[Baypiggies] Trivial OOP pattern problem

David Ginsburg dav9dg at gmail.com
Fri Jun 15 08:56:39 CEST 2012

The problem with this is if a method needs to return a value other than
self, you break it.
This is probably hacky, but you could accomplish what you want doing the

import inspect
import functools

def make_chainable_context(cls):

    def return_self_decorator(func):

        def wrapper(self, *args, **kwargs):
            to_return = func(self, *args, **kwargs)
            if to_return is None:
                to_return = self
            return to_return
        return wrapper

    for method_name, method in inspect.getmembers(cairo.Context,
        setattr(cls, method_name, return_self_decorator(method))
    return cls

class ChainableContext(cairo.Context):

This way methods that return something other than None are not broken.


On Thu, Jun 14, 2012 at 11:28 PM, Brian Palmer <bpalmer at gmail.com> wrote:

> On Thu, Jun 14, 2012 at 10:52 PM, Ian Zimmerman <itz at buug.org> wrote:
>> Isaac> Hi, Why do you need each of the methods to return the instance?
>> Isaac> Can you just wrap the method calls in a function and return the
>> Isaac> instance?
>> I am not married to the idea of a wrapper class, but I don't understand
>> what you propose instead.  What would the example code look like under
>> your scheme?
> Your initial solution seems like it'd work:
> class Wrap(object):
>   def __init__(self, ctx):
>     self.ctx = ctx
>   def __getattr__(self, name):
>     def myf(*args, **kwargs):
>       getattr(self.ctx, name)(*args, **kwargs)
>       return self
>     return myf
> There are benefits to being less dynamic, but this might be good enough.
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