Developing new Dialects

Note

When studying this file, it's probably a good idea to also familiarize with the README.unittests.rst file, which discusses SQLAlchemy's usage and extension of the pytest test runner.

While SQLAlchemy includes many dialects within the core distribution, the trend for new dialects should be that they are published as external projects. SQLAlchemy has since version 0.5 featured a "plugin" system which allows external dialects to be integrated into SQLAlchemy using standard setuptools entry points. As of version 0.8, this system has been enhanced, so that a dialect can also be "plugged in" at runtime.

On the testing side, SQLAlchemy includes a "dialect compliance suite" that is usable by third party libraries, in the source tree at lib/sqlalchemy/testing/suite. There's no need for a third party dialect to run through SQLAlchemy's full testing suite, as a large portion of these tests do not have dialect-sensitive functionality. The "dialect compliance suite" should be viewed as the primary target for new dialects.

Dialect Layout

The file structure of a dialect is typically similar to the following:

sqlalchemy-<dialect>/
                     setup.py
                     setup.cfg
                     sqlalchemy_<dialect>/
                                          __init__.py
                                          base.py
                                          <dbapi>.py
                                          requirements.py
                     test/
                                          __init__.py
                                          conftest.py
                                          test_suite.py
                                          test_<dialect_specific_test>.py
                                          ...

An example of this structure can be seen in the MS Access dialect at https://github.com/gordthompson/sqlalchemy-access .

Key aspects of this file layout include:

setup.py - should specify setuptools entrypoints, allowing the dialect to be usable from create_engine(), e.g.:
```
entry_points = {
 'sqlalchemy.dialects': [
      'access.pyodbc = sqlalchemy_access.pyodbc:AccessDialect_pyodbc',
      ]
}
```
Above, the entrypoint access.pyodbc allow URLs to be used such as:
```
create_engine("access+pyodbc://user:pw@dsn")
```
setup.cfg - this file contains the traditional contents such as [tool:pytest] directives, but also contains new directives that are used by SQLAlchemy's testing framework. E.g. for Access:
```
[tool:pytest]
addopts= --tb native -v -r fxX --maxfail=25 -p no:warnings
python_files=test/*test_*.py

[sqla_testing]
requirement_cls=sqlalchemy_access.requirements:Requirements
profile_file=test/profiles.txt

[db]
default=access+pyodbc://admin@access_test
sqlite=sqlite:///:memory:
```
Above, the [sqla_testing] section contains configuration used by SQLAlchemy's test plugin. The [tool:pytest] section include directives to help with these runners. When using pytest the test/conftest.py file will bootstrap SQLAlchemy's plugin.
test/conftest.py - This script bootstraps SQLAlchemy's pytest plugin into the pytest runner. This script can also be used to install your third party dialect into SQLAlchemy without using the setuptools entrypoint system; this allows your dialect to be present without any explicit setup.py step needed. The other portion invokes SQLAlchemy's pytest plugin:
```
from sqlalchemy.dialects import registry
import pytest

registry.register("access.pyodbc", "sqlalchemy_access.pyodbc", "AccessDialect_pyodbc")

pytest.register_assert_rewrite("sqlalchemy.testing.assertions")

from sqlalchemy.testing.plugin.pytestplugin import *
```
Where above, the registry module, introduced in SQLAlchemy 0.8, provides an in-Python means of installing the dialect entrypoint(s) without the use of setuptools, using the registry.register() function in a way that is similar to the entry_points directive we placed in our setup.py. (The pytest.register_assert_rewrite is there just to suppress a spurious warning from pytest.)
requirements.py - The requirements.py file is where directives regarding database and dialect capabilities are set up. SQLAlchemy's tests are often annotated with decorators that mark tests as "skip" or "fail" for particular backends. Over time, this system has been refined such that specific database and DBAPI names are mentioned less and less, in favor of @requires directives which state a particular capability. The requirement directive is linked to target dialects using a Requirements subclass. The custom Requirements subclass is specified in the requirements.py file and is made available to SQLAlchemy's test runner using the requirement_cls directive inside the [sqla_testing] section.

For a third-party dialect, the custom Requirements class can usually specify a simple yes/no answer for a particular system. For example, a requirements file that specifies a database that supports the RETURNING construct but does not support nullable boolean columns might look like this:
```
# sqlalchemy_access/requirements.py

from sqlalchemy.testing.requirements import SuiteRequirements

from sqlalchemy.testing import exclusions

class Requirements(SuiteRequirements):
    @property
    def nullable_booleans(self):
        """Target database allows boolean columns to store NULL."""
        # Access Yes/No doesn't allow null
        return exclusions.closed()

    @property
    def returning(self):
        return exclusions.open()
```
The SuiteRequirements class in sqlalchemy.testing.requirements contains a large number of requirements rules, which attempt to have reasonable defaults. The tests will report on those requirements found as they are run.

The requirements system can also be used when running SQLAlchemy's primary test suite against the external dialect. In this use case, a --dburi as well as a --requirements flag are passed to SQLAlchemy's test runner so that exclusions specific to the dialect take place:
```
cd /path/to/sqlalchemy
pytest -v \
  --requirements sqlalchemy_access.requirements:Requirements \
  --dburi access+pyodbc://admin@access_test
```
test_suite.py - Finally, the test_suite.py module represents a stub test suite, which pulls in the actual SQLAlchemy test suite. To pull in the suite as a whole, it can be imported in one step:
```
# test/test_suite.py

from sqlalchemy.testing.suite import *
```
That's all that's needed - the sqlalchemy.testing.suite package contains an ever expanding series of tests, most of which should be annotated with specific requirement decorators so that they can be fully controlled. In the case that the decorators are not covering a particular test, a test can also be directly modified or bypassed. In the example below, the Access dialect test suite overrides the get_huge_int() test:
```
from sqlalchemy.testing.suite import *

from sqlalchemy.testing.suite import IntegerTest as _IntegerTest

class IntegerTest(_IntegerTest):

    @testing.skip("access")
    def test_huge_int(self):
        # bypass this test because Access ODBC fails with
        # [ODBC Microsoft Access Driver] Optional feature not implemented.
        return
```

AsyncIO dialects

As of version 1.4 SQLAlchemy supports also dialects that use asyncio drivers to interface with the database backend.

SQLAlchemy's approach to asyncio drivers is that the connection and cursor objects of the driver (if any) are adapted into a pep-249 compliant interface, using the AdaptedConnection interface class. Refer to the internal asyncio driver implementations such as that of asyncpg, asyncmy and aiosqlite for examples.

Going Forward

The third-party dialect can be distributed like any other Python module on PyPI. Links to prominent dialects can be featured within SQLAlchemy's own documentation; contact the developers (see AUTHORS) for help with this.

While SQLAlchemy includes many dialects built in, it remains to be seen if the project as a whole might move towards "plugin" model for all dialects, including all those currently built in. Now that SQLAlchemy's dialect API is mature and the test suite is not far behind, it may be that a better maintenance experience can be delivered by having all dialects separately maintained and released.

As new versions of SQLAlchemy are released, the test suite and requirements file will receive new tests and changes. The dialect maintainer would normally keep track of these changes and make adjustments as needed.