[Python-Dev] exec/with thunk-handling proposal

[Jack Jansen]

On Tuesday, Feb 4, 2003, at 12:54 Europe/Amsterdam, holger krekel wrote:

>>>     exec expr [with params]: suite
>>
>> Gut reaction: ugh!
>
> then i guess you rather prefer new keywords.  I thought that there
> is a general reluctance to introduce new keywords *and* many
> people dislike 'exec' for its existence.

I agree with Michael's "Ugh!". And actually I extend the "Ugh!" to 
Guido's proposal
of "expr [= expr] [lambda]: suite".

While I think that the reluctance to introduce new keywords is a good 
thing in principle
it shouldn't go at the expense of the clarity of the language. In other 
words, if there
is a clear construct that doesn't require a new keyword (cf. import as) 
that is a good thing,
but if the clarity of Python would be compromised then I think we 
should bite the bullet
and add a new keyword.

We should think of people who come fresh to Python in three years time. 
What "if" does
and what the parameters are is immediately clear. Same for "for". Same 
for "class", "def",
assignments and what-have-you. And then suddenly they come across
     foo():
         ...
or
     exec foo():
         ...
The only construct I can think of that would give the user an idea of 
what was happening is
"with expr [= expr]". And even that will become ugly once you want to 
support both thunks
with a new scope and parameters, and thunks in the existing scope.

Last week I made a suggestion that was completely ignored, so let me 
try again (i.e. if everyone thinks
it's silly please shoot it down in stead of ignoring it:-): how about 
adding a meta-keyword
that would be used to turn an identifier into a keyword. For sake of 
the example let's use
"keyword" as the meta-keyword, although that's a bad choice, probably. 
Here's how it would be
used: if we decide to go with "with" as the thunk-keyword what happens 
is that in Python 2.4
you would have to type it as
     keyword(with) expr:
         suite
or, alternatively, do something like "from future import with_keyword". 
In 2.4, using "with" as an identifier
would give a FutureWarning. In Python 2.5 "with" would become a 
reserved word, but keyword(with) would
still continue to work. Whether the latter is for one release (with 
accompanying FutureWarning) or
for all eternity is open to discussion.
--
Jack Jansen, <Jack.Jansen@cwi.nl>, http://www.cwi.nl/~jack
If I can't dance I don't want to be part of your revolution -- Emma 
Goldman