[Python-Dev] Tricky way of of creating a generator via a comprehension expression

Nick Coghlan ncoghlan at gmail.com
Thu Nov 23 01:00:58 EST 2017

On 23 November 2017 at 15:33, Stephen J. Turnbull <
turnbull.stephen.fw at u.tsukuba.ac.jp> wrote:

> However, my model of comprehensions is exactly a for loop that appends
> to an empty list repeatedly, but doesn't leak iteration variables.

Not since Python 3.0. Instead, they create a nested function, the same way
generator expressions do (which is why the name resolution semantics are
now identical between the two cases).

The differences in structure between the four cases (genexp, list/set/dict
comprehensions) then relate mainly to what the innermost loop does:

    result.append(expr) # list comp
    result.add(expr) # set comp
    result[k] = v # dict comp
    yield expr # genexp

Thus, when the expression itself is a yield expression, you get:

    result.append(yield expr) # list comp
    result.add(yield expr) # set comp
    result[k] = (yield v) # dict comp
    yield (yield expr) # genexp


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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