Use importlib_metadata; add namespace for mariadb

The ``importlib_metadata`` library is used to scan for setuptools
entrypoints rather than pkg_resources.   as importlib_metadata is a small
library that is included as of Python 3.8, the compatibility library is
installed as a dependency for Python versions older than 3.8.

Unfortunately setuptools "attr:" is broken because it tries to import
the module; seems like this is fixed as part of
https://github.com/pypa/setuptools/pull/1753 however this is too recent
to rely upon for now.

Added a new dialect token "mariadb" that may be used in place of "mysql" in
the :func:`_sa.create_engine` URL.  This will deliver a MariaDB dialect
subclass of the MySQLDialect in use that forces the "is_mariadb" flag to
True.  The dialect will raise an error if a server version string that does
not indicate MariaDB in use is received.   This is useful for
MariaDB-specific testing scenarios as well as to support applications that
are hardcoding to MariaDB-only concepts.  As MariaDB and MySQL featuresets
and usage patterns continue to diverge, this pattern may become more
prominent.

Fixes: #5400
Fixes: #5496
Change-Id: I330815ebe572b6a9818377da56621397335fa702

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/__init__.py b/lib/sqlalchemy/dialects/__init__.py
index 86f567eb5..4a79608d9 100644
--- a/lib/sqlalchemy/dialects/__init__.py
+++ b/lib/sqlalchemy/dialects/__init__.py
@@ -15,6 +15,7 @@ __all__ = (
     "sybase",
 )
 
+
 from .. import util
 
 
@@ -44,6 +45,15 @@ def _auto_fn(name):
             except ImportError:
                 module = __import__("sqlalchemy.dialects.sybase").dialects
                 module = getattr(module, dialect)
+        elif dialect == "mariadb":
+            # it's "OK" for us to hardcode here since _auto_fn is already
+            # hardcoded.   if mysql / mariadb etc were third party dialects
+            # they would just publish all the entrypoints, which would actually
+            # look much nicer.
+            module = __import__(
+                "sqlalchemy.dialects.mysql.mariadb"
+            ).dialects.mysql.mariadb
+            return module.loader(driver)
         else:
             module = __import__("sqlalchemy.dialects.%s" % (dialect,)).dialects
             module = getattr(module, dialect)