Replace whitespaces with an underscore

S.replace(” “, “_”)

Replace whitespaces with an underscore and vice versa.

import re

S = 'Python Exercises'
S = S.replace(" ", "_")

print(S)
S = S.replace("_", " ")
print(S)

Output:

Python_Exercises
Python Exercises

See also

https://www.w3resource.com/python-exercises/re/python-re-exercise-23.php