In Python 3.6, invalid escape sequence were deprecated in string literals (bytes and str): issue #71551, commit 110b6fe.

What's New in Python 3.6: Deprecated Python behavior:

A backslash-character pair that is not a valid escape sequence now generates a DeprecationWarning. Although this will eventually become a SyntaxError, that will not be for several Python releases. (Contributed by Emanuel Barry in bpo-27364.)

I propose now raises a SyntaxError, rather than a DeprecationWarning (which is silent in most cases).

Example:

$ python3 -W default -c 'print(list("\z"))'
<string>:1: DeprecationWarning: invalid escape sequence '\z'
['\\', 'z']
$ python3 -W default -c 'print(list(b"\z"))'
<string>:1: DeprecationWarning: invalid escape sequence '\z'
[92, 122]

Note: Python REPL ate some DeprecationWarning which makes manual testing harder. It was fixed last month by commit 426d72e in issue gh-96052.

Python 3.11 now emits a deprecation warning for invalid octal escape sequence (issue gh-81548):

Octal escapes in string and bytes literals with value larger than 0o377 now produce DeprecationWarning. In a future Python version they will be a SyntaxWarning and eventually a SyntaxError. (Contributed by Serhiy Storchaka in gh-81548.)

Example:

$ python3.11 -Wdefault -c 'print(list(b"\777"))'
<string>:1: DeprecationWarning: invalid octal escape sequence '\777'
[255]

• PR: gh-98401: Invalid escape sequences emits SyntaxWarning #99011