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1. vstinner

   added

   type-featureA feature request or enhancementA feature request or enhancement

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Jul 24, 2026
2. vstinner

   added a commit that references this issue on Jul 24, 2026

   pythongh-154610: Use repr() to format the attribute name in Attribute…

   ...

   8f1c531
3. serhiy-storchaka commented on Jul 24, 2026

   

   serhiy-storchaka

   on Jul 24, 2026

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   How does it work with StrEnum?
4. vstinner commented on Jul 24, 2026

   

   vstinner

   on Jul 24, 2026

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   How does it work with StrEnum?

   My PR changes how StrEnum instances and str subclasses are formatted.

   Currently, PyErr_Format(PyExc_AttributeError, "...%U", ..., name) calls PyUnicode_FromFormatV() which calls _PyUnicodeWriter_WriteStr() to render %U. The _PyUnicodeWriter_WriteStr(writer, obj) call expects a str instance and writes str.__str__(obj) even if the __str__() method is overridden in a str subclass.

   ---

   StrEnum example:

   import enum
   
   class Build(enum.StrEnum):
       DEBUG = enum.auto()
       OPTIMIZED = enum.auto()
   
   getattr(object(), Build.DEBUG)

   Before (Python 3.15):

   AttributeError: 'object' object has no attribute 'debug'
   

   After:

   AttributeError: 'object' object has no attribute <Build.DEBUG: 'debug'>
   

   ---

   Example of a str subclass:

   class MyStr(str):
       def __repr__(self):
           return '<custom repr>'
   
   getattr(object(), MyStr('abc'))

   Before (Python 3.15):

   AttributeError: 'object' object has no attribute 'abc'
   

   After:

   AttributeError: 'object' object has no attribute <custom repr>
   
5. serhiy-storchaka commented on Jul 24, 2026

   

   serhiy-storchaka

   on Jul 24, 2026

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   __str__() should not be called for %U. We have %S for this.
6. serhiy-storchaka commented on Jul 24, 2026

   

   serhiy-storchaka

   on Jul 24, 2026

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   My concerns:

   • Such errors are often used in EAFP code. Error message is generated, but not used. The faster it executes and the less third-party code that can fail is executed, the better.
   • Such errors can happen in stressful cases, for example in process of the interpreter shutdown or when it is failing from memory starvation. The less third-party code that can fail is executed, the better.

   %U rarely fails (especially if we forbid execution of __str__()). %R is more complex, it usually involves several memory allocations.
7. vstinner commented on Jul 24, 2026

   

   vstinner

   on Jul 24, 2026

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   Sometimes, I wish that we could more easily extend PyUnicode_FromFormat() formats. For example, it would be nice to have a format similar to %U but which would quote the string: similar to repr(str) but don't call __repr__() method on str subclasses. Adding a suffix such as %Uq format is not possible, since currently, it's parsed as %U format + q literal string. There is %#U where # usually means "alternative format". But I'm not sure if # relationship to "repr/quote" is obvious enough.

   Note: PEP 737 added %T, %#T, %N and %#N where # only changes the separator: use colon (:) instead of dot (.).
8. serhiy-storchaka commented on Jul 25, 2026

   

   serhiy-storchaka

   on Jul 25, 2026

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   I thought about adding variants of %c and %s which escape non-printable characters.
9. serhiy-storchaka commented on Jul 25, 2026

   

   serhiy-storchaka

   on Jul 25, 2026

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   Implemented in #154669.
10. vstinner commented on Sep 17, 2026

    

    vstinner

    on Sep 17, 2026

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    I created PR gh-157685 to add !R to f-string and add %#R to PyUnicode_FromFormat().

    I also created https://discuss.python.org/t/add-r-conversion-to-f-string-and-str-format-to-format-error-messages/109098 discussion. Changing string formatting concerns a lot of people, so I prefer to involve a larger audience.