# Numpy functions may not do what you think

Numpy has the ability to mask arrays and ignore their values for certain computations, called “masked arrays”. They contain a `.mask`

attribute which is a boolean array, `True`

where the value should be masked and `False`

otherwise.

Numpy also comes with a suite of functions which can handle this masking naturally. Typically for a function in the `np.`

namespace, there is a masked-array-aware version under the `np.ma.`

namespace:

```
np.median => np.ma.median
np.average => np.ma.average
```

A crucial thing to remember however is that **standard numpy functions ignore the mask for a masked array**.

This caught me out when sigma clipping values using `astropy.stats.sigma_clip`

- which masks out values outside the sigma range. To ignore the sigma clipped values I should have used `np.ma.median`

instead of `np.median`

.

2016-05-06