Alambic: modifier le type de colonne à l'AIDE de

Je suis d'essayer d'utiliser un alambic pour convertir un SQLAlchemy PostgreSQL ARRAY(Texte) pour un PEU(variable=True) terrain pour une de mes colonnes de la table.

La colonne est actuellement définie comme:

cols = Column(ARRAY(TEXT), nullable=False, index=True)

Je veux le changer pour:

cols = Column(BIT(varying=True), nullable=False, index=True)

Changement des types de colonnes ne semble pas être pris en charge par défaut, donc je suis en train de modifier l'alambic script à la main. C'est ce que j'ai actuellement:

def upgrade():
    op.alter_column(
        table_name='views',
        column_name='cols',
        nullable=False,
        type_=postgresql.BIT(varying=True)
    )


def downgrade():
    op.alter_column(
        table_name='views',
        column_name='cols',
        nullable=False,
        type_=postgresql.ARRAY(sa.Text())
    )

Toutefois, l'exécution de ce script donne l'erreur:

Traceback (most recent call last):
File "/home/home/.virtualenvs/deus_lex/bin/alembic", line 9, in <module>
load_entry_point('alembic==0.7.4', 'console_scripts', 'alembic')()
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/config.py", line 399, in main
CommandLine(prog=prog).main(argv=argv)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/config.py", line 393, in main
self.run_cmd(cfg, options)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/config.py", line 376, in run_cmd
**dict((k, getattr(options, k)) for k in kwarg)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/command.py", line 165, in upgrade
script.run_env()
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/script.py", line 382, in run_env
util.load_python_file(self.dir, 'env.py')
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/util.py", line 242, in load_python_file
module = load_module_py(module_id, path)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/compat.py", line 79, in load_module_py
mod = imp.load_source(module_id, path, fp)
File "./scripts/env.py", line 83, in <module>
run_migrations_online()
File "./scripts/env.py", line 76, in run_migrations_online
context.run_migrations()
File "<string>", line 7, in run_migrations
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/environment.py", line 742, in run_migrations
self.get_context().run_migrations(**kw)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/migration.py", line 305, in run_migrations
step.migration_fn(**kw)
File "/home/home/deus_lex/winslow/scripts/versions/2644864bf479_store_caselist_column_views_as_bits.py", line 24, in upgrade
type_=postgresql.BIT(varying=True)
File "<string>", line 7, in alter_column
File "<string>", line 1, in <lambda>
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/util.py", line 387, in go
return fn(*arg, **kw)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/operations.py", line 470, in alter_column
existing_autoincrement=existing_autoincrement
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 147, in alter_column
existing_nullable=existing_nullable,
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 105, in _exec
return conn.execute(construct, *multiparams, **params)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 729, in execute
return meth(self, multiparams, params)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 69, in _execute_on_connection
return connection._execute_ddl(self, multiparams, params)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 783, in _execute_ddl
compiled
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 958, in _execute_context
context)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1159, in _handle_dbapi_exception
exc_info
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
reraise(type(exception), exception, tb=exc_tb)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 951, in _execute_context
context)
File "/home/home/.virtualenvs/deus_lex/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 436, in do_execute
cursor.execute(statement, parameters)
sqlalchemy.exc.ProgrammingError: (ProgrammingError) column "cols" cannot be cast automatically to type bit varying
HINT:  Specify a USING expression to perform the conversion.
'ALTER TABLE views ALTER COLUMN cols TYPE BIT VARYING' {}

Comment puis-je modifier mon script avec l'AIDE de l'expression?