Adding method to class at run-time: bad style?

Grant Edwards invalid at invalid
Wed Apr 8 00:44:23 CEST 2009

On 2009-04-07, Scott David Daniels <Scott.Daniels at Acm.Org> wrote:

>>      File "/usr/lib/python2.5/site-packages/", line 2016, in add_to_form
>>        Control.add_to_form(self, form)
>>    TypeError: unbound method add_to_form() must be called with FancyControl instance as first argument (got CheckboxControl instance instead)
> OK, that is one of the problems with monkey-patching.  What happened
> is ClientForm apparently defines subclasses of Control such as
> CheckboxControl.

Right.  Control is just a base class, and changing it to a
"different" class after sub-classes have been defined pulls the
rug out from under things.  Had I thought about it for a
minute, I should have known that would happen.  The working
patch tweaks the class in-place, so it works OK.

> The monkey-patching only happens after the ClientForm module
> has been executed (first import), and the monkey-patching
> happens after all of that.  So now the "tack it into the
> class" method looks a bit better if you cannot simply add your
> requirement to the ClientForm source.

That's obviously the "right thing", but it makes portability
more of an issue (I would have to archive/distribute ClientForm
source and users would have to install the customized version
of ClientForm).

Of course there's always the chance that my version of
monkey-patching will stop working with a different version of
ClientForm.  We'll burn that bridge when we come to it.

Grant Edwards                   grante             Yow! BARBARA STANWYCK makes
                                  at               me nervous!!

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