better exception when attempting reflection in batch with --sql

Migrated issue, originally created by Tim Mitchell

Given a migration from

# original table
class Customer(Base):
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(Unicode, nullable=False)

to this

# new table
class Customer(Base):
    id = Column(Integer, primary_key=True, autoincrement=True)
    first_name = Column(Unicode, nullable=False)
    last_name = Column(Unicode, nullable=False)

with upgrade step

# upgrade step in migration script
def upgrade():
    op.add_column('Customer', sa.Column('first_name', sa.Unicode(), nullable=True))
    op.add_column('Customer', sa.Column('last_name', sa.Unicode(), nullable=True))

    conn = op.get_bind()
    conn.execute('''UPDATE Customer SET
                     first_name = SUBSTR(name, 0, instr(name, ' ')),
                     last_name = SUBSTR(name, instr(name, ' ')+1)
                     ''')

    with op.batch_alter_table("Customer") as batch_op:  # sqlite requires this
        batch_op.alter_column('first_name', nullable=False)
        batch_op.alter_column('last_name', nullable=False)
        batch_op.drop_column('name')

I get this traceback

  File "migrations\env.py", line 95, in <module>
    run_migrations_offline()
  File "migrations\env.py", line 72, in run_migrations_offline
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "C:\envs\pycon\lib\site-packages\alembic\runtime\environment.py", line 788, in run_migrations
    self.get_context().run_migrations(**kw)
  File "C:\envs\pycon\lib\site-packages\alembic\runtime\migration.py", line 312, in run_migrations
    step.migration_fn(**kw)
  File "C:\Users\tim.mitchell\Source\pycon-talk\migrations\versions\39bb7539f0ef_split_name_column.py", line 32, in upgrade
    batch_op.drop_column('name')
  File "C:\Python-279-amd64\Lib\contextlib.py", line 24, in __exit__
    self.gen.next()
  File "C:\envs\pycon\lib\site-packages\alembic\operations\base.py", line 299, in batch_alter_table
    impl.flush()
  File "C:\envs\pycon\lib\site-packages\alembic\operations\batch.py", line 69, in flush
    *self.reflect_args, **self.reflect_kwargs)
  File "C:\envs\pycon\lib\site-packages\sqlalchemy\sql\schema.py", line 416, in __new__
    metadata._remove_table(name, schema)
  File "C:\envs\pycon\lib\site-packages\sqlalchemy\util\langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "C:\envs\pycon\lib\site-packages\sqlalchemy\sql\schema.py", line 411, in __new__
    table._init(name, metadata, *args, **kw)
  File "C:\envs\pycon\lib\site-packages\sqlalchemy\sql\schema.py", line 484, in _init
    self._autoload(metadata, autoload_with, include_columns)
  File "C:\envs\pycon\lib\site-packages\sqlalchemy\sql\schema.py", line 494, in _autoload
    autoload_with.run_callable(
AttributeError: 'MockConnection' object has no attribute 'run_callable'

Source files attached.

Attachments: pycon-talk.zip

Start by reproducing the traceback from migrations/env.py and the migration script, focusing on batch_alter_table and the reflection path shown in alembic/operations/batch.py. Check how SQLAlchemy reflection is invoked in --sql mode; done means this case produces an actionable exception rather than the MockConnection AttributeError.