype, b[1].dtype
# object int32 int32