### How to drop 'NAN' values from a list

```
def remove_sel(l1, l2):
return [z1 for z1, z2 in zip(l1, l2) if z2>=0]
```

works nicely to remove negatives values of a list but if my list is `z=[1, 2, 3, 'NAN']`

Ps I wonder if my list is `z`

or `z'=['1','2', '3','NAN']`

I have tried

```
math import isnan
zz = [x for x in z if not isnan(x)]
```