- The precision used when coercing a returned floating point value to

Python ``Decimal`` via string is now configurable.  The
flag ``decimal_return_scale`` is now supported by all :class:`.Numeric`
and :class:`.Float` types, which will ensure this many digits are taken
from the native floating point value when it is converted to string.
If not present, the type will make use of the value of ``.scale``, if
the type supports this setting and it is non-None.  Otherwise the original
default length of 10 is used. [ticket:2867]

doc/build/changelog/changelog_09.rst

@@ -14,6 +14,23 @@
 .. changelog::
     :version: 0.9.0b2
 
+    .. change::
+        :tags: feature, sql
+        :tickets: 2867
+
+        The precision used when coercing a returned floating point value to
+        Python ``Decimal`` via string is now configurable.  The
+        flag ``decimal_return_scale`` is now supported by all :class:`.Numeric`
+        and :class:`.Float` types, which will ensure this many digits are taken
+        from the native floating point value when it is converted to string.
+        If not present, the type will make use of the value of ``.scale``, if
+        the type supports this setting and it is non-None.  Otherwise the original
+        default length of 10 is used.
+
+        .. seealso::
+
+            :ref:`feature_2867`
+
     .. change::
         :tags: bug, schema
         :tickets: 2868

doc/build/changelog/migration_09.rst

@@ -877,6 +877,50 @@ rendering::
 
 :ticket:`722`
 
+.. _feature_2867:
+
+Floating Point String-Conversion Precision Configurable for Native Floating Point Types
+---------------------------------------------------------------------------------------
+
+The conversion which SQLAlchemy does whenever a DBAPI returns a Python
+floating point type which is to be converted into a Python ``Decimal()``
+necessarily involves an intermediary step which converts the floating point
+value to a string.  The scale used for this string conversion was previously
+hardcoded to 10, and is now configurable.  The setting is available on
+both the :class:`.Numeric` as well as the :class:`.Float`
+type, as well as all SQL- and dialect-specific descendant types, using the
+parameter ``decimal_return_scale``.    If the type supports a ``.scale`` parameter,
+as is the case with :class:`.Numeric` and some float types such as
+:class:`.mysql.DOUBLE`, the value of ``.scale`` is used as the default
+for ``.decimal_return_scale`` if it is not otherwise specified.   If both
+``.scale`` and ``.decimal_return_scale`` are absent, then the default of
+10 takes place.  E.g.::
+
+    from sqlalchemy.dialects.mysql import DOUBLE
+    import decimal
+
+    data = Table('data', metadata,
+        Column('double_value',
+                    mysql.DOUBLE(decimal_return_scale=12, asdecimal=True))
+    )
+
+    conn.execute(
+        data.insert(),
+        double_value=45.768392065789,
+    )
+    result = conn.scalar(select([data.c.double_value]))
+
+    # previously, this would typically be Decimal("45.7683920658"),
+    # e.g. trimmed to 10 decimal places
+
+    # now we get 12, as requested, as MySQL can support this
+    # much precision for DOUBLE
+    assert result == decimal.Decimal("45.768392065789")
+
+
+:ticket:`2867`
+
+
 .. _change_2824:
 
 Column Bundles for ORM queries

lib/sqlalchemy/dialects/mysql/base.py

@@ -398,7 +398,8 @@ class _FloatType(_NumericType, sqltypes.Float):
             raise exc.ArgumentError(
                 "You must specify both precision and scale or omit "
                 "both altogether.")
-
+        if scale is not None:
+            kw.setdefault('decimal_return_scale', scale)
         super(_FloatType, self).__init__(precision=precision, asdecimal=asdecimal, **kw)
         self.scale = scale
 
@@ -490,6 +491,14 @@ class DOUBLE(_FloatType):
     def __init__(self, precision=None, scale=None, asdecimal=True, **kw):
         """Construct a DOUBLE.
 
+        .. note::
+
+            The :class:`.DOUBLE` type by default converts from float
+            to Decimal, using a truncation that defaults to 10 digits.  Specify
+            either ``scale=n`` or ``decimal_return_scale=n`` in order to change
+            this scale, or ``asdecimal=False`` to return values directly as
+            Python floating points.
+
         :param precision: Total digits in this number.  If scale and precision
           are both None, values are stored to limits allowed by the server.
 
@@ -515,6 +524,14 @@ class REAL(_FloatType, sqltypes.REAL):
     def __init__(self, precision=None, scale=None, asdecimal=True, **kw):
         """Construct a REAL.
 
+        .. note::
+
+            The :class:`.REAL` type by default converts from float
+            to Decimal, using a truncation that defaults to 10 digits.  Specify
+            either ``scale=n`` or ``decimal_return_scale=n`` in order to change
+            this scale, or ``asdecimal=False`` to return values directly as
+            Python floating points.
+
         :param precision: Total digits in this number.  If scale and precision
           are both None, values are stored to limits allowed by the server.
 

lib/sqlalchemy/dialects/oracle/cx_oracle.py

@@ -232,10 +232,7 @@ class _OracleNumeric(sqltypes.Numeric):
 
         if dialect.supports_native_decimal:
             if self.asdecimal:
-                if self.scale is None:
-                    fstring = "%.10f"
-                else:
-                    fstring = "%%.%df" % self.scale
+                fstring = "%%.%df" % self.decimal_return_scale
 
                 def to_decimal(value):
                     if value is None:

lib/sqlalchemy/dialects/postgresql/pg8000.py

@@ -39,7 +39,8 @@ class _PGNumeric(sqltypes.Numeric):
     def result_processor(self, dialect, coltype):
         if self.asdecimal:
             if coltype in _FLOAT_TYPES:
-                return processors.to_decimal_processor_factory(decimal.Decimal)
+                return processors.to_decimal_processor_factory(
+                                    decimal.Decimal, self.decimal_return_scale)
             elif coltype in _DECIMAL_TYPES or coltype in _INT_TYPES:
                 # pg8000 returns Decimal natively for 1700
                 return None

lib/sqlalchemy/dialects/postgresql/psycopg2.py

@@ -191,7 +191,8 @@ class _PGNumeric(sqltypes.Numeric):
     def result_processor(self, dialect, coltype):
         if self.asdecimal:
             if coltype in _FLOAT_TYPES:
-                return processors.to_decimal_processor_factory(decimal.Decimal)
+                return processors.to_decimal_processor_factory(
+                                decimal.Decimal, self.decimal_return_scale)
             elif coltype in _DECIMAL_TYPES or coltype in _INT_TYPES:
                 # pg8000 returns Decimal natively for 1700
                 return None

lib/sqlalchemy/processors.py

@@ -66,7 +66,7 @@ def py_fallback():
                 return decoder(value, errors)[0]
         return process
 
-    def to_decimal_processor_factory(target_class, scale=10):
+    def to_decimal_processor_factory(target_class, scale):
         fstring = "%%.%df" % scale
 
         def process(value):
@@ -119,7 +119,7 @@ try:
         else:
             return UnicodeResultProcessor(encoding).process
 
-    def to_decimal_processor_factory(target_class, scale=10):
+    def to_decimal_processor_factory(target_class, scale):
         # Note that the scale argument is not taken into account for integer
         # values in the C implementation while it is in the Python one.
         # For example, the Python implementation might return

lib/sqlalchemy/sql/sqltypes.py

@@ -409,6 +409,7 @@ class BigInteger(Integer):
     __visit_name__ = 'big_integer'
 
 
+
 class Numeric(_DateAffinity, TypeEngine):
     """A type for fixed precision numbers.
 
@@ -453,7 +454,10 @@ class Numeric(_DateAffinity, TypeEngine):
 
     __visit_name__ = 'numeric'
 
-    def __init__(self, precision=None, scale=None, asdecimal=True):
+    _default_decimal_return_scale = 10
+
+    def __init__(self, precision=None, scale=None,
+                  decimal_return_scale=None, asdecimal=True):
         """
         Construct a Numeric.
 
@@ -468,6 +472,18 @@ class Numeric(_DateAffinity, TypeEngine):
           datatypes - the Numeric type will ensure that return values
           are one or the other across DBAPIs consistently.
 
+        :param decimal_return_scale: Default scale to use when converting
+         from floats to Python decimals.  Floating point values will typically
+         be much longer due to decimal inaccuracy, and most floating point
+         database types don't have a notion of "scale", so by default the
+         float type looks for the first ten decimal places when converting.
+         Specfiying this value will override that length.  Types which
+         do include an explicit ".scale" value, such as the base :class:`.Numeric`
+         as well as the MySQL float types, will use the value of ".scale"
+         as the default for decimal_return_scale, if not otherwise specified.
+
+         .. versionadded:: 0.9.0
+
         When using the ``Numeric`` type, care should be taken to ensure
         that the asdecimal setting is apppropriate for the DBAPI in use -
         when Numeric applies a conversion from Decimal->float or float->
@@ -487,6 +503,10 @@ class Numeric(_DateAffinity, TypeEngine):
         """
         self.precision = precision
         self.scale = scale
+        self.decimal_return_scale = decimal_return_scale \
+                                    if decimal_return_scale is not None \
+                                    else self.scale if self.scale is not None \
+                                    else self._default_decimal_return_scale
         self.asdecimal = asdecimal
 
     def get_dbapi_type(self, dbapi):
@@ -525,12 +545,10 @@ class Numeric(_DateAffinity, TypeEngine):
                           'storage.' % (dialect.name, dialect.driver))
 
                 # we're a "numeric", DBAPI returns floats, convert.
-                if self.scale is not None:
-                    return processors.to_decimal_processor_factory(
-                                decimal.Decimal, self.scale)
-                else:
-                    return processors.to_decimal_processor_factory(
-                                decimal.Decimal)
+                return processors.to_decimal_processor_factory(
+                            decimal.Decimal,
+                            self.scale if self.scale is not None
+                            else self._default_decimal_return_scale)
         else:
             if dialect.supports_native_decimal:
                 return processors.to_float
@@ -576,7 +594,8 @@ class Float(Numeric):
 
     scale = None
 
-    def __init__(self, precision=None, asdecimal=False, **kwargs):
+    def __init__(self, precision=None, asdecimal=False,
+                        decimal_return_scale=None, **kwargs):
         """
         Construct a Float.
 
@@ -587,6 +606,17 @@ class Float(Numeric):
           defaults to ``False``.   Note that setting this flag to ``True``
           results in floating point conversion.
 
+        :param decimal_return_scale: Default scale to use when converting
+         from floats to Python decimals.  Floating point values will typically
+         be much longer due to decimal inaccuracy, and most floating point
+         database types don't have a notion of "scale", so by default the
+         float type looks for the first ten decimal places when converting.
+         Specfiying this value will override that length.  Note that the
+         MySQL float types, which do include "scale", will use "scale"
+         as the default for decimal_return_scale, if not otherwise specified.
+
+         .. versionadded:: 0.9.0
+
         :param \**kwargs: deprecated.  Additional arguments here are ignored
          by the default :class:`.Float` type.  For database specific
          floats that support additional arguments, see that dialect's
@@ -596,13 +626,17 @@ class Float(Numeric):
         """
         self.precision = precision
         self.asdecimal = asdecimal
+        self.decimal_return_scale = decimal_return_scale \
+                                    if decimal_return_scale is not None \
+                                    else self._default_decimal_return_scale
         if kwargs:
             util.warn_deprecated("Additional keyword arguments "
                                 "passed to Float ignored.")
 
     def result_processor(self, dialect, coltype):
         if self.asdecimal:
-            return processors.to_decimal_processor_factory(decimal.Decimal)
+            return processors.to_decimal_processor_factory(
+                                    decimal.Decimal, self.decimal_return_scale)
         else:
             return None
 

lib/sqlalchemy/testing/requirements.py

@@ -393,6 +393,14 @@ class SuiteRequirements(Requirements):
 
         return exclusions.closed()
 
+    @property
+    def precision_generic_float_type(self):
+        """target backend will return native floating point numbers with at
+        least seven decimal places when using the generic Float type.
+
+        """
+        return exclusions.open()
+
     @property
     def floats_to_four_decimals(self):
         """target backend can return a floating-point number with four

lib/sqlalchemy/testing/suite/test_types.py

@@ -356,6 +356,16 @@ class NumericTest(_LiteralRoundTripFixture, fixtures.TestBase):
             filter_=lambda n: n is not None and round(n, 5) or None
         )
 
+
+    @testing.requires.precision_generic_float_type
+    def test_float_custom_scale(self):
+        self._do_test(
+            Float(None, decimal_return_scale=7, asdecimal=True),
+            [15.7563827, decimal.Decimal("15.7563827")],
+            [decimal.Decimal("15.7563827"),],
+            check_scale=True
+        )
+
     def test_numeric_as_decimal(self):
         self._do_test(
             Numeric(precision=8, scale=4),

test/dialect/mysql/test_types.py

@@ -10,7 +10,7 @@ from sqlalchemy.testing import fixtures, AssertsCompiledSQL, AssertsExecutionRes
 from sqlalchemy import testing
 from sqlalchemy.testing.engines import utf8_engine
 import datetime
-
+import decimal
 
 class TypesTest(fixtures.TestBase, AssertsExecutionResults, AssertsCompiledSQL):
     "Test MySQL column types"
@@ -147,6 +147,23 @@ class TypesTest(fixtures.TestBase, AssertsExecutionResults, AssertsCompiledSQL):
                 res
             )
 
+    @testing.provide_metadata
+    def test_precision_float_roundtrip(self):
+        t = Table('t', self.metadata,
+                    Column('scale_value', mysql.DOUBLE(precision=15, scale=12, asdecimal=True)),
+                    Column('unscale_value', mysql.DOUBLE(decimal_return_scale=12, asdecimal=True))
+            )
+        t.create(testing.db)
+        testing.db.execute(
+            t.insert(), scale_value=45.768392065789,
+            unscale_value=45.768392065789
+        )
+        result = testing.db.scalar(select([t.c.scale_value]))
+        eq_(result, decimal.Decimal("45.768392065789"))
+
+        result = testing.db.scalar(select([t.c.unscale_value]))
+        eq_(result, decimal.Decimal("45.768392065789"))
+
     @testing.exclude('mysql', '<', (4, 1, 1), 'no charset support')
     def test_charset(self):
         """Exercise CHARACTER SET and COLLATE-ish options on string types."""

test/requirements.py

@@ -540,6 +540,13 @@ class DefaultRequirements(SuiteRequirements):
                 ]
                 )
 
+    @property
+    def precision_generic_float_type(self):
+        """target backend will return native floating point numbers with at
+        least seven decimal places when using the generic Float type."""
+
+        return fails_if('mysql', 'mysql FLOAT type only returns 4 decimals')
+
     @property
     def floats_to_four_decimals(self):
         return fails_if("mysql+oursql", "Floating point error")