[ python-Bugs-1467929 ] %-formatting and dicts

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Bugs item #1467929, was opened at 2006-04-10 15:39
Message generated for change (Comment added) made by akuchling
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Category: Python Interpreter Core
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: M.-A. Lemburg (lemburg)
Assigned to: Anthony Baxter (anthonybaxter)
Summary: %-formatting and dicts

Initial Comment:
This looks like a bug in the way the %-formatting code
works or is it a feature ?

>>> '%s %(a)s' % {'a': 'xyz'}
"{'a': 'xyz'} xyz"

>>> u'%s %(a)s' % {'a': 'xyz'}
u"{'a': 'xyz'} xyz"

Note that both strings and Unicode are affected.

Python 2.3 and 2.4 also show this behavior.

I would have expected an exception or the %-formatter
simply ignoring the first %s.




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>Comment By: A.M. Kuchling (akuchling)
Date: 2006-07-25 08:35

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So, what should '%s' % {} do?  Should it be 1) '{}' or 2) an
error because the argument is a mapping but the format
specifier doesn't have a '(key)'?

I've attached a draft patch that fixes stringobject.c; if
the approach is deemed OK, I'll apply it to unicodeobject.c,
too.  PyString_Format() records the type of argument being
processed (a tuple or a mapping) and raises ValueError if
you mix them, at the cost of two extra comparisons for each
format specifier processed.  This preserves the current
behaviour of '%s' % dictionary.

Questions: 1) is the approach reasonably clear?  2) are the
additional two comparisons unacceptably slow?  3) Is
ValueError the right exception?  4) can someone come up with
a better exception message than "both keyed and unkeyed
format specifiers used"?



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Comment By: Martin v. Löwis (loewis)
Date: 2006-07-24 16:07

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IMO, it's correct to break backwards compatibility, as the
current behaviour clearly violates the spec; I'm not sure
whether it's good to break the behaviour *now* (i.e. with no
further betas before the release of 2.5).

Deferring to the release manager.

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Comment By: Georg Brandl (gbrandl)
Date: 2006-07-24 09:37

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The library ref specifies that if a dict is supplied, the
format specifiers MUST include a mapping key, so the right
thing to do would be to raise an exception.

Is it worth breaking backwards compatibility, Martin?

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Comment By: Hasan Diwan (hdiwan650)
Date: 2006-04-14 04:33

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It looks as though it's a feature... The first %s will print
the whole dictionary as a string, the second, only that
value looked up by the key.

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