[Python-ideas] 'where' statement in Python?

[Terry Reedy]

On 7/21/2010 7:24 AM, Nick Coghlan wrote:
> On Wed, Jul 21, 2010 at 9:16 PM, Stefan Behnel<stefan_ml at behnel.de>  wrote:
>> Terry Reedy, 20.07.2010 21:49:
>>>
>>> I did not comment then because I thought the idea of cluttering python
>>> with augmented local namespace blocks, with no functional gain, was
>>> rejected and dead, and hence unnecessary of comment.
>>> -10
>>> For me, the idea would come close to destroying (what remains of) the
>>> simplicity that makes Python relatively easy to learn. It seems to be
>>> associated with the (to me, cracked) idea that names are pollution.
>>
>> Actually, it's about *giving* names to subexpressions, that's quite the
>> opposite.
>
> I think Terry's point was that you can already give names to
> subexpressions by assigning them to variables in the current scope,
> but some people object to that approach due to "namespace pollution".

Right.

> I agree with him that avoiding namespace pollution isn't a particular
> strong argument though (unless you have really long scripts and

Okay, we can leave that issue aside.

> functions), which is why I've tried to emphasize the intended
> readability benefits.

whereas I am trying to emphasize the reading horror for people whose 
brains are wired differently from yours. The backwards conditional 
expressions are nearly impossible for me to read, which is to say, 
painful. To some, something like

e = fe(a,b,c, p1) where:
   c = fc(a, d, p2 where:
     d = fd(a, p1) where:
       a = fa(p1, p2)
   b = fb(a,p2)

where p1,p2 are input parameters;

looks about as bad (and it was a real effort to write). I would rather 
something like that were in a branch dialect, Ypthon with its own 
extension (.yp).

Algorithm book authors usually want their books read by lots of people. 
When they invent a pseudocode language, they usually invent something 
lots of people can read. (Knuth's MIX was something of an exception.) It 
is often so close to (a subset of) Python that it is ridiculous that 
they do not just use (a subset) Python so it is not 'pseudo'. I cannot 
remember seeing anything like the above. I believe the reason is because 
it would be, on average, less readable and harder to understand.

-- 
Terry Jan Reedy