`KeyError` hides the exception raised by str(key)

[Locked-chess-official]

Bug report

Bug description:

>>>a = object.__new__(type(exit))
>>>b = {}
>>>b[a]
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    b[a]
    ~^^^
KeyError: <exception str() failed>

The exception AttributeError was hidden:

Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    b[a]
    ~^^^
AttributeError: 'Quitter' object has no attribute 'name'

The exception in str() shouldn't be ignore. May because of the lack of the attribute or the wrong type that __str__ return.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[Locked-chess-official]

Here the exception texts are same, but the really exception is different:

>>> class A:
...     def __repr__(self):
...         return
...
>>> a = A()
>>> b = {}
>>> b[a]
Traceback (most recent call last):
  File "<python-input-3>", line 1, in <module>
    b[a]
    ~^^^
KeyError: <exception str() failed>

[Locked-chess-official]

Test for SystemExit:

[vstinner]

What you saw is a feature, not a bug. I suggest to close the issue.

If you write regular Python code:

a = object.__new__(type(exit))
b = {}
try:
    b[a]
except KeyError as exc:
    print(exc)

The exception is not ignored:

Traceback (most recent call last):
  File "/home/vstinner/python/main/x.py", line 4, in <module>
    b[a]
    ~^^^
KeyError: <exception str() failed>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/vstinner/python/main/x.py", line 6, in <module>
    print(exc)
    ~~~~~^^^^^
  File "/home/vstinner/python/main/Lib/_sitebuiltins.py", line 18, in __repr__
    return 'Use %s() or %s to exit' % (self.name, self.eof)
                                       ^^^^^^^^^
AttributeError: 'Quitter' object has no attribute 'name'

What you saw is the sys.unraisablehook which failed to format the exception as a string. In this case, it writes <exception str() failed> since it cannot raise any new exception.

[Locked-chess-official]

What you saw is a feature, not a bug. I suggest to close the issue.

No. I think that in this, it is needed to print this error too. In the project, if the keyerror raised and the message is "<exception str() failed>", what will you think? Some serious problems are hidden then something broken.

Errors should never pass silently. --The Zen of Python, by Tim Peters

[Locked-chess-official]

What you saw is a feature, not a bug. I suggest to close the issue.

If you write regular Python code:

a = object.new(type(exit))
b = {}
try:
b[a]
except KeyError as exc:
print(exc)
The exception is not ignored:

Traceback (most recent call last):
  File "/home/vstinner/python/main/x.py", line 4, in <module>
    b[a]
    ~^^^
KeyError: <exception str() failed>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/vstinner/python/main/x.py", line 6, in <module>
    print(exc)
    ~~~~~^^^^^
  File "/home/vstinner/python/main/Lib/_sitebuiltins.py", line 18, in __repr__
    return 'Use %s() or %s to exit' % (self.name, self.eof)
                                       ^^^^^^^^^
AttributeError: 'Quitter' object has no attribute 'name'

What you saw is the sys.unraisablehook which failed to format the exception as a string. In this case, it writes <exception str() failed> since it cannot raise any new exception.

Here may be a compromise: If the built-in exceptions normally (not due to the user action) need to use str() or repr() then it raise a Exception, the count plus 1 and continue use sys.excepthook, and if the count is above 5 then use sys.unraisablehook to handle. This may keep safely and solved some ignore exceptions.

[Locked-chess-official]

I opened an ideal here, and I want to see the other programmer how to think about it.
Ideal

[serhiy-storchaka]

The problem here is that if str() on exception object is failed, a new exception is raised, which overrides the exception corresponding to the original error. This is not good, because it not only looses information, but swallows an error. If it was KeyboardInterrupt, pressing Ctrl-C did not have effect. If it was MemoryError, RecursionError, UnicodeDecodeError, which are common in such cases, the user looses possibility to catch them and do something with this.

The code should raise an original exception. If we need more context, we can use exception notes.

[Locked-chess-official]

The problem here is that if str() on exception object is failed, a new exception is raised, which overrides the exception corresponding to the original error. This is not good, because it not only looses information, but swallows an error. If it was KeyboardInterrupt, pressing Ctrl-C did not have effect. If it was MemoryError, RecursionError, UnicodeDecodeError, which are common in such cases, the user looses possibility to catch them and do something with this.

The code should raise an original exception. If we need more context, we can use exception notes.

Yes. I think that the original exception should be kept and add more detail about the new exception.