You can always call the built-in dir function to get methods defined on an object.
This function lists variables assigned in the caller's scope when called with no argument.
It returns a list of all the attributes available for any object passed to it.
In general, leading and trailing double underscores is the naming pattern Python uses for implementation details.
The names without the underscores in this list are the callable methods on string objects.
The dir function simply gives the methods' names.
To ask what they do, you can pass them to the help function:
help(S.replace) Help on built-in function replace: replace(...) S.replace(old, new[, count]) -> str Return a copy of S with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced.