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https://github.com/zzzeek/sqlalchemy

THIS IS NOT THE OFFICIAL REPO - PLEASE SUBMIT PRs ETC AT: http://github.com/sqlalchemy/sqlalchemy
https://github.com/zzzeek/sqlalchemy

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THIS IS NOT THE OFFICIAL REPO - PLEASE SUBMIT PRs ETC AT: http://github.com/sqlalchemy/sqlalchemy

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README 

        
========================

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-/

                         setup.py

                         setup.cfg

                         sqlalchemy_/

                                              __init__.py

                                              base.py

                                              .py

                                              requirements.py

                         test/

                                              __init__.py

                                              conftest.py

                                              test_suite.py

                                              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.