The AST _does_ correctly represent the Python string object in the source, though. After:

>>> s = """
... Hello \n world
... """

we have a Python object `s` of type `str`, which contains exactly three newlines, zero "n" characters, and zero backslashes. So:

>>> s == '\nHello \n world\n'
True


If the AST Str node value were '\nHello \\\n world\n' as you suggest, that would represent a different string to `s`: one containing two newline characters, one "n" and one backslash.

If you need to operate directly on the source as text, then the AST representation probably isn't what you want.