Implement generic Double and related fixed types

Added :class:`.Double`, :class:`.DOUBLE`, :class:`.DOUBLE_PRECISION`
datatypes to the base ``sqlalchemy.`` module namespace, for explicit use of
double/double precision as well as generic "double" datatypes. Use
:class:`.Double` for generic support that will resolve to DOUBLE/DOUBLE
PRECISION/FLOAT as needed for different backends.

Implemented DDL and reflection support for ``FLOAT`` datatypes which
include an explicit "binary_precision" value. Using the Oracle-specific
:class:`_oracle.FLOAT` datatype, the new parameter
:paramref:`_oracle.FLOAT.binary_precision` may be specified which will
render Oracle's precision for floating point types directly. This value is
interpreted during reflection. Upon reflecting back a ``FLOAT`` datatype,
the datatype returned is one of :class:`_types.DOUBLE_PRECISION` for a
``FLOAT`` for a precision of 126 (this is also Oracle's default precision
for ``FLOAT``), :class:`_types.REAL` for a precision of 63, and
:class:`_oracle.FLOAT` for a custom precision, as per Oracle documentation.

As part of this change, the generic :paramref:`_sqltypes.Float.precision`
value is explicitly rejected when generating DDL for Oracle, as this
precision cannot be accurately converted to "binary precision"; instead, an
error message encourages the use of
:meth:`_sqltypes.TypeEngine.with_variant` so that Oracle's specific form of
precision may be chosen exactly. This is a backwards-incompatible change in
behavior, as the previous "precision" value was silently ignored for
Oracle.

Fixes: #5465
Closes: #7674
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7674
Pull-request-sha: 5c68419e5aee2e27bf21a8ac9eb5950d196c77e5

Change-Id: I831f4af3ee3b23fde02e8f6393c83e23dd7cd34d

1 files changed, 3 insertions, 6 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index 698ea277f..e265cd0f7 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -1449,6 +1449,7 @@ from ...types import BIGINT
 from ...types import BOOLEAN
 from ...types import CHAR
 from ...types import DATE
+from ...types import DOUBLE_PRECISION
 from ...types import FLOAT
 from ...types import INTEGER
 from ...types import NUMERIC
@@ -1575,10 +1576,6 @@ class BYTEA(sqltypes.LargeBinary):
     __visit_name__ = "BYTEA"
 
 
-class DOUBLE_PRECISION(sqltypes.Float):
-    __visit_name__ = "DOUBLE_PRECISION"
-
-
 class INET(sqltypes.TypeEngine):
     __visit_name__ = "INET"
 
@@ -2896,8 +2893,8 @@ class PGTypeCompiler(compiler.GenericTypeCompiler):
         else:
             return "FLOAT(%(precision)s)" % {"precision": type_.precision}
 
-    def visit_DOUBLE_PRECISION(self, type_, **kw):
-        return "DOUBLE PRECISION"
+    def visit_double(self, type_, **kw):
+        return self.visit_DOUBLE_PRECISION(type, **kw)
 
     def visit_BIGINT(self, type_, **kw):
         return "BIGINT"