Coerce float Python type to Float; ensure Python float coming back

Added some extra strictness to the handling of Python "float" values
passed to SQL statements.  A "float" value will be associated with the
:class:`.Float` datatype and not the Decimal-coercing :class:`.Numeric`
datatype as was the case before, eliminating a confusing warning
emitted on SQLite as well as unecessary coercion to Decimal.

Change-Id: I1bb1810ff1d198c0d929ccba5656e55401d74119
Fixes: #4017

2 files changed, 19 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/sqltypes.py b/lib/sqlalchemy/sql/sqltypes.py
index 7a3c50549..06b5e5c19 100644
--- a/lib/sqlalchemy/sql/sqltypes.py
+++ b/lib/sqlalchemy/sql/sqltypes.py
@@ -2604,7 +2604,7 @@ MATCHTYPE = MatchType()
 
 _type_map = {
     int: Integer(),
-    float: Numeric(),
+    float: Float(),
     bool: BOOLEANTYPE,
     decimal.Decimal: Numeric(),
     dt.date: Date(),
diff --git a/lib/sqlalchemy/testing/suite/test_types.py b/lib/sqlalchemy/testing/suite/test_types.py
index ee757e1ca..de32e77a4 100644
--- a/lib/sqlalchemy/testing/suite/test_types.py
+++ b/lib/sqlalchemy/testing/suite/test_types.py
@@ -431,6 +431,24 @@ class NumericTest(_LiteralRoundTripFixture, fixtures.TestBase):
             filter_=lambda n: n is not None and round(n, 5) or None
         )
 
+    @testing.fails_on("mysql", "until we do #4020")
+    def test_float_coerce_round_trip(self):
+        expr = 15.7563
+
+        val = testing.db.scalar(
+            select([literal(expr)])
+        )
+        eq_(val, expr)
+
+    # TODO: this one still breaks on MySQL
+    # def test_decimal_coerce_round_trip(self):
+    #    expr = decimal.Decimal("15.7563")
+    #
+    #    val = testing.db.scalar(
+    #        select([literal(expr)])
+    #    )
+    #    eq_(val, expr)
+
     @testing.requires.precision_numerics_general
     def test_precision_decimal(self):
         numbers = set([