Initialise numpy array of unknown length

Question

I want to be able to \'build\' a numpy array on the fly, I do not know the size of this array in advance.

For example I want to do something like this:

Answer 1

For posterity, I think this is quicker:

a = np.array([np.array(list()) for _ in y])

You might even be able to pass in a generator (i.e. [] -> ()), in which case the inner list is never fully stored in memory.

---

Responding to comment below:

>>> import numpy as np
>>> y = range(10)
>>> a = np.array([np.array(list) for _ in y])
>>> a
array([array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object),
       array(, dtype=object)], dtype=object)