Code generator and visitor pattern

[Carl Banks]

On Jul 15, 11:45 am, Karsten Wutzke <kwut... at web.de> wrote:
> On 15 Jul., 20:28, Thomas Jollans <tho... at jollans.com> wrote:
>
> > On 07/15/2010 07:58 PM, Karsten Wutzke wrote:
>
> > > Hello,
>
> > > this is obviously a Python OO question:
>
> > > Since Python isn't stringly typed,
>
> > I expect this is an innocent typo, and you mean strictly.
>
> > > single-dispatch isn't available per se. So is the "double-dispatch" Visitor pattern,
>
> Yes, typo, I meant strictly.
>
>
>
>
>
> > Wait, what?
> > First of all, python is strictly typed in that every object has exactly
> > one type, which is different from other types. So you can't do "1"+2, as
> > you can in some other languages.
>
> > Anyway, this is interesting: Tell me more about how Python's dynamic
> > nature makes it impossible to do whatever you're trying to do. I'm
> > baffled. What are you trying to do, anyway?
>
> > > which is usually used
> > > in OO systems to implement code generators. So, what is the de facto
> > > method in Python to handle source code generation?
>
> > WHOA! Now if that isn't a Gedankensprung. Also, I'm still very far from
> > your train of thought, apparently: Now, the thing that code generators
> > probably share is that they write code to files. It depends on what I'm
> > trying to do of course, but I expect there's a good chance that if I
> > wrote a code generator in Python, it wouldn't be particularly
> > object-oriented at all.
>
> Well, I'm most experienced in OO, so writing OO in Python seems like
> the way to start with Python. The visitor pattern uses single-
> dispatch, that is, it determines which method to call be the type of
> object passed in. I did some reading and it turned out that Python
> can't do it without some tricks (function decorators and 3rd party
> code). For what I'm doing, I can't, or rather don't want to rely on
> 3rd party code (that has reasons). Thus, the visitor OO pattern must
> be replaced by some other way.

Oh brother.

Around these parts, we consider the main use of most Design Patterns
to be to work around limitations of other languages.  Visitor Pattern
is probably the worst example of it.

In Python it's completely unnecessary (at least in its boilerplate-
heavy incarnation as used in C++), and the fact that Python isn't
strongly typed, as you put it, is exactly the reason why.

Say you have a bunch of unrelated types that define a calculate
method, you have a variable x that could be any of these types.
Here's how you would do that in Python:

    x.calculate()

Bam, that's it.  Visitor Pattern in Python.  You don't have to create
a bunch of homemade dispatching boilerplate like you do in C++.


Carl Banks