How can I verify when a copy is made in Python?

Question

In Python 3.x, I'm working with large numpy arrays. I would like to have confirmation (without having to actually do some kind of experiment) that methods I have written are either working with a copy of the array OR are working with a direct reference to the array.

I would like confirmation also that the array in question has or has not been modified.

In C++, if I wanted to ensure that an object was NOT modified, I could pass it as a const... in Python, I seem to have no such assurances and have to be extra careful.

So, to sum up: I need a way to tell whether a copy has or has not been produced of a numpy array. I need a way to tell if an array (or any object for that matter) has been modified. I would prefer a fast, automatic way rather than having to do an experiment.

Answer 1

You can use np.ndarray.flags:

>>> a = np.arange(5)
>>> a.flags
  C_CONTIGUOUS : True
  F_CONTIGUOUS : True
  OWNDATA : True
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False

For example, you can set an array to not be writeable, by using np.setflags; In that case an attempt to modify the array will fail:

>>> a.setflags(write=False)  # sets the WRITEABLE flag to False
>>> a[2] = 10                # the modification will fail
ValueError: assignment destination is read-only

Another useful flag is the OWNDATA, which for example can indicate that the array is in fact a view on another array, so does not own its data:

>>> a = np.arange(5)
>>> b = a[::2]
>>> a.flags['OWNDATA']
True
>>> b.flags['OWNDATA']
False