Re: string interpolation mystery in Python 2.6
Alan G Isaac wrote:
George Brandl explained it to me this way:
It's probably best explained with a bit of code:
>>> class C(object):
... def __str__(self): return '[str]'
... def __unicode__(self): return '[unicode]'
...
>>> "%s %s" % ('foo', C())
'foo [str]'
>>> "%s %s" % (u'foo', C())
u'foo [unicode]'
I.e., as soon as a Unicode element is interpolated into
a string, further interpolations automatically request
Unicode via __unicode__, if it exists.
Even more fun (until you know what is going on):
>>> c = C()
>>> "%s %s %s" % (c, u'c', c)
u'[str] c [unicode]'
--Scott David Daniels
Scott David dani...@acm.org
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Re: string interpolation mystery in Python 2.6
On 9/11/2009 9:42 PM, Steven D'Aprano wrote:
However, I must admit I'm perplexed why the original example is calling
__unicode__() in the first place! Given the line:
raise self.severe('Problems with "%s" directive path:\n%s: %s.'
% (self.name, error.__class__.__name__, error))
it looks to me like it should be calling error.__str__() not
error.__unicode(). Making the suggested edit:
raise self.severe('Problems with "%s" directive path:\n%s: %s.'
% (self.name, error.__class__.__name__, str(error)))
should have no effect. But it (apparently) does. This brings us back to
Alan's original question:
"MYSTERY: how can "%s"%error be different from "%s"%str(error) in Python
2.6?"
George Brandl explained it to me this way:
It's probably best explained with a bit of code:
>>> >>> class C(object):
... def __str__(self): return '[str]'
... def __unicode__(self): return '[unicode]'
...
>>> "%s %s" % ('foo', C())
'foo [str]'
>>> "%s %s" % (u'foo', C())
u'foo [unicode]'
I.e., as soon as a Unicode element is interpolated into a string, further
interpolations automatically request Unicode via __unicode__, if it
exists.
Pretty subtle ...
Cheers,
Alan Isaac
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Re: string interpolation mystery in Python 2.6
2009/9/12 Steven D'Aprano : > On Fri, 11 Sep 2009 15:19:05 -0700, Chris Rebert wrote: > >> Sounds like IOError or one of its ancestors defines both __str__() and >> __unicode__ () special methods but has them produce different output. > > > That's what it looks like to me too, which I wouldn't call either a bug > or a feature. I don't think Python makes any promises regarding exception > messages. > > There are some report about the apparently same issue in the bug tracker. http://bugs.python.org/issue5274 http://bugs.python.org/issue6108 vbr -- http://mail.python.org/mailman/listinfo/python-list
Re: string interpolation mystery in Python 2.6
On Fri, 11 Sep 2009 15:19:05 -0700, Chris Rebert wrote:
> On Fri, Sep 11, 2009 at 3:12 PM, Alan G Isaac
> wrote:
>> Michael Foord came up with a much simpler illustration. With Python
>> 2.6::
[snip]
> Sounds like IOError or one of its ancestors defines both __str__() and
> __unicode__ () special methods but has them produce different output.
That's what it looks like to me too, which I wouldn't call either a bug
or a feature. I don't think Python makes any promises regarding exception
messages.
However, I must admit I'm perplexed why the original example is calling
__unicode__() in the first place! Given the line:
raise self.severe('Problems with "%s" directive path:\n%s: %s.'
% (self.name, error.__class__.__name__, error))
it looks to me like it should be calling error.__str__() not
error.__unicode(). Making the suggested edit:
raise self.severe('Problems with "%s" directive path:\n%s: %s.'
% (self.name, error.__class__.__name__, str(error)))
should have no effect. But it (apparently) does. This brings us back to
Alan's original question:
"MYSTERY: how can "%s"%error be different from "%s"%str(error) in Python
2.6?"
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Re: string interpolation mystery in Python 2.6
On Fri, Sep 11, 2009 at 3:12 PM, Alan G Isaac wrote:
> Michael Foord came up with a much simpler
> illustration. With Python 2.6::
>
> >>> try:
> ... open('flooble')
> ... except Exception as e:
> ... pass
> ...
> >>> e
> IOError(2, 'No such file or directory')
> >>> unicode(e)
> u"(2, 'No such file or directory')"
> >>> str(e)
> "[Errno 2] No such file or directory: 'flooble'"
> >>> u'%s' % e
> u"(2, 'No such file or directory')"
> >>> '%s' % e
> "[Errno 2] No such file or directory: 'flooble'"
Sounds like IOError or one of its ancestors defines both __str__() and
__unicode__ () special methods but has them produce different output.
Cheers,
Chris
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Re: string interpolation mystery in Python 2.6
Michael Foord came up with a much simpler
illustration. With Python 2.6::
>>> try:
... open('flooble')
... except Exception as e:
... pass
...
>>> e
IOError(2, 'No such file or directory')
>>> unicode(e)
u"(2, 'No such file or directory')"
>>> str(e)
"[Errno 2] No such file or directory: 'flooble'"
>>> u'%s' % e
u"(2, 'No such file or directory')"
>>> '%s' % e
"[Errno 2] No such file or directory: 'flooble'"
Cheers,
Alan Isaac
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string interpolation mystery in Python 2.6
MYSTERY: how can "%s"%error be different from "%s"%str(error) in Python 2.6?
APOLOGY: I tried to strip this down, but could not find a simple way to
reproduce the problem. This way works, however. (There is a discussion on
the docutils-develop list.) Although there are several steps, we are talking
about less than 5 minutes to document the puzzle. Please use the specific
revision (or earlier) in the directions, as later revisions implement a
work around.
1. Check out revision 6121 from docutils
http://docutils.sourceforge.net/docs/dev/repository.html
2. Install docutils under Python 2.6
3. Process the file below [1]_ using the rst2html.py script
(found in Python26/Scripts on Windows platforms)
4. Note the IOError::
temp.rst:: (SEVERE/4) Problems with "include" directive path:
IOError: (2, 'No such file or directory').
Exiting due to level-4 (SEVERE) system message.
5. Make the following change and *no other* changes:
In docutils/parsers/rst/directives/misc.py (line 66-67) change ::
raise self.severe('Problems with "%s" directive path:\n%s: %s.'
% (self.name, error.__class__.__name__, error))
to ::
raise self.severe('Problems with "%s" directive path:\n%s: %s.'
% (self.name, error.__class__.__name__,
str(error)))
6. Process the same file the same way. Note the change in the IOError::
temp.rst:: (SEVERE/4) Problems with "include" directive path:
IOError: [Errno 2] No such file or directory: 'doesnotexist.rst'.
Exiting due to level-4 (SEVERE) system message.
7. Try this again in Python 2.5. The correct (filename reported) error report
is produced both times. So this is a Python 2.6 change.
Clues? Bug or feature?
I'm going to hazard a guess that there was an undocumented (in What's New)
change
to the __unicode__ method of BaseException.
Thanks,
Alan Isaac
.. [1] Following is the rst file to process:
Test
This is just a test.
.. include:: doesnotexist.rst
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