NumPy resize method

Question

Can anyone explain this to me? (Python 3.3.2, numpy 1.7.1):

>>> a = np.array([[1,2],[3,4]])
>>> a    # just a peek
array([[1, 2],
       [3, 4]])
>>> a.resize(3,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: cannot resize an array references or is referenced
by another array in this way.  Use the resize function
>>> a = np.array([[1,2],[3,4]])
>>> a.resize(3,2)
>>> a
array([[1, 2],
       [3, 4],
       [0, 0]])
>>> a = np.array([[1,2],[3,4]])
>>> print(a)   # look properly this time
[[1 2]
 [3 4]]
>>> a.resize(3,2)
>>> a
array([[1, 2],
       [3, 4],
       [0, 0]])

Why does taking a peek at the array create a reference to it? (or, at least, why does that reference persist after I'm done looking?) Also, is it just me or does that Exception need a bit of a rewrite?

Answer 1

From the documentation (emphasis mine):

The purpose of the reference count check is to make sure you do not use this array as a buffer for another Python object and then reallocate the memory. However, reference counts can increase in other ways so if you are sure that you have not shared the memory for this array with another Python object, then you may safely set refcheck to False.

Your "peek", unlike print, does not decrement the reference count afterwards. This is because, in the interpreter, the result of the last computation is assigned to _. Try:

print(_) # shows array
a.resize((3, 2), refcheck=False) # works

Alternatively, if you do any other computation (e.g. just 1 + 2) in-between, this will dereference your array from _.