- 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]

8 files changed, 86 insertions, 18 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/base.py b/lib/sqlalchemy/dialects/mysql/base.py
index 6883be5af..6ffc1319a 100644
--- a/lib/sqlalchemy/dialects/mysql/base.py
+++ b/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.
 
diff --git a/lib/sqlalchemy/dialects/oracle/cx_oracle.py b/lib/sqlalchemy/dialects/oracle/cx_oracle.py
index d59aab8f7..0c6d257dc 100644
--- a/lib/sqlalchemy/dialects/oracle/cx_oracle.py
+++ b/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:
diff --git a/lib/sqlalchemy/dialects/postgresql/pg8000.py b/lib/sqlalchemy/dialects/postgresql/pg8000.py
index 0e503746c..cd9c545f3 100644
--- a/lib/sqlalchemy/dialects/postgresql/pg8000.py
+++ b/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
diff --git a/lib/sqlalchemy/dialects/postgresql/psycopg2.py b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
index 02eda094e..9995a1f5a 100644
--- a/lib/sqlalchemy/dialects/postgresql/psycopg2.py
+++ b/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
diff --git a/lib/sqlalchemy/processors.py b/lib/sqlalchemy/processors.py
index bf95d146b..f51bdfdee 100644
--- a/lib/sqlalchemy/processors.py
+++ b/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
diff --git a/lib/sqlalchemy/sql/sqltypes.py b/lib/sqlalchemy/sql/sqltypes.py
index 8f22ae81c..7cf5a6dca 100644
--- a/lib/sqlalchemy/sql/sqltypes.py
+++ b/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
 
diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index 408c3705e..e48fa2c00 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -394,6 +394,14 @@ 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
         significant digits (such as 15.7563) accurately
diff --git a/lib/sqlalchemy/testing/suite/test_types.py b/lib/sqlalchemy/testing/suite/test_types.py
index 3eb105ba3..b147f891a 100644
--- a/lib/sqlalchemy/testing/suite/test_types.py
+++ b/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),