How to create a numpy array of lists?

Question

I want to create a numpy array in which each element must be a list, so later I can append new elements to each.

I have looked on google and here on stack overflow a

Answer 1

If you really need a 1-d array of lists you will have to wrap your lists in your own class as numpy will always try to convert your lists to arrays inside of an array (which is more efficient but obviously requires constant size-elements), for example through

class mylist:

    def __init__(self, l):
        self.l=l

    def __repr__(self): 
        return repr(self.l)

    def append(self, x):
        self.l.append(x)

and then you can change any element without changing the dimension of others

>>> x = mylist([1,2,3])
>>> y = mylist([1,2,3])
>>> import numpy as np
>>> data = np.array([x,y])
>>> data
array([[1,2,3], [1,2,3]], dtype=object)
>>> data[0].append(2)
>>> data
array([[1,2,3,2], [1,2,3]], dtype=object)

Update

As suggested by ali_m there is actually a way to force numpy to simply create a 1-d array for references and then feed them with actual lists

>>> data = np.empty(2, dtype=np.object)
>>> data[:] = [1, 2, 3], [1, 2, 3]
>>> data
array([[1, 2, 3], [1, 2, 3]], dtype=object)
>>> data[0].append(4)
>>> data
array([[1, 2, 3, 4], [1, 2, 3]], dtype=object)