I am trying to replace an empty string in a list with a known string (say 'NAN'). I am using the following command

```
a = ['','','asdf']    
["nan" if x =='' else x for x in a]

```

The code is working, when it is used standalone, however when I am trying to employ it in my main code, it is not working. My main code is as follows: 

```
data = [ ('plant_data.xlsx', 0, []),('sorg.xlsx', 1, ['','','asdf'])]#,('sloc.xlsx', 1, ['307-040N'])];

for fl in data:
    filename = fl[0];
    filename = filename[:-5];

    f = open('IC1_Results\%s.txt' %filename,'w');

    if fl[1] == 0:
        f.write("All Part Numbers exist");
        f.close()

    elif fl[1] == 1:
        a = fl[2];
        print type(a)

        ["nan" if x == '' else x for x in a]

        print fl[2],a

```