Hello. Perhaps looking at some examples of what nested itercomps might look like (because they _will_ be used if they're available...) using each of the leading syntaxes would be useful in trying to decide which form, if any, is most acceptable (or least unacceptable, whichever the case may be): # (1) without parentheses: B(y) for y in A(x) for x in myIterable # (2) for clarity, we'll add some optional parentheses: B(y) for y in (A(x) for x in myIterable) # (3) OK. Now, with required parentheses: (B(y) for y in (A(x) for x in myIterable)) # (4) And, now with the required "yield:" and parentheses: (yield: B(y) for y in (yield: A(x) for x in myIterable)) #(5) And, finally, for completeness, using the rejected PEP 289 syntax: [yield B(y) for y in [yield A(x) for x in myIterable]] Hope that's useful, Sean p.s. I'm only a Python user, and not a developer, so if my comments are not welcome here, please let me know, and I will refrain in future. Thanks for your time.