why do we need np.squeeze()?

Question

Very often, arrays are squeezed with np.squeeze(). In the documentation, it says

Remove single-dimensional entries from the shape of a.

Answer 1

When you squeeze a (2,1) array, you get (2,) which works as both (2,1) and (1,2):

>>> a = np.ones(2)
>>> a.shape
(2,)
>>> a.T.shape
(2,)
>>> X = np.ones((2,2))*2
>>> np.dot(a,X)
[4. 4.]
>>> np.dot(X,a)
[4. 4.]

This cannot happen with a (2,1) array:

>>> b = np.ones((2,1))
>>> np.dot(b,X)
Traceback (most recent call last):
ValueError: shapes (2,1) and (2,2) not aligned: 1 (dim 1) != 2 (dim 0)