How to create a void matrix (not filled with ones or zeros) in Python of expected dimensions M times N?

Question

In order to create a void vector a of N elements in Python we use:

a = [None] * N

How about creating a matrix of M times N not filled with ones or zeros? thank you!

Answer 1

a more "matrixy" answer is to use numpy's object dtype: for example:

>>> import numpy as np
>>> np.ndarray(shape=(5,6), dtype=np.object)
array([[None, None, None, None, None, None],
       [None, None, None, None, None, None],
       [None, None, None, None, None, None],
       [None, None, None, None, None, None],
       [None, None, None, None, None, None]], dtype=object)

But, as wim suggests, this might be inefficient, if you're using this to do math.

>>> mat = np.empty(shape=(5,6))
>>> mat.fill(np.nan)
>>> mat
array([[ nan,  nan,  nan,  nan,  nan,  nan],
       [ nan,  nan,  nan,  nan,  nan,  nan],
       [ nan,  nan,  nan,  nan,  nan,  nan],
       [ nan,  nan,  nan,  nan,  nan,  nan],
       [ nan,  nan,  nan,  nan,  nan,  nan]])
>>>

If you're really using more python objecty things, and don't intend to fill the matrix, you can use something nicer; a dict!

>>> from collections import defaultdict
>>> mat = defaultdict(lambda: None)
>>> mat[4,4]
>>> mat[4,4] is None
True