On 2/16/2010 8:56 AM, Leo Breebaart wrote:
Chris Rebert<c...@rebertia.com> writes:
On Mon, Feb 15, 2010 at 10:29 AM, Leo Breebaart<l...@lspace.org> wrote:
I have a base class Foo with a number of derived classes FooA,
FooB, FooC, etc. Each of these derived classes needs to read
(upon initialisation) text from an associated template file
FooA.tmpl, FooB.tmpl, FooC.tmpl, etc.
[...]
But, since this information is the same for every instance of
each derived class, I was wondering if there was a way to achieve
the same thing outside of the __init__ function, and just have
these assignments be done as a class attribute (i.e. so that I
can refer to FooA.template_body, etc.)
Metaclasses to the rescue!:
class WithTemplateAttrs(type):
def __new__(cls, name, bases, dct):
klass =3D type.__new__(cls, name, bases, dct)
klass.template_filename =3D "%s.tmpl" % name
klass.template_body =3D read_body_from(klass.template_filename)
return klass
class Foo(object):
__metaclass__ =3D WithTemplateAttrs
#rest of class body here
Now just have FooA, FooB, etc. subclass Foo as before. They'll
automatically get the attributes generated.
Thanks for the feedback! I am thrilled that an actual real-life
issue I'm having may be resolvable by metaclasses (which so far
I've only admired from afar but never considered relevant to my
day-to-day work),
I thought this a really nice example of metaclass use too.
> but unfortunately I'm still struggling to get
this to work.
If I add your code, what happens is that the Foo class will try
to read "Foo.tmpl", which does not exist -- it is only the
derived classes FooA etc, that need to execute this code, not Foo
itself.
My simpleminded solution to the above would be to create a dummy
Foo.tmpl file, so it does exist. Or, in __new__ above, conditionalize
the fetch: if name != 'Foo': ...
Terry Jan Reedy
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