Python 3.9.10 (main, Jan 15 2022, 18:56:52)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import re
>>> re.sub(r"{\w}", "\s", "Hello! {user}")
Traceback (most recent call last):
  File "/usr/lib/python3.9/sre_parse.py", line 1039, in parse_template
    this = chr(ESCAPES[this][1])
KeyError: '\\s'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/re.py", line 210, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/usr/lib/python3.9/re.py", line 327, in _subx
    template = _compile_repl(template, pattern)
  File "/usr/lib/python3.9/re.py", line 318, in _compile_repl
    return sre_parse.parse_template(repl, pattern)
  File "/usr/lib/python3.9/sre_parse.py", line 1042, in parse_template
    raise s.error('bad escape %s' % this, len(this))
re.error: bad escape \s at position 0
>>>

Isn't '\s' covered by: " Unknown escapes of ASCII letters are reserved for future use and treated as errors" (https://docs.python.org/3/library/re.html#re.sub)?

There seem to be a number of escaping problems in the OP's example.

Is this what was intended?

>>> re.sub(r"\{(\w+)\}", r"\1", "Hello! {user}")
'Hello! user'

Yes, I assume that's what the OP intended, but then stumbled across the error with '\s'.

In any event, I don't think there's a bug here so I'm going to close this. @siddheshsathe: if you disagree, please respond here.

I'd just like to point out that to a user it could _look_ like a bug, that an error occurred while reporting, because the traceback isn't giving a 'clean' report; the stuff about the KeyError is an internal detail.

I agree the error message could be better. Also, "s.error('bad escape %s' % this, len(this))" should probably be "from None", since as @mrabarnett notes the KeyError is an implementation detail.