Prevent numpy from creating a multidimensional array

This behavior has been discussed a number of times before (e.g. Override a dict with numpy support). np.array tries to make as high a dimensional array as it can. The model case is nested lists. If it can iterate and the sublists are equal in length it will ‘drill’ on down.

Here it went down 2 levels before encountering lists of different length:

In [250]: np.array([[[1,2],[3]],[1,2]],dtype=object)
array([[[1, 2], [3]],
       [1, 2]], dtype=object)
In [251]: _.shape
Out[251]: (2, 2)

Without a shape or ndmax parameter it has no way of knowing whether I want it to be (2,) or (2,2). Both of those would work with the dtype.

It’s compiled code, so it isn’t easy to see exactly what tests it uses. It tries to iterate on lists and tuples, but not on sets or dictionaries.

The surest way to make an object array with a given dimension is to start with an empty one, and fill it

In [266]: A=np.empty((2,3),object)
In [267]: A.fill([[1,'one']])
In [276]: A[:]={1,2}
In [277]: A[:]=[1,2]   # broadcast error

Another way is to start with at least one different element (e.g. a None), and then replace that.

There is a more primitive creator, ndarray that takes shape:

In [280]: np.ndarray((2,3),dtype=object)
array([[None, None, None],
       [None, None, None]], dtype=object)

But that’s basically the same as np.empty (unless I give it a buffer).

These are fudges, but they aren’t expensive (time wise).

================ (edit), Enh: Object array creation function. is an enhancement request. Also the error message for accidentally irregular arrays is confusing.

The developer sentiment seems to favor a separate function to create dtype=object arrays, one with more control over the initial dimensions and depth of iteration. They might even strengthen the error checking to keep np.array from creating ‘irregular’ arrays.

Such a function could detect the shape of a regular nested iterable down to a specified depth, and build an object type array to be filled.

def objarray(alist, depth=1):
    shape=[]; l=alist
    for _ in range(depth):
        l = l[0]
    arr = np.empty(shape, dtype=object)
    return arr

With various depths:

In [528]: alist=[[Test([1,2,3])], [Test([3,2,1])]]
In [529]: objarray(alist,1)
Out[529]: array([[Test([1, 2, 3])], [Test([3, 2, 1])]], dtype=object)
In [530]: objarray(alist,2)
array([[Test([1, 2, 3])],
       [Test([3, 2, 1])]], dtype=object)
In [531]: objarray(alist,3)  
array([[[1, 2, 3]],

       [[3, 2, 1]]], dtype=object)
In [532]: objarray(alist,4)
TypeError: object of type 'int' has no len()

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