test for isnull FunctionElement with concatenation

test_mssql_concat_isnull

from sqlalchemy import select, Boolean, String
from sqlalchemy.sql.expression import FunctionElement

from sqlalchemy.ext.compiler import compiles
from sqlalchemy.sql import column
from sqlalchemy.testing import fixtures, AssertsCompiledSQL
from sqlalchemy.sql.functions import ReturnTypeFromArgs



class isnull(ReturnTypeFromArgs):

    """
    Provide MSSQL's and Oracle's `isnull` function for use within SQLAlchemy
    queries.  Registered by a compiler directive within this module.

    Usage:
    >>> from jivacore.db.custom_handlers import isnull
    >>> session.query(isnull(None, 100)).first()
    >>> (100,)
    """
    name = 'isnull'


def add_mssql_isnull_handler():
    "Enable SQLAlchemy expression compilation for MSSQL ISNULL"

    @compiles(isnull, 'mssql')
    def compile(element, compiler, **kw):
        """
        MSSQL has a built-in "ISNULL" function designed to provide a default value.

        The first arg must be a column or expression; the second arg is the default
        to be used if the first arg evaluates to a NULL value.
        """
        if len(element.clauses) > 2:
            raise TypeError("isnull only supports two arguments on MS-SQL")
        return "isnull(%s)" % compiler.process(element.clauses)


class UserDefinedTest(fixtures.TestBase, AssertsCompiledSQL):
    __dialect__ = 'mssql+mxodbc'

    def test_null_concat(self):
        add_mssql_isnull_handler()
        self.assert_compile(select([isnull(column('spam'), '') +
                            isnull(column('spamspam'), '').label("bothcols"), ]),
                            'SELECT isnull(spam, :param_1) + isnull(spamspam, :param_2) AS anon_1')

Ok, after fixes based on help from @zzzeek this test is now passing.