Getting in to metaprogramming

[Hendrik van Rooyen]

 "Aaron Brady" <castironpi at gmail.com> wrote:

>On Nov 26, 10:41 pm, "Hendrik van Rooyen" <m... at microcorp.co.za>
>wrote:
>> "Steven D'Aprano" <steau> wrote:
>>
>>
>>
>> > Well, I don't know about "any problem". And it's not so much about
>> > whether metaprograms can solve problems that can't be solved by anything
>> > else, as whether metaprograms can solve problems more effectively than
>> > other techniques.
>>
>> > If you include factory functions, class factories, the builder design
>> > pattern, metaclasses, etc. as "metaprogramming", then I use it all the
>> > time, and find it an excellent technique to use.
>>
>> > But if you mean using a Python program to generate Python source code,
>> > then I can't think of any time I used it. Which doesn't mean that others
>> > don't find it helpful, only that I haven't yet.
>>
>> I am using the term in the restricted sense of Python writing Python source.
>>
>> Given that, can anybody think of an example that you could not do with
>> a class? (excepting the "stored procedure" aspect)
>>
>> Or can I claim a new a new meta - rule - I would call it van Rooyen's
folly...
>
>The example I think of is the Visitor Pattern of Gamma and all.  One
>class's method calls another's method with its own class's name in the
>name.
>
>class Visitor:
>  def visit_A( self, arg ):...
>  def visit_B( self, arg ):...
>
>class A:
>  def visit( self, vis ):
>    vis.visit_A( self )
>
>class B:
>  def visit( self, vis ):
>    vis.visit_B( self )
>
>As you can see, the 'visit' method is mechanical for classes A and B.
>One might want to autogenerate those in some languages, but Python has
>introspection:
>
>class BaseAB:
>  def visit( self, vis ):
>    getattr( vis, 'visit_%s'% self.__class__.__name__ )( self )
>
>And it's easier to modify the default behavior this way than in
>autogenerated code too.

This is kind of the wrong way around - this is an example of
OO jiggery pokery that will be difficult to do by generating the
source code - are there things that can be done by generating
the source code that cannot be done with OO?

- Hendrik