[Python-Dev] Naming comprehension syntax [was Re: Informal educator feedback on PEP 572 ...] (original) (raw)

Steven D'Aprano steve at pearwood.info
Tue Jul 3 20:10:27 EDT 2018

On Wed, Jul 04, 2018 at 11:13:20AM +1200, Greg Ewing wrote:

Terry Reedy wrote: >If we had followed the math precedent, instead of language>, we would have set builders, list builders, dict builders, and >generator builders.

I was intending to suggest something like that back when comprehensions were first being discussed, but people raced ahead and adopted the term "comprehension" before I got the chance. "List builder" and "dict builder" make a lot of sense, but "generator builder" not so much -- it is a generator, not something that builds a generator. In fact it doesn't build anything in the sense that the others do. So maybe "generator expression" is the best we could have done.

But [expr for x in seq] is a list, just as (expr for ...) is a generator. If you don't believe me, try it:

py> type([x for x in (1,)]) <class 'list'>

py> type(x for x in (1,)) <class 'generator'>

So I think the similarity is complete. Further, if we think of "list builder" as an abbreviation of "list builder syntax", we have:

Of course, there are other ways to build lists, such as calling the constructor, or using a list display ("list literal", except it isn't always a literal). But they're not builder syntax :-)

In hindsight, I think "spam builder (syntax)" would have been better than the rather mysterious technical word "comprehension" and the not very felicitous term "generator expression".

-- Steve

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