subclassing Decimal

[jbauer]

I was interested in playing around with Decimal and
subclassing it. For example, if I wanted a special
class to permit floats to be automatically converted
to strings.

from decimal import Decimal

class MyDecimal(Decimal):
def __init__(self, value):
if isinstance(value, float):
... initialize using str(float) ...

In the classic days, I would have added something
like this to MyDecimal __init__:

Decimal.__init__(self, str(value))

But I'm unfamiliar with the __new__ protocol.

Jeff Bauer
Rubicon, Inc.

 

[Steven Bethard]

I was interested in playing around with Decimal and
subclassing it. For example, if I wanted a special
class to permit floats to be automatically converted
to strings.

from decimal import Decimal

class MyDecimal(Decimal):
def __init__(self, value):
if isinstance(value, float):
... initialize using str(float) ...

In the classic days, I would have added something
like this to MyDecimal __init__:

Decimal.__init__(self, str(value))

But I'm unfamiliar with the __new__ protocol.

__new__ is called to create a new instance of the class. It is a
staticmethod that gets passed as its first parameter the class to be
created. You should be able to do something like[1]:

py> import decimal
py> class MyDecimal(decimal.Decimal):
.... def __new__(cls, value):
.... if isinstance(value, float):
.... value = str(value)
.... return super(MyDecimal, cls).__new__(cls, value)
....
py> MyDecimal(3.0)
Decimal("3.0")

STeVe

[1] If you're really afraid of super for some reason, you can replace
the line:
return super(MyDecimal, cls).__new__(cls, value)
with
return decimal.Decimal.__new__(cls, value)

 

[jbauer]

STeVe,

Thanks for the response. Very clear.

yea-though-I-walk-thru-the-valley-of-__new__-I-will-fear-no-super-ly
y'rs,

-Jeff

 

[Nick Coghlan]

yea-though-I-walk-thru-the-valley-of-__new__-I-will-fear-no-super-ly

That's beautiful }:>

Cheers,
Nick.