I've taken the liberty of replying back to the list rather than in
private. Denis, if you mean to deliberately reply privately, please say
so at the start of the email, otherwise I will assume it was an
accident.
On Fri, 12 Mar 2010 09:56:11 pm spir wrote:
> Side-question: Why use super() when we know it can only be unicode?
super is necessary for multiple inheritance to work correctly:
class SpecialString(MyOtherStringClass, Unicode):
...
will have hard-to-find bugs if you don't use super. But if you are
absolutely sure that you will never directly or indirectly use multiple
inheritance, then you could replace the calls to super with:
unicode.__new__(...)
But why bother? super does the right thing for both single and multiple
inheritance.
> And why use cls when we know it can only be Unicode?
Because you might want to subclass Unicode, and if you use cls then
everything will just work correctly, but if you hard-code the name of
the class, things will break.
Actually, my code has a bug. I wrote:
return super(Unicode, cls).__new__(Unicode, *args)
in the __new__ method, but that hard-codes the name of the class. Let's
try it:
>>> type(Unicode()) # Unicode class as defined in my previous post.
<class '__main__.Unicode'>
>>> class K(Unicode):
... pass
...
>>> type(K())
<class '__main__.Unicode'>
Broken! I hang my head in shame :(
So you need to replace the above return with:
return super(Unicode, cls).__new__(cls, *args)
and then it will work correctly:
>>> class K(Unicode):
... pass
...
>>> type(K())
<class '__main__.K'>
You might be tempted to change the first reference to Unicode to cls as
well, but sadly that does not work. The reason is complicated, and to
be honest I don't remember it, but you will probably find it by
googling for "python super gotchas".
--
Steven D'Aprano
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