[PyKDE] Can I use QString as key in Python dictionary?

[Gerard Vermeulen]

On Thu, 02 Jun 2005 08:08:34 +0200
Torsten Marek <shlomme at gmx.net> wrote:

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> Sebastian Kügler schrieb:
> > On Thursday 02 June 2005 04:10, Matej Cepl wrote:
> > 
> >>I have this simple dialog in PyQt, but whenever I press any item in
> >>the list (or click OK), I get this error:
> >>
> >>        chelcicky:pycatsel$ ./pycatsel
> >>        Traceback (most recent call last):
> >>          File
> >>"/home/matej/archiv/programky/vim-qadas/pycatsel/selectCode.py",\
> >>line 148, in Quit
> >>            note = unicode(self.dictCategories[key])
> >>        KeyError: <__main__.qt.QString object at 0xb7e12c5c>
> >>        chelcicky:pycatsel$
> >>
> >>Does anybody have any idea, what's wrong? Can I use QString as a key
> >>in Python dictionary (both key and dict's values are QStrings)?
> > 
> > 
> >>>>from qt import *
> >>>>QString("foo") is QString("foo")
> > 
> > False
> > 
> > As you can see these two QString are different objects, so there's 
> > actually no such key.
> > 
> > 
> >>>>k = QString("bla")
> >>>>mydict = {QString("bla"):"foobar"}
> >>>>mydict[k]
> > 
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in ?
> > KeyError: <qt.QString object at 0xb7df350c>
> > 
> > As you can see, it yields a KeyError because k is actually no key in 
> > mydict. However, you can use a QString as a key, but you have to use 
> > exactly that reference (not a *similar* one):
> > 
> > 
> >>>>k = QString("bla")
> >>>>mydict = {k:"foobar"}
> >>>>mydict[k]
> > 
> > 'foobar'
> 
> One could think about changing the QString implementation of __hash__ to use the
> strings's contents only. This shouldn't be too difficult (although I didn't try
> it out, so I'm probably wrong.).
> 
A Python string is immutable and a QString is mutable.

You'll may get undefined behaviour if you change a QString key while
manipulating a dictionary containing that QString key.

Gerard