Günther Jena Günther Jena - 5 months ago 6
Python Question

Python Class with integer emulation

Given is the following example:

class Foo(object):
def __init__(self, value=0):
self.value=value

def __int__(self):
return self.value


I want to have a class Foo, which acts as an integer (or float). So I want to do the following things:

f=Foo(3)
print int(f)+5 # is working
print f+5 # TypeError: unsupported operand type(s) for +: 'Foo' and 'int'


The first statement
print int(f)+5
is working, cause there are two integers. The second one is failing, because I have to implement
__add__
to do this operation with my class.

So to implement the integer behaviour, I have to implement all the integer emulating methods. How could I get around this. I tried to inherit from
int
, but this attempt was not successful.

Update

Inheriting from
int
fails, if you want to use a
__init__
:

class Foo(int):
def __init__(self, some_argument=None, value=0):
self.value=value
# do some stuff

def __int__(self):
return int(self.value)


If you then call:

f=Foo(some_argument=3)


you get:

TypeError: 'some_argument' is an invalid keyword argument for this function


Tested with Python 2.5 and 2.6

Answer

You need to override __new__, not __init__:

class Foo(int):
    def __new__(cls, some_argument=None, value=0):
        i = int.__new__(cls, value)
        i._some_argument = some_argument
        return i

    def print_some_argument(self):
        print self._some_argument

Now your class work as expected:

>>> f = Foo(some_argument="I am a customized int", value=10)
>>> f
10
>>> f + 8
18
>>> f * 0.25
2.5
>>> f.print_some_argument()
I am a customized int

More information about overriding new can be found in Unifying types and classes in Python 2.2.