MySQL DOUBLE accepts deprecated params that dont propagate from the base DOUBLE type, document this caveat

[MacBath]

Describe the bug

Hi im trying to get startet with sql alchemy and alembic and just found a bug.

during the translation of a double precision=17, and decimal_return_scale=7 into mysql i always get an exception. sqlalchemy.exc.ArgumentError: You must specify both precision and scale or omit both altogether.

When i check the kw at dialects/mysql/types.py i see that the parameter there is still the parameter decimal_return_scale=7, but named argument scale is None.

Optional link from https://docs.sqlalchemy.org which documents the behavior that is expected

No response

SQLAlchemy Version in Use

2.0.28

DBAPI (i.e. the database driver)

mysqlclient

Database Vendor and Major Version

MariaDB 10.*

Python Version

3.11

Operating system

Windows

To Reproduce

from sqlalchemy.orm import DeclarativeBase, MappedAsDataclass, Mapped, mapped_column
from sqlalchemy.types import DOUBLE
from datetime import datetime
from sqlalchemy import create_engine


class Base(MappedAsDataclass, DeclarativeBase):
    """Base Class for ORM Table Modell"""

    pass


metadata = Base.metadata


class mytable(Base):
    __tablename__ = "tablename"

    _unix_dt: Mapped[float] = mapped_column(
        type_=DOUBLE(precision=17, decimal_return_scale=7),
        primary_key=True,
    )
    _dt: Mapped[datetime] = mapped_column(nullable=True, server_default=None)


def main():
    engine = create_engine("mysql+mysqldb://root:secret@localhost/dummdb")
    Base.metadata.create_all(engine)


if __name__ == "__main__":
    main()

Error

raceback (most recent call last):
  File "HOMEDIR\AppData\Local\Programs\Python\Python311\Lib\runpy.py", line 198, in _run_module_as_main
    return _run_code(code, main_globals, None,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "HOMEDIR\AppData\Local\Programs\Python\Python311\Lib\runpy.py", line 88, in _run_code
    exec(code, run_globals)
  File "HOMEDIR\.vscode\extensions\ms-python.python-2024.2.1\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher/../..\debugpy\__main__.py", line 39, in <module>
    cli.main()
  File "HOMEDIR\.vscode\extensions\ms-python.python-2024.2.1\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher/../..\debugpy/..\debugpy\server\cli.py", line 430, in main
    run()
  File "HOMEDIR\.vscode\extensions\ms-python.python-2024.2.1\pythonFiles\lib\python\debugpy\adapter/../..\debugpy\launcher/../..\debugpy/..\debugpy\server\cli.py", line 284, in run_file
    runpy.run_path(target, run_name="__main__")
  File "HOMEDIR\.vscode\extensions\ms-python.python-2024.2.1\pythonFiles\lib\python\debugpy\_vendored\pydevd\_pydevd_bundle\pydevd_runpy.py", line 321, in run_path
    return _run_module_code(code, init_globals, run_name,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "HOMEDIR\.vscode\extensions\ms-python.python-2024.2.1\pythonFiles\lib\python\debugpy\_vendored\pydevd\_pydevd_bundle\pydevd_runpy.py", line 135, in _run_module_code
    _run_code(code, mod_globals, init_globals,
  File "HOMEDIR\.vscode\extensions\ms-python.python-2024.2.1\pythonFiles\lib\python\debugpy\_vendored\pydevd\_pydevd_bundle\pydevd_runpy.py", line 124, in _run_code
    exec(code, run_globals)
  File "Project.git\classes\dummy.py", line 32, in <module>
    main()
  File "Project.git\classes\dummy.py", line 28, in main
    Base.metadata.create_all(engine)
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\schema.py", line 5825, in create_all
    bind._run_ddl_visitor(
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\engine\base.py", line 3254, in _run_ddl_visitor
    conn._run_ddl_visitor(visitorcallable, element, **kwargs)
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\engine\base.py", line 2460, in _run_ddl_visitor
    visitorcallable(self.dialect, self, **kwargs).traverse_single(element)
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\visitors.py", line 664, in traverse_single
    return meth(obj, **kw)
           ^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\ddl.py", line 918, in visit_metadata
    self.traverse_single(
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\visitors.py", line 664, in traverse_single
    return meth(obj, **kw)
           ^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\ddl.py", line 956, in visit_table
    )._invoke_with(self.connection)
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\ddl.py", line 314, in _invoke_with
    return bind.execute(self)
           ^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\engine\base.py", line 1421, in execute
    return meth(
           ^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\ddl.py", line 180, in _execute_on_connection
    return connection._execute_ddl(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\engine\base.py", line 1529, in _execute_ddl
    compiled = ddl.compile(
               ^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\elements.py", line 307, in compile
    return self._compiler(dialect, **kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\ddl.py", line 69, in _compiler
    return dialect.ddl_compiler(dialect, self, **kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\compiler.py", line 865, in __init__
    self.string = self.process(self.statement, **compile_kwargs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\compiler.py", line 910, in process
    return obj._compiler_dispatch(self, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\visitors.py", line 141, in _compiler_dispatch
    return meth(self, **kw)  # type: ignore  # noqa: E501
           ^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\compiler.py", line 6601, in visit_create_table
    processed = self.process(
                ^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\compiler.py", line 910, in process
    return obj._compiler_dispatch(self, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\visitors.py", line 141, in _compiler_dispatch
    return meth(self, **kw)  # type: ignore  # noqa: E501
           ^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\compiler.py", line 6632, in visit_create_column
    text = self.get_column_specification(column, first_pk=first_pk)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\dialects\mysql\base.py", line 1818, in get_column_specification        
    column.type._unwrapped_dialect_impl(self.dialect),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\type_api.py", line 882, in _unwrapped_dialect_impl
    return self.dialect_impl(dialect)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\type_api.py", line 868, in dialect_impl
    return self._dialect_info(dialect)["impl"]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\type_api.py", line 977, in _dialect_info
    impl = self._gen_dialect_impl(dialect)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\type_api.py", line 992, in _gen_dialect_impl
    return dialect.type_descriptor(self)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\engine\default.py", line 589, in type_descriptor
    return type_api.adapt_type(typeobj, self.colspecs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\type_api.py", line 2323, in adapt_type
    return typeobj.adapt(impltype)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\sql\type_api.py", line 1032, in adapt
    return util.constructor_copy(
           ^^^^^^^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\util\langhelpers.py", line 1414, in constructor_copy
    return cls(*args, **kw)
           ^^^^^^^^^^^^^^^^
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\dialects\mysql\types.py", line 185, in __init__
    super().__init__(
  File "Project.git\.venv\Lib\site-packages\sqlalchemy\dialects\mysql\types.py", line 42, in __init__
    raise exc.ArgumentError(
sqlalchemy.exc.ArgumentError: You must specify both precision and scale or omit both altogether.

Additional context

No response

[CaselIT]

Hi,

Sorry I guess this issue was missed. I think you will need to use the mysql double directly here. Also note that the setting decimal_return_scale is ignored if you don't set asdecimal=True

@zzzeek Do you think that double should do a better job here while adapting to the driver native type?

[zzzeek]

https://dev.mysql.com/doc/refman/8.3/en/floating-point-types.html

"FLOAT(M,D)and DOUBLE(M,D) are nonstandard MySQL extensions; and are deprecated. You should expect support for these variants to be removed in a future version of MySQL. "

Since these are deprecated, it's not critical that our own generic DOUBLE doesnt support these. I would say the answer is to not use these deprecated extensions. We can document this caveat

[zzzeek]

we should likely alter the exception message in mysql.DOUBLE also to reflect this