s = '(x + 2 - y)'

'(x + 2 - y)'

pattern = '(?P<char>[(+)])'
repl = '\g<char>'

>>> s = '(x + 2 - y)'
>>> pattern = '(?P<char>[(+)])'
>>> repl = '\g<char>'
>>>
>>> re.sub(pattern, repl, s)
'\g<char>x \g<char> 2 - y\g<char>'
>>>

>>> repl = '\\g<char>'
>>> re.sub(pattern, repl, s)
'\g<char>x \g<char> 2 - y\g<char>'

>>> repl = r'\\g<char>'
>>> re.sub(pattern, repl, s)
'\\g<char>x \\g<char> 2 - y\\g<char>'

>>> repl = r'\g<char>'
>>> re.sub(pattern, repl, s)
'\g<char>x \g<char> 2 - y\g<char>'

>>> repl = '\g<char>'
>>> re.sub(pattern, repl, s)
'\g<char>x \g<char> 2 - y\g<char>'

>>> repl = '\1'
>>> re.sub(pattern, repl, s)
'\x01x \x01 2 - y\x01'

>>> repl = '\g<1>1'
>>> re.sub(pattern, repl, s)
'\g<1>1x \g<1>1 2 - y\g<1>1'