[issue2651] Strings passed to KeyError do not round trip

[Anthony Sottile]

Anthony Sottile  added the comment:

(I know this is old and closed but I've just run into the same thing with 
pwd.getpwnam in issue41767)


KeyError: "getpwnam(): name not found: 'test'"


would it be possible / make sense to extend `KeyError` with a 
named-only-argument which would preserve the raw string in this case?

something like (yeah the name is probably not great, I haven't had time to 
think much about this)

raise KeyError("getpwnam(): name not found 'test'", preserve_msg=True)

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa  added the comment:

I agree with Inadasan. I was eager to fix this until I actually got to it at 
the '16 core sprint. I think there's too little value in fixing this versus 
possible backwards compatibility breakage.

--
resolution:  -> wont fix
stage: patch review -> resolved
status: open -> closed

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[issue2651] Strings passed to KeyError do not round trip

[INADA Naoki]

INADA Naoki  added the comment:

Even if we fixed stdlib, there are many KeyError(missing) idiom used in 3rd 
party code.

KeyError: '42'
KeyError: '42\n'
KeyError: 42

All of the above are looks different.  I think it's good.
Is it really worth enough to break it?

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[issue2651] Strings passed to KeyError do not round trip

[Ben Doremus]

Ben Doremus  added the comment:

Did this patch die?  I ran into the same issue noted in the SO post.  It's 
bizarre that KeyError is the only error message to handle things this way.

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[issue2651] Strings passed to KeyError do not round trip

[Terry J. Reedy]

Terry J. Reedy  added the comment:

A new Stackoverflow question gives a better illustration of how special-casing 
KeyError can be a nuisance.
https://stackoverflow.com/questions/46892261/new-line-on-error-message-in-idle-python-3-3/46899120#46899120
>From a current repository build instead of 3.3:

>>> s = 'line\nbreak'
>>> raise Exception(s)
Traceback (most recent call last):
  File "", line 1, in 
Exception: line
break
>>> raise KeyError(s)
Traceback (most recent call last):
  File "", line 1, in 
KeyError: 'line\nbreak'
>

The OP wanted to get the line break to break without fudging the code to catch 
Exception rather than KeyError.  I proposed catching KeyError and then 
'print(err.args[0]' instead of 'print(err)'.

Why this makes a difference, and why KeyError is unique in needing this, is 
obvious after I found this issue and read the code comment quoted by Amaury.  
But it sure wasn't before.

The rational for applying repr only applies when there is exactly one arg of 
value ''.  So I think the fix should be to only apply it when args *is* ('',).  
There is no reason to quote a non-blank message -- and suppress any formatting 
a user supplies.

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa added the comment:

So actually the issue long predates Python 2.5...

https://hg.python.org/cpython/rev/0401a0ead1eb

Now I'm not so sure it's worth touching it anymore ;)

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[issue2651] Strings passed to KeyError do not round trip

[Antoine Pitrou]

Antoine Pitrou added the comment:

> No, the suggestion is to only adopt the first part of the patch from 2010, 
> which is to revert KeyError to behave like LookupError again

That ship has sailed long ago. 2.7, 3.4 and 3.5 (the three major Python 
versions currently in use) all have the same behaviour, and nobody seems to 
complain very loudly:

>>> {}['foo']
Traceback (most recent call last):
  File "", line 1, in 
{}['foo']
KeyError: 'foo'

>>> KeyError('foo')
KeyError('foo')
>>> print(KeyError('foo'))
'foo'

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa added the comment:

No, the suggestion is to only adopt the first part of the patch from 2010, 
which is to revert KeyError to behave like LookupError again:

  >>> print(LookupError('key'))
  key
  >>> print(KeyError('key'), 'now')
  'key' now
  >>> print(KeyError('key'), 'in 3.6')
  key in 3.6

In other words, there is no descriptive message while stringifying KeyError. 
Having an API with two arguments was disruptive because it moved the key from 
e.args[0] to e.args[1].

Raymond, would it be acceptable to create a two-argument form where the 
*second* argument is the message? That way we could make descriptive error 
messages for dicts, sets, etc. possible. In this world:

  >>> print(KeyError('key', 'key {!r} not found in dict'), 'in 3.6')
  key 'key' not found in dict in 3.6

Do you think any code depends on `str(e) == str(e.args[0])`?

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[issue2651] Strings passed to KeyError do not round trip

[Antoine Pitrou]

Antoine Pitrou added the comment:

What are you suggesting? Something like:

  KeyError: key 'foobar' not found in dict

? If so, +1 from me.

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[issue2651] Strings passed to KeyError do not round trip

[Raymond Hettinger]

Raymond Hettinger added the comment:

The current behavior seems to be a recurring source of confusion and bugs, so 
something needs to change.  I'm thinking that the least egregious thing to do 
is to remove (in 3.6) the special case code for KeyError.  The small downside 
is that KeyError('')  wouldn't look as good.  The benefit is that we eliminate 
an unexpected special case and make KeyErrors behave like all the other 
exceptions including LookupError. 

I'm against the two-args solution as being too disruptive.  Since args[0] is 
currently the only reliable way of extracting the key, prepending a message 
field will likely break much existing code that really wants to access the key.

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Changes by Łukasz Langa :


--
versions: +Python 3.6 -Python 3.5

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[issue2651] Strings passed to KeyError do not round trip

[Raymond Hettinger]

Changes by Raymond Hettinger :


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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa added the comment:

Agreed that this can be addressed now for Python 3.5.

--
assignee:  - lukasz.langa
versions: +Python 3.5 -Python 3.2, Python 3.3

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[issue2651] Strings passed to KeyError do not round trip

[Martin Panter]

Martin Panter added the comment:

The moratorium is over as far as I understand, and PEP 3151 (OSError changes) 
has already been implemented.

My understanding is that exception messages are not generally part of the API. 
I think that avoiding this surprising quirk is more important than retaining 
backwards compatibility with the formatted exception message. But if it cannot 
be changed, we can at least document the quirk.

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[issue2651] Strings passed to KeyError do not round trip

[David]

David vencabot_tep...@hotmail.com added the comment:

I'm +1 for fixing this behavior for the same reasons that are mentioned in the 
OP: consistency and predictability. I raised this issue as #14086, and I was 
referred to this issue before closing mine as a duplicate.

It took me a while to figure out why I was getting unexpected escaped quotation 
marks in my strings, and it turned out that it was because I was passing 
strings back and forth as Exception arguments (tagging built-in Exceptions with 
a little bit of extra information when they occurred and re-raising), and every 
time that it occurred with a KeyError (and only with a KeyError), the string 
would grow another pair of quotation marks.

In my issue, I bring up the documentation in the Python Tutorial about 
Exception.args and Exception.__str__(); it states very plainly and simply (as 
it should be) that the __str__() method is there to be able to conveniently 
print Exception arguments without calling .args, and, when an unhandled 
Exception stops Python, the tail-end of the message (the details) of the 
exception will be the arguments that it was given. This is not the case with 
KeyError.

str(KeyError(Foo)) should be equal to Foo, as it would be with any other 
Exception and as is the documented behavior of built-in Exceptions, at least in 
the tutorial (which I realize isn't the be-all, end-all document). The 
documented behavior makes more sense.

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[issue2651] Strings passed to KeyError do not round trip

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


--
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versions: +Python 3.3

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[issue2651] Strings passed to KeyError do not round trip

[Meador Inge]

Changes by Meador Inge mead...@gmail.com:


--
stage: needs patch - patch review

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[issue2651] Strings passed to KeyError do not round trip

[Julian Berman]

Changes by Julian Berman julian+python@grayvines.com:


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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa luk...@langa.pl added the comment:

For the record, I am -1 for this change after discussion on #python-dev. There 
are three major arguments against the proposed approach:

1. Moratorium. If we don't stick to the rules set by ourselves, nobody will 
take us seriously. No exceptions, this has to wait.

2. Backwards incompatibility. The current implementation is backwards 
incompatible and doesn't provide a reliable way of returning `e.key`. There was 
an alternative discussed about including key= and message= keyword arguments to 
the exception. This would require writing an additional helper routine in C for 
dealing with keyword arguments and is thus a bit of a broader problem.

3. PEP 3151. While still in draft form, this PEP prepares a solution for more 
generic and consistent behaviour of exceptions. It mentions IOError and OSError 
at the moment but Alexander is right: if the PEP 3151 approach chooses a new 
form of argument handling for exceptions, lookup errors should try to conform 
with that.

For the above reasons, I would resolve this issue as 'after moratorium' and 
prepare a superseder that gathers all PEP 3151 related issues in the tracker.

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[issue2651] Strings passed to KeyError do not round trip

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

Unassigning because others seem to be ahead of me reviewing this patch.   I 
will keep an eye on this, though.

Please note that I marked issue #614557 to depend on this one.  Adding 'key' 
attribute to exceptions is the subject of that issue.

While I support these features, I would really like to see a more comprehensive 
redesign as a part of PEP 3151.  I think it would be a mistake to rush 
something in before PEP 3151 is implemented and then discover that it was not 
the best choice as a part of bigger picture.

--
assignee: belopolsky - 

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[issue2651] Strings passed to KeyError do not round trip

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Changes by Łukasz Langa luk...@langa.pl:


Removed file: http://bugs.python.org/file18356/issue2651.diff

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa luk...@langa.pl added the comment:

Corrected patch attached. You're right, I left in ReST doc changes for 
configparser. Sorry for that.

--
Added file: http://bugs.python.org/file18371/issue2651.diff

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[issue2651] Strings passed to KeyError do not round trip

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

In KeyError_str, I think the following code shouldn't be deleted:

-if (PyTuple_GET_SIZE(self-args) == 1) {
-return PyObject_Repr(PyTuple_GET_ITEM(self-args, 0));

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Changes by Łukasz Langa luk...@langa.pl:


Removed file: http://bugs.python.org/file18371/issue2651.diff

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa luk...@langa.pl added the comment:

Patch updated to include a roundtrip test in test_exceptions.

--
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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa luk...@langa.pl added the comment:

FTR regarding for Antoine's comment above: that code should in fact be removed 
:)

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[issue2651] Strings passed to KeyError do not round trip

[Antoine Pitrou]

Antoine Pitrou pit...@free.fr added the comment:

Latest patch looks good. Note that you could use PyUnicode_FromFormat() instead 
of building the format string manually.

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa luk...@langa.pl added the comment:

Patch for py3k ready. Includes patches for the stdlib where tests actually 
failed.

--
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[issue2651] Strings passed to KeyError do not round trip

[Éric Araujo]

Éric Araujo mer...@netwok.org added the comment:

Looks like you didn’t narrow your diff command, since unrelated changes to 
configparser appear there. svn diff file1 path/file2 :)

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[issue2651] Strings passed to KeyError do not round trip

[Mark Lawrence]

Mark Lawrence breamore...@yahoo.co.uk added the comment:

@Łukasz: please provide an updated patch.

--
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stage:  - needs patch

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[issue2651] Strings passed to KeyError do not round trip

[Łukasz Langa]

Łukasz Langa luk...@langa.pl added the comment:

Alexander, Brett, I could update the patch but first I need thumbs up that this 
is going to be accepted and some eventual code breaks will be patched (again, I 
can do that but it has to be accepted on time).

Brett, what to do?

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[issue2651] Strings passed to KeyError do not round trip

[Alexander Belopolsky]

Alexander Belopolsky belopol...@users.sourceforge.net added the comment:

KeyError.patch is out of date.  Is anyone motivated enough to update it for 
py3k?  I like the idea, but don't have spare cycles at the moment.

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[issue2651] Strings passed to KeyError do not round trip

[Alexander Belopolsky]

Changes by Alexander Belopolsky belopol...@users.sourceforge.net:


--
assignee:  - belopolsky
nosy: +belopolsky

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[issue2651] Strings passed to KeyError do not round trip

[Alexander Belopolsky]

Changes by Alexander Belopolsky belopol...@users.sourceforge.net:


--
versions: +Python 3.2 -Python 2.5

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[issue2651] Strings passed to KeyError do not round trip

[Antoine Pitrou]

Antoine Pitrou [EMAIL PROTECTED] added the comment:

Wouldn't it be nice to also store the offending key as a key attribute?
Writing key_error.key is a lot intuitive than key_error.args[0] (or is
it key_error.args[1] ? I've already forgotten :-)).

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[issue2651] Strings passed to KeyError do not round trip

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc [EMAIL PROTECTED] added the comment:

Attached patch changes KeyError: when it has two arguments, they are
formatted with %s: %r. Otherwise the base repr is called, and this
allows the round trip.

Standard objects (dict, set, UserDict, namedtuple, defaultdict, weak
dictionaries) now raise something like KeyError(not in dict, key). 

At least one place in the stdlib relied on the key being the first
argument to KeyError() (in ConfigParser.py)

I don't know if this incompatibility will break much code. At least we
can say that the current behavior is not documented.

--
keywords: +patch
Added file: http://bugs.python.org/file10069/KeyError.patch

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[issue2651] Strings passed to KeyError do not round trip

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc [EMAIL PROTECTED] added the comment:

Here is another form of the same inconsistency:

 [].pop(0)
IndexError: pop from empty list
 {}.pop(0)
KeyError: 'pop(): dictionary is empty'

And my preferred one:
 unicodedata.lookup('')
KeyError: 'undefined character name \'\''

KeyError is special in that dict lookup raises the equivalent of
KeyError(key). Since the key may be any kind of (hashable) object, it's
preferable to repr() it.

I can see 3 solutions to the problem:

1- imitate IndexError for lists: the exception do not contain the key.

2- dict lookup builds the complete string message, and raise it
   raise KeyError(key not found: %r % key)
then KeyError.__str__ can be removed.

3- like IOError, KeyError has msg and key attributes. then dict
lookup raises
   raise KeyError(key not found, key)
and KeyError.__str__ is something like:
   if self.key is not None:
   return %s: %r % (self.msg, self.key)
   else
   return str(self.msg)

Choice 1 is not an improvement.
Choice 2 has the correct behavior but leads to performance problems;
KeyErrors are very very common in the interpreter (namespace lookups...)
and formatting the message is costly.
Choice 3 may cause regression for code that use exception.args[0], but
otherwise seems the best to me. I'll try to come with a patch.

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[issue2651] Strings passed to KeyError do not round trip

[Rick Harris]

New submission from Rick Harris [EMAIL PROTECTED]:

Here is a bug in Python 2.5 which would be nice to fix for Py3k (since
we are already breaking compatibility):

Take a string:
s = Hello

Create a KeyError exception with that string:
e = KeyError(s)

Counterintuitively, casting the exception to a string doesn't return the
same string:
str(e) != s

Instead, when KeyError is cast to a string it affixes single-quotes
around the string.

I have create a test which shows that the other built-in exceptions
(except for 3 Unicode Errors which seem to be unusual in that they don't
accept just a string), do indeed round-trip the string unaltered.

This actually caused a bug (in an old version of zope.DocumentTemplate).

I am including the test case I wrote for now; I will begin looking into
a solution shortly and hopefully whip up a patch.

--
components: Interpreter Core
files: testExceptionStringRoundTrip.py
messages: 65586
nosy: rharris
severity: normal
status: open
title: Strings passed to KeyError do not round trip
type: behavior
versions: Python 2.5
Added file: http://bugs.python.org/file10048/testExceptionStringRoundTrip.py

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[issue2651] Strings passed to KeyError do not round trip

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc [EMAIL PROTECTED] added the comment:

Here is a relevant comment inside the KeyError_str function:
/* If args is a tuple of exactly one item, apply repr to args[0].
   This is done so that e.g. the exception raised by {}[''] prints
 KeyError: ''
   rather than the confusing
 KeyError
   alone.  The downside is that if KeyError is raised with an
explanatory
   string, that string will be displayed in quotes.  Too bad.
   If args is anything else, use the default BaseException__str__().
*/

Why is it so important to round trip?

--
nosy: +amaury.forgeotdarc

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[issue2651] Strings passed to KeyError do not round trip

[Rick Harris]

Rick Harris [EMAIL PROTECTED] added the comment:

I think it is important to round-trip for at least two reasons:

1) Consistency. Other built-in exceptions behave this way, why should
KeyError be any different? Okay, technically 3 UnicodeErrors don't allow
just strings to be passed in (perhaps they should :-); but for common
exception classes, they all behave the same way.

To quote PEP-20: Special cases aren't special enough to break the rules

2) Intuitiveness. Decorating the string with quotes is unexpected; it
has already caused at least one bug and could cause more.

Ensuring intuitive round-trip behavior is an important enough issue that
is explicitly discussed in PEP 327 for the decimal type.

Why can't intuitiveness be restored to KeyError in Py3K?

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