[Python-ideas] Possible PEP 380 tweak (original) (raw)

Guido van Rossum guido at python.org
Sat Oct 30 01:45:28 CEST 2010

On Fri, Oct 29, 2010 at 4:30 PM, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

Guido van Rossum wrote:

I went over that myself in detail and ended up deciding that for "yield-from" nothing should be changed! The expansion in the PEP remains the same. In that case, the proposal has nothing to do with PEP 380 and needn't be mentioned in it -- except perhaps to point out that using it in the presence of yield-from may not produce the expected result.

The connection is that it works well with returning values from generators, which is specified in PEP 380. So I think this does belong there.

But since this PEP also specifies "return value" it would be nice if there was a convenient way to capture this value, As long as you're willing to accept that if the generator you're closing is delegating using yield-from, the return value from the inner generator will get lost. To put it another way, if you design a generator to be used in this way (i.e. its caller using close() to finish it and get a value), you may find it awkward or impossible to later refactor it in certain ways.

Only if after the refactoring the outer generator would need the return value of the interrupted yield-from expression in order to compute its return value. I think that's reasonable. (It might be possible to tweak the yield-from expansion so that the return value is assigned before GeneratorExit is re-raised, but that sounds fragile, and doesn't always apply, e.g. if the return value is not assigned to a local variable.)

-- --Guido van Rossum (python.org/~guido)

More information about the Python-ideas mailing list