py3k concerns. An example

[Carl Banks]

On Apr 18, 11:58 am, Aaron Watters <aaron.watt... at gmail.com> wrote:
> Why is the migration to py3k a concern?
> For example I have libraries which use string%dictionary
> substitution where the dictionary is actually an object
> which emulates a dictionary.  The __getitem__ for
> the object can be very expensive and is only called when
> needed by the string substitution.
>
> In py3k string%dictionary is going away.  Why?
> I have no idea.
>
> The replacement is a string.format(...) method
> which supports dictionary calling.
>     string.format(**dictionary)
> But dictionary
> calling doesn't support dictionary emulation.
> So in the example below the substitution works
> but the call fails.
>
> === code
>
> class fdict(dict):
>     def __getitem__(self, item):
>         return "got("+item+")"
>
> def fn(**d):
>     print d["boogie"]
>
> if __name__=="__main__":
>     fd = fdict()
>     print "attempting string substitution with fake dictionary"
>     print
>     print "hello there %(boogie)s" % fd # <-- works
>     print
>     print "now attempting function call with fake dictionary"
>     print
>     fn(**fd) # <-- fails
>
> === output
>
> % python2.6 dtest.py
> attempting string substitution with fake dictionary
>
> hello there got(boogie)
>
> now attempting function call with fake dictionary
>
> Traceback (most recent call last):
>   File "dtest.py", line 17, in <module>
>     fn(**fd)
>   File "dtest.py", line 7, in fn
>     print d["boogie"]
> KeyError: 'boogie'
>
> ==== end of output
>
> Consequently there is no simple way to translate
> my code, I think.  I suspect you will find this kind of subtle
> issue in many places.  Or worse, you won't find it
> until after your program has been installed
> in production.
>
> It's a damn shame because
> if string%dict was just left in it wouldn't be an issue.
>
> Also, if making f(**d) support dict emulation
> has any negative  performance implications
> then I don't want it please.
>
> sigh.  -- Aaron Watters


If you don't like Python 3, DON'T USE IT.

It's been stated repeatedly that 2.x and 3.x are going to be supported
in parallel for years.

Refusing to use 3, thus casting your brain-share vote against it, is
far more likely to have an effect than you coming here and making
everyone's life miserable with your pitiful whining.


Carl Banks