ColumnClause pretends to be iterable and then isn't

[Former user]

Updating to SQLAlchemy 0.8, I'm getting regressions because ColumnClause.__iter__ now exists (defined in ColumnOperators in 1e29a74bd62c6c872819388e430901c35f84f07c) only to raise an exception. This sucker-punches code that checks for iterableness either by hasattr(o, '__iter__') or isinstance(o, collections.Iterable) ... such as this VALUES compiler that supports tuples/lists/iterables on PostgreSQL, representing them as ARRAYs:

@compiles(values)
def compile_values(element, compiler, asfrom=False, **kw):
    v = u"VALUES %s" % ", ".join(
        u"(%s)" % u", ".join(
            u'ARRAY[%s](%s)' % ','.join(
                (u"'%s'" % subel.replace(u"'", u"''").replace(u"%", u"%%"))
                if isinstance(subel, unicode)
                else str(subel) if isinstance(subel, int)
                else compiler.process(subel)
                for subel in elem
            ) if hasattr(elem, '__iter__')
            and elem.__iter__.__func__ is not blocking_iter_func
            else (u"'%s'" % elem.replace(u"'", u"''").replace(u"%", u"%%"))
            if isinstance(elem, unicode)
            else u'NULL' if elem is None
            else compiler.process(elem) if isinstance(elem, ColumnElement)
            else repr(elem)
            for elem in tup)
        for tup in element.list 
    ) 
    if asfrom: 
        v = "(%s)" % v 
    return v

I get what the method is there for, but it satisfies the conventional ways of checking for iterableness, while it's actually there to assert ''non-''iterableness. That's not very neat.

I'm provisionally working around this as a special case:

from sqlalchemy.sql.operators import ColumnOperators
blocking_iter_func = ColumnOperators.__iter__.__func__

    ... if hasattr(elem, '__iter__') and elem.__iter__.__func__ is not blocking_iter_func

(and the wind carries a soft murmur of kittens screaming in the distance...)

[Mike Bayer]

what are you going to do about strings in Python 3? they have __iter__() also. I'm not sure if checking for __iter__/Iterable and nothing else to detect lists/tuples/sets is really feasible due to that.

[Former user]

Hopefully nothing, for a while. :) (Our codebase is not a library, does not need to run on Python 3 until either (a) we want it to, or (b) libraries we want to use no longer support 2.7.x, neither of which seems imminent.)

We're not only detecting lists/tuples/sets, it's also generators, and our own types with __iter__ defined. So just isinstance(o, (list, tuple, set)) or isinstance(o, collections.Sequence) wouldn't hack it. In these (fairly common) cases we're looking for anything iterable that isn't a string. Presumably we'll just end up saying that verbatim, i.e. isinstance(o, collections.Iterable) and not isinstance(o, basestring), which isn't much neater, but at least explicit.

[Mike Bayer]

there's not much option here except to take that whole thing out, and wait for the stackoverflow issues when people call list() on their custom columns...

[Mike Bayer]

33d3e11cbf596c64abc3ced1d5aa01989afe0ad8 0.8

d993bdeac8674db88e 0.9

[Mike Bayer]

Removing milestone: 0.8.xx (automated comment)