Constructor overloading

[Sergey Krushinsky]

Hello all,

Is there a common way to emulate constructor overloading in Python class?

For instanse, I have 3 classes:
1/ Polar - to hold polar coordinates;
2/ Cartesian - to hold cartesian coordinates;
3/ Coordinates3D, which holds synchronized instances of the both in 
__p__ and __c__ fields respectively.

I want to design Coordinates3D so that when instantiated with Polar 
argument, self.__p__=argument passed to constructor, and self.__c__ is 
calculated. When argument is Cartesian, self.__c__=argument, and 
self.__p__ is calculated. Runtime type checking works, but maybe there 
is a better way?

Thanks in advance,
Sergey