I have list of lists which contains a mix of empty and non-empty sublists. 
The total length of the list is 240.

```
_remain = [['24'],
 ['24'],
 ['17'],
 [],
 ['17'],
 [],
 ['17'],...]

```

And I've tried to flatten the list of lists into one list with:

`[name for sublist in _remain for name in sublist]`

But when I do this I get a list with length 220. The empty sublists are gone.

My desire is to get a flattened list, replacing empty lists with `np.nan` so that I can insert it into a pandas `DataFrame`.

The resultant list I would like to get:

```
['24',
 '24',
 'np.nan',
 '17',
 'np.nan',
 '17',...]

```

What should i try?