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djc@miles ~ $ python2.7
Python 2.7 (r27:82500, Oct 4 2010, 10:01:41)
[GCC 4.3.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> {}.pop('a')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
KeyError: 'pop(): dictionary is empty'
>>> {'a': 'b'}.pop('c')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
KeyError: 'c'
IMO the former exception should be in line with normal KeyErrors.

There have been several requests for KeyError to grow "key" attribute that will always contain the key that caused the error. See issue 1182143, for example. If this is done, I think it would be natural to unify the args as well for empty and non-empy case. Without adding key access API, however, I am at most -0 on changing the error message. Traditionally, the error messages are not considered part of language specification, so this is not a bug.

I agree with the OP's request.
d.pop(k) is conceptually equivalent to:
 r = d[k] # raises KeyError(k)
 del d[k]
 return r
The current message was probably borrowed from dict.popitem(). But that is much different since the dict.pop(k) method is key-specific (instead of size related).
This fix should be backported (as it doesn't change guaranteed behaviors but does improve the debugability).

This is the case where fixing an issue is easier than arguing that it is not a bug. :-) (Changing to "behavior" not because I agree that it is a bug, but for consistency with targeting 2.7)
A (1-line) patch attached.

Attaching a patch with tests.

I wonder if it would be worthwhile to unify missing key processing as in issue10221a-with-tests.diff.

See r85967, r85968 and r85969.

Raymond,
Did you mean to exclude unit test additions from your commit? See issue10221-with-tests.diff.

We don't usually test the content of error messages because they are not a guaranteed behavior.

On Mon, Nov 1, 2010 at 2:13 PM, Raymond Hettinger
<report@bugs.python.org> wrote:
..
>
> We don't usually test the content of error messages because they are not a guaranteed behavior.
>
I recall that when I asked about this on #python-dev, someone pointed
to places where content of error messages is tested in python unit
tests. I also asked whether such tests should be marked as cpython
details, and the answer was know. Unfortunately I don't remember who
was telling me that, but it sounded reasonable enough that I submitted
the tests.
I think if testing error messages is a grey area, in this case it is
reasonable to have a test for at least two reasons:
1. It was reported as a bug, so users already expect this behavior.
2. There is no prose in the error message, just the key, so it is not
as arbitrary as other error messages.
Note that in my tests I deliberately tested only e.args[0] and not
str(e) or e.args[1:].