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Nick Keighley wrote:

> Hi,
>
> I'm a Python beginner and I'm would like to write a function that
> returns a class (perhaps a tad ambitious...). I've looked through
> the FAQ and perused "Python In A Nutshell" (not a good book to
> start with?). The only example I found in PiaN used a simple

If you're an experienced programmer with other languages then
the Nutshell should be helpful -- if you're a newbie to programming,
you should start with easier books (but with the kind of tasks
you're setting yourself I guess you aren't). I didn't particularly
emphasize metaprogramming in the Nutshell -- it's more of a "gee
whiz" kind of thing and the Nutshell aims to cover solid, everyday,
bread-and-butter usage.

> if statement to return one of a selection of pre-existing classes.
> I'd like to generate a class on-the-fly from a parameter (a dictionary).

How do you want to use that dict -- as the class's dict? Then either:

def makeaclass(fromdict):
class X: pass
X.__dict__ = fromdict
return X

or

def makeaclass(fromdict):
class X: pass
X.__dict__.update(fromdict)
return X

might be helpful.


> Can Python do this sort of stuff? Does the mean I have to mess with
> the dreaded meta-classes?

Metaclasses may be easier to use than sufficiently-general class-building
functions, actually -- no reason to dread them. But anyway, yes, Python
IS pretty good at metaprogramming, too, both with and without metaclasses.


Alex

Alex Martelli <> wrote in message news:<5SR0b.21604$>...
> Nick Keighley wrote:

thamks all for help and suggestions.

> > I'm a Python beginner and I'm would like to write a function that
> > returns a class (perhaps a tad ambitious...). I've looked through
> > the FAQ and perused "Python In A Nutshell" (not a good book to
> > start with?). The only example I found in PiaN used a simple
>
> If you're an experienced programmer with other languages then
> the Nutshell should be helpful -- if you're a newbie to programming,
> you should start with easier books (but with the kind of tasks
> you're setting yourself I guess you aren't).

no, not a programming beginner. A Python beginner. I was specifically
trying to do something that was hard (impossible) to do in "normal"
language (eg. C). The idea was try and generate types (ie. classes)
that had a
limited set of values. I'd found the C enum emulators in

http://aspn.activestate.com/ASPN/Coo...n/Recipe/67107

but they didn't quite do what I wanted.

> [...] I didn't particularly
> emphasize metaprogramming in the Nutshell -- it's more of a "gee
> whiz" kind of thing and the Nutshell aims to cover solid, everyday,
> bread-and-butter usage.
>
> > if statement to return one of a selection of pre-existing classes.
> > I'd like to generate a class on-the-fly from a parameter (a dictionary).
>
> How do you want to use that dict -- as the class's dict? Then either:
>
> def makeaclass(fromdict):
> class X: pass
> X.__dict__ = fromdict
> return X
>
> or
>
> def makeaclass(fromdict):
> class X: pass
> X.__dict__.update(fromdict)
> return X
>
> might be helpful.
>
>
> > Can Python do this sort of stuff? Does the mean I have to mess with
> > the dreaded meta-classes?
>
> Metaclasses may be easier to use than sufficiently-general class-building
> functions, actually -- no reason to dread them. But anyway, yes, Python
> IS pretty good at metaprogramming, too, both with and without metaclasses.


--
Nick Keighley