I want to replace single quotes with double quotes in a list

You cannot change how str works for list.

How about using JSON format which use " for strings.

>>> animals = ['dog','cat','fish']
>>> print(str(animals))
['dog', 'cat', 'fish']

>>> import json
>>> print(json.dumps(animals))
["dog", "cat", "fish"]

import json

...

textfile.write(json.dumps(words))