做数据库迁移时提示 name 'String' is not defined

在做migrate的upgrade操作时提示 name 'String' is not defined这个问题，搞不懂问题出在哪

报错如下

(venv) E:\python project\test1>python manage.py db upgrade
INFO  [alembic.runtime.migration] Context impl MySQLImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 61ea10b5db1e -> ce1f8dd4a89a, empty message
Traceback (most recent call last):
  File "manage.py", line 33, in <module>
    manager.run()
  File "E:\python project\test1\venv\lib\site-packages\flask_script\__init__.py", line 417, in run
    result = self.handle(argv[0], argv[1:])
  File "E:\python project\test1\venv\lib\site-packages\flask_script\__init__.py", line 386, in handle
    res = handle(*args, **config)
  File "E:\python project\test1\venv\lib\site-packages\flask_script\commands.py", line 216, in __call__
    return self.run(*args, **kwargs)
  File "E:\python project\test1\venv\lib\site-packages\flask_migrate\__init__.py", line 96, in wrapped
    f(*args, **kwargs)
  File "E:\python project\test1\venv\lib\site-packages\flask_migrate\__init__.py", line 271, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "E:\python project\test1\venv\lib\site-packages\alembic\command.py", line 298, in upgrade
    script.run_env()
  File "E:\python project\test1\venv\lib\site-packages\alembic\script\base.py", line 489, in run_env
    util.load_python_file(self.dir, "env.py")
  File "E:\python project\test1\venv\lib\site-packages\alembic\util\pyfiles.py", line 98, in load_python_file
    module = load_module_py(module_id, path)
  File "E:\python project\test1\venv\lib\site-packages\alembic\util\compat.py", line 184, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "migrations\env.py", line 96, in <module>
    run_migrations_online()
  File "migrations\env.py", line 90, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "E:\python project\test1\venv\lib\site-packages\alembic\runtime\environment.py", line 846, in run_migrations
    self.get_context().run_migrations(**kw)
  File "E:\python project\test1\venv\lib\site-packages\alembic\runtime\migration.py", line 520, in run_migrations
    step.migration_fn(**kw)
  File "E:\python project\test1\migrations\versions\ce1f8dd4a89a_.py", line 25, in upgrade
    sa.Column('domainabbr', sa.String(length=String(length=2)), nullable=True),
NameError: name 'String' is not defined

别光贴错误，还需要看看你的模型