[Python-Dev] RE: code blocks using 'for' loops and generators

[Jim Jewett]

It may be time to PEP (or re-PEP), if only to clarify what people are 
actually asking for.

Brian Sabbey's example from message 
http://mail.python.org/pipermail/python-dev/2005-March/052202.html
*seems* reasonably clear, but I don't see how it relates in any way to
"for" loops or generators, except as one (but not the only) use case.

    def add(thunk1, thunk2, other):
        print thunk1(1,2) + thunk2(3,4) + other

    with x,y from add(100):
        value x*y
    also a,b:           # yikes??
        value a*b   # this is thunk2

To make my own understanding explicit:

(1)  Calls for "Ruby blocks" or "thunks" are basically calls for 
placeholders in a function.  These placeholders will be filled
with code from someplace else, but will execute in the function's
own local namespace.  

(2)  A function as a parameter isn't good enough, because the 
passed-in function can't see bindings in the surrounding larger 
function.  (It still sees the lexical scope it which it was defined.)

(3)  A function as a parameter isn't good enough, because the
passed-in function can't modify bindings in the surrounding
larger function.  

(4)  A thunk could be used today be creating a string (rather than
a pre-compiled function) and substituting in the thunk's string
(rather than the code to which it would compile) before execution,
though this would be ugly.

(5)  A thunk *might* be do-able with exec or eval if a function's
locals were still a dictionary.

(6)  This has nothing whatsoever to do with for loops or generators,
except as a common use case.  In particular, thunks may be used
to lock/unlock or unwind-protect some resource.  Generators are
not the only place this is needed, but they have made the "what
do you mean, __del__ might never get called?" problem even worse,
and Ruby often solves these with a Ruby Block.

(7)  A __leave__ or __exit__ special method really turns into another
name for __del__.  It might still be useful if it came with semantics
of "I explicitly don't care what order cycles are broken; call me as
soon as my object is garbage and who cares if it gets resurrected."
There have been recipes (from Tim?) for emulating this by using 
a proxy to the resource, so that no __del__ cycles can form.

-jJ