this might be doable for 1.4 but need to evaluate:

# 1. expanding bound parameters in all cases
# 2. don't need "expanding", compiler knows it's the right side
#    of an in op
# 3. if the parameter *has* a value, compiler renders the
#    bound params inline anyway (makes more sense with str() of
#    queries)
# 4. if statement is to be cached, then never render params
# return BindParameter(None, args, expanding=True, type_=expr.type)

compiler has a flag, "expanding_in=[auto|True|False]". StrSQLCompiler sets it to false. "auto" means if the bindparam inside of in_() has a value, we render as individual params. True is when we invoke for caching.

Mike Bayer has proposed a fix for this issue in the master branch:

Use expanding IN for all literal value IN expressions https://gerrit.sqlalchemy.org/1607