mysql default clause doesnt have a dispatcher

[zzzeek]

moved from sqlalchemy/sqlalchemy#5574

# Insert code here
from sqlalchemy import Column
from sqlalchemy import Float
from sqlalchemy import Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Session
from sqlalchemy import create_engine

#url = "sqlite:////tmp/sqlite.db"  ## different error
url = "mysql://test:test@localhost/test"

# first let's create the old table

BasePrep = declarative_base()
class TestPrep(BasePrep):
    __tablename__ = "test123"
    id = Column(Integer, primary_key=True)
    label_x = Column(Float, nullable=False, server_default="0")
    label_y = Column(Float, nullable=False, server_default="0")
enginePrep = create_engine(url)
BasePrep.metadata.create_all(enginePrep)

# this is the new one

Base = declarative_base()
class Test(Base):
    __tablename__ = "test123"
    id = Column(Integer, primary_key=True)
    label_x = Column(Float, default=0)  # dies
    label_y = Column(Float, nullable=True)  # also dies

# now try to migrate this

from alembic.runtime.migration import MigrationContext
from alembic.autogenerate import api
from alembic.operations.base import Operations

engine = create_engine(url)

session = Session(engine)
ctx = MigrationContext(dialect=session.bind.dialect, connection=session.connection(), opts={})
res = api.produce_migrations(ctx,Test.__table__.metadata)
ops = Operations(ctx, ctx.impl)

# If there's an Alembic method to do this without looping, I haven't found it.
def all_ops(op):
    if hasattr(op,"ops"):
        for o in op.ops:
            yield from all_ops(o)
    else:
        yield op
for op in all_ops(res.upgrade_ops):
    print(type(op),vars(op))
    ops.invoke(op)

[sqla-tester]

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

Use regular renderer for MySQL server default https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/2221