- 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]

4 files changed, 23 insertions, 7 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