[Python-Dev] Anonymous blocks: Thunks or iterators?
jimjjewett at gmail.com
Fri Apr 29 20:23:05 CEST 2005
Guido van Rossum:
> -- but it's more efficient, since calling yield doesn't create a frame.
Neither should a thunk.
> The other problem with thunks is that once we think of them as the
> anonymous functions they are, we're pretty much forced to say that a
> return statement in a thunk returns from the thunk rather than from
> the containing function.
Why should a thunk be a function? We already have first class
functions. What we're missing is a way to pass around a suite.
if a > 4:
b = a
c = process(a) # thunk line 1
print a # thunk line 2
return # thunk line 3
We don't have a good way to package up "c = process(a); print a; return"
The return should exit the whole function, not just (part of) the if clause.
>> I'd like to reconsider a thunk implementation. It
>> would be a lot simpler, doing just what is required
>> without any jiggery pokery with exceptions and
>> break/continue/return statements. It would be easy
>> to explain what it does and why it's useful.
> I don't know. In order to obtain the required local variable sharing
> between the thunk and the containing function I believe that every
> local variable used or set in the thunk would have to become a 'cell'
> (our mechanism for sharing variables between nested scopes).
Cells only work if you have a complete set of names at compile-time.
Your own resource-example added "item" to the namespace inside
a block. If you don't know which blocks could be used with a pattern,
cells are out.
That said, the compiler code is already two-pass. Once to find names,
and another time to resolve them. This just means that for thunks
(and functions that call them) the adjustment will be to LOAD_NAME
instead of getting a LOAD_FAST index.
More information about the Python-Dev