Raw f-string format spec undocumented behavior change between 3.11 and 3.12 #137314
Description
Bug report
Bug description:
I found this while working on astral-sh/ruff#19546 .
Between 3.11 and 3.12, the behavior of raw f-string format specifiers changed.
On 3.11 and earlier, the rawness of the f-string is respected in the format specifier:
PS ~>uvx python@3.11 -c @'
class UnchangedFormat:
def __format__(self, format):
return format
print("Non-raw output:", repr(f"{UnchangedFormat():\xFF}"))
print("Raw output:", repr(rf"{UnchangedFormat():\xFF}"))
'@
Non-raw output: 'ÿ'
Raw output: '\\xFF'On 3.12 and later, the rawness of the f-string is not respected:
PS ~>uvx python@3.12 -c @'
class UnchangedFormat:
def __format__(self, format):
return format
print("Non-raw output:", repr(f"{UnchangedFormat():\xFF}"))
print("Raw output:", repr(rf"{UnchangedFormat():\xFF}"))
'@
Non-raw output: 'ÿ'
Raw output: 'ÿ'This new behavior looks to have been carried to t-strings on 3.14 and later:
PS ~>uvx python@3.14 -c @'
class UnchangedFormat:
def __format__(self, format):
return format
print("Non-raw output:", repr(t"{UnchangedFormat():\xFF}"))
print("Raw output:", repr(rt"{UnchangedFormat():\xFF}"))
'@
Non-raw output: Template(strings=('', ''), interpolations=(Interpolation(<__main__.UnchangedFormat object at 0x00000183F9904EC0>, 'UnchangedFormat()', None, 'ÿ'),))
Raw output: Template(strings=('', ''), interpolations=(Interpolation(<__main__.UnchangedFormat object at 0x00000183F9908A50>, 'UnchangedFormat()', None, 'ÿ'),))I could not find any information/documentation about how the rawness of f-strings affect their format specifiers, despite looking in:
- The input output tutorial f-string section
- The lexical analysis section on f-strings
- PEP 498 – Literal String Interpolation
- PEP 701 – Syntactic formalization of f-strings
- The 3.12 what's new section
- Multiple CPython github issue searches
- Asking in the Python discord
CPython versions tested on:
3.12, 3.11, 3.14
Operating systems tested on:
Windows