How to rename columns in Pandas df with underscores

The correct answer in your case directly gets the needed string, for example:

#!/usr/bin/env pythonimport timeprint(time.strftime('%Y-%m-%d %H.%M.%S'))

Or, if you need fractions of a second:

#!/usr/bin/env pythonfrom datetime import datetimeprint(datetime.now().strftime('%Y-%m-%d %H.%M.%S.%f'))

Answer to the question in the title

Q: Changing values of string elements in a loop

Strings are immutable in Python