Down casting Python objects

Steve Holden steve at
Wed Mar 10 20:06:05 CET 2010

Andrey Fedorov wrote:
> On Wed, Mar 10, 2010 at 12:24 AM, Rami Chowdhury
> <rami.chowdhury at <mailto:rami.chowdhury at>> wrote:
>     Could you tell us *why* you need to down-cast x? Explicit
>     type-casting is usually unnecessary in Python...
> Sure! It's related to the longer question I unwisely asked during PyCon
> [1] (when no one had time to read it, I suppose).
> I have a couple of different flavors of request objects which I'd like
> to make conform to a single interface. So Request `x' come in, I
> determine which kind of request I'm dealing with, and would like to
> "down-cast" it to ARequest or BRequest, classes which will provide
> appropriate accessor properties for the rest of the code to use.
> An option clearly in line with Python's docs might be for `x' to be an
> attribute of an ARequest instance, but that would complicate the code of
> ARequest. What I'm looking for is a way of adding mix-in's at runtime,
> if that makes sense.
The technique you need is not inheritance (which you are really trying
to abuse here, IMHO) but delegation.

Since all requests start out as of the same type, why not just give each
request a handler attribute and set it to a BarHandler or FooHandler
instance as appropriate. Then your generic code can call the handler's
methods, obtaining Foo- or Bar-style behavior as appropriate. If the
handler needs to manipulate the request object then the request object
can pass itself in as a method argument.

Technically your requests are delegating the handling to a subsidiary
object which implements the correct behavior for the request type.

Steve Holden           +1 571 484 6266   +1 800 494 3119
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