use of singletons in 0.9 causes non-intuitive folding of constants

[Mike Bayer]

from sqlalchemy import select, null

from sqlalchemy.sql.elements import Null

print select([null(), null()])

print select([Null(), Null()])

SELECT NULL AS anon_1
SELECT NULL AS anon_1, NULL AS anon_2

this is due to the singleton.

it's totally a regression, so this is critical that something is figured out.

[Mike Bayer]

interestingly, this is OK:

stmt = select([None, None])

I think we have to just get rid of the singleton

[Mike Bayer]

simple and all the tests pass so I think we should just go with it. i think its OK for 0.9.8.

[Mike Bayer]

gotta be really conservative with 0.9 at this point

[Mike Bayer]

• Reversing a change that was made in 0.9, the "singleton" nature of the "constants" :func:.null, :func:.true, and :func:.false has been reverted. These functions returning a "singleton" object had the effect that different instances would be treated as the same regardless of lexical use, which in particular would impact the rendering of the columns clause of a SELECT statement. fixes #3170

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