Strange behavior with numpy arrays. Can't add and store float array to int array

Question

So.. I'm confused. Was it always this way? I've lost an hour trying to find who the hell was reiniting my array after I added a float array.

>>> import numpy as np
>>> a = np.array([1,2])
>>> a
array([1, 2])
>>> a += [0.1, 0.1]
>>> a
array([1, 2])
>>> a += np.array([0.1, 0.1])
>>> a
array([1, 2])
>>> a += np.array([0.1, 0.1])
>>> a
array([1, 2])
>>> a = a + [0.1, 0.1]
>>> a
array([ 1.1,  2.1])
>>> a = np.array([1,2]).astype(float)
>>> a += [0.1,0.2]
>>> a
array([ 1.1,  2.2])

EDIT: Well, this was totally unexpected. My life will never be the same and I will start making lots of float arrays now.