Python becoming less Lisp-like

Mike C. Fletcher mcfletch at rogers.com
Wed Mar 16 06:23:33 CET 2005


Thomas Bellman wrote:

>Torsten Bronger <bronger at physik.rwth-aachen.de> wrote:
>  
>
Just to amplify Thomas' statements...

...

>And inflexibility will always make some situations horribly
>daunting to get out of.
>
>Powerful constructs like these can, in some cases, enable a
>skilled package writer to design an API that reduces the amount
>of boiler plate code needed for using the package.
>  
>
...
The following is from the (draft of) my paper for my pycon presentation 
(upcoming).

> We see similar patterns throughout the class-type redesign, the 
> introduction of the __new__ method which allowed for “hooking” the 
> creation (as distinct from initialization) of an object, the 
> __getattribute__ method for hooking all attribute access. In each 
> case, the operations were previously “part of the interpreter” and 
> unavailable for customization by the Python programmer, the protocols 
> generalized the behavior, making objects themselves responsible for 
> what was previously implemented by the interpreter, and thus was 
> always exactly the same.
>
> That sameness was arguably a strong advantage for Python in the early 
> days of its life-cycle. When you were handed an object (particularly, 
> any object defined by a class), it had simple, predictable behavior. 
> The (one) class implementation was straightforward enough that most 
> new users didn't particularly get lost in it. Of course, the flip side 
> was the presence of “types”, which could behave very differently, and 
> the need for people who wanted to customize the core behavior of an 
> object (for instance to change how attribute traversal was handled) to 
> write C extensions. And then there was the strange exception for 
> attribute access for functions, but that was so natural and practical 
> it seldom bit anyone who wasn't doing meta-programming.
>
> Effectively, there were a few bits of “magic” that the Python 
> programmer needed to understand, but the variety and complexity of the 
> magical patterns was limited. The magic incantations could be learned 
> readily by the acolyte, but there was a hard cap on what could be done 
> without becoming a “wizard” and programming in C. The limits on 
> functionality made Python a smaller and less complex language, but it 
> also tended to frustrate the “wizards”, who, after all, might know C, 
> but would rather either program in Python (or spend more of their time 
> looking at really deep C, rather than creating yet another 
> customization to support something just beyond the reach of the acolytes).
>
> So, with Python 2.2, the wizards of Python made it possible to 
> override and customize the behavior of objects in ways that were 
> previously impossible. They brought things that were previously 
> “magic” (for example, the order of attribute lookup that somehow 
> created bound instance methods) into the realm of the understandable 
> by rationalizing the process.
>
metaclasses and descriptors are primarily of interest to 
metaprogrammers, people whose target audience is other programmers. Most 
programmers can get along perfectly fine ignoring them save for knowing 
what the particular ones they *use* do. Metaprogramming is not 
*necessarily* a *dark* art, but it can certainly lead the way to 
darkness if done improperly or wantonly. When done properly, you can 
build systems that seem to perfectly map a problem domain into the 
familiar embrace of everyday Python with just the right "lived in" 
touches to make it seem as though the language was designed from the 
start to support that domain [leaving aside any questions of syntactic 
variants, which is its own involved debate].

When you have metaprogramming to do, it is *such* a relief when you can 
get it done in Python without resorting to C.

Have fun all,
Mike

________________________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://www.vrplumber.com
  http://blog.vrplumber.com
                              PyCon is coming...




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