Bug report

Between Python 3.12 alpha 7 and beta 1 the behaviour of format specifiers containing a backslash (e.g.\n,\u2603) in f-strings changed.

Previously, the__format__ method would get e.g. the newline character, but it now receives a backslash and ann character (i.e. the string"\n")

The following script demonstrates this:

# issue.pyclassF:def__format__(self,format_spec):print("spec:",repr(format_spec))returnformat_spec.join(["a","b","c"])print("out: ",repr((f"{F():\n}")))print("out: ",repr((f"{F():\u2603}")))print("out: ",repr((format(F(),"\n"))))print("out: ",repr((format(F(),"\u2603"))))

$python3.11 issue.pyspec: '\n'out:  'a\nb\nc'spec: '☃'out:  'a☃b☃c'spec: '\n'out:  'a\nb\nc'spec: '☃'out:  'a☃b☃c'

$python3.12 issue.pyspec: '\\n'out:  'a\\nb\\nc'spec: '\\u2603'out:  'a\\u2603b\\u2603c'spec: '\n'out:  'a\nb\nc'spec: '☃'out:  'a☃b☃c'

In both cases theformat() function works correctly - I only encounter the issue with f-strings.

I suspect this is related to the PEP 701 changes to f-strings, but I couldn't find anything in the discussion of that PEP to suggest this was a deliberate change.

Your environment

• CPython versions tested on: Python 3.11.2, 3.12.0alpha7, 3.12.0beta1
• Operating system and architecture: Ubuntu 20.04 (AMD64)

Linked PRs

• gh-105194: Fix format specifier escaped characters in f-strings #105231
• [3.12] gh-105194: Fix format specifier escaped characters in f-strings (GH-105231) #105234