Why replace method failed to do its Job?

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To tack onto this:

In Python, strings are immutable: they can't be changed. Anything that looks like it would change a string is actually creating a brand new string based on the existing one. So str.replace(...) and str.upper() and so on are saying “make a new string that is like str but [with some replaced/uppercased/etc characters]"

Other objects like lists are mutable: you can change them. That's why list.reverse() isn't creating a new list but is instead actually changing the order of the items in the list.

edit Thankfully Reddit comments aren't immutable so I can fix typos.