[Sqlalchemy-tickets] Issue #4017: float should coerce to Float (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4017: float should coerce to Float
https://bitbucket.org/zzzeek/sqlalchemy/issues/4017/float-should-coerce-to-float

Michael Bayer:

it seems to be ancient history that we slowly added support for Decimal, but never changed the coercion of plain "float" to be Float, not Numeric w/ decimals:

```
#!python


_type_map = {
    int: Integer(),
    float: Numeric(),
    bool: BOOLEANTYPE,
    decimal.Decimal: Numeric(),
    dt.date: Date(),
    dt.datetime: DateTime(),
    dt.time: Time(),
    dt.timedelta: Interval(),
    util.NoneType: NULLTYPE
}

```

So the more expensive Decimal is added into situations where plain float is used, and also on SQLite we of course get the warning as well as the wrong type back:

```
#!python


from sqlalchemy import create_engine, literal, select

e = create_engine("sqlite://")

print repr(e.scalar(select([literal(4.56)])))
```

```
#!


$ python test.py 
/home/classic/dev/sqlalchemy/lib/sqlalchemy/sql/sqltypes.py:596: SAWarning: Dialect sqlite+pysqlite does *not* support Decimal objects natively, and SQLAlchemy must convert from floating point - rounding errors and other issues may occur. Please consider storing Decimal numbers as strings or integers on this platform for lossless storage.
  'storage.' % (dialect.name, dialect.driver))
Decimal('4.5600000000')

```

ouch!

this is easy to fix, but may or may not have hard implications for current applications.  Consider for 1.2 if possible.