The question is about filtering a list, keeping only those
elements in the list that correspond to positive numbers.

This means filtering out all elements that are not positive numbers.

In particular all strings must be filtered out.

If the list is known to consist only of numbers and 'NAN',
or only of numbers and any strings, it is easy to filter out strings.

To exclude values that are equal to the string 'NAN':

```
zz = [x for x in z if x != 'NAN' and x > 0]
```

To exclude all strings:

```
zz = [x for x in z if not isinstance(x, str) and x > 0]
```

If list elements can be anything, first check which elements
are real numbers, and test positivity for those.

```
import numbers
zz = [x for x in z if isinstance(x, numbers.Real) and x > 0]
```