Consider the following string that represents a line from a tab delimited txt file:

```
line = 'pf\t2\t0\t9\t0\t9\t9\n'

```

I would like to join the first two elements from this string using an underscore and then write the line back to file. I am using the following simple script to do it:

```
newLabel = '_'.join(line.split('\t')[:2])
newLine = line.split('\t')
newLine[:2] = newLabel

```

What I would expect is the following:

```
['pf_2', '0', '9', '0', '9', '9\n']

```

Instead I am getting:

```
['p', 'f', '_', '2', '0', '9', '0', '9', '9\n']

```

Maybe I am missing something obvious here but why does python split the joined string again? What is the best way to achieve what I want?
Thanks!