Monthly Archives: June 2013

Making Numpy arrays read-only

The CMB analysis framework that we’re writing at the moment needs to cache certain data (e.g. sky maps, maps of noise properties etc.) for quick access during various stages of the analysis. It’s important that other parts of the program don’t alter the cached information directly, since it could fall out of sync with the current state of the MCMC chain. To protect the cache, then, we need to make certain arrays read-only.

An easy way of doing this for Numpy arrays is to use theĀ setflags() method on the array. The flag for modifying read/write access is, unsurprisingly, write, so to make an array called arr read-only, you would simply call arr.setflags(write=False). If you subsequently try to modify the array, it raises a ValueError, like so:

>>> a = np.arange(6)
>>> a.setflags(write=False)
>>> a[4] = 6
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: assignment destination is read-only