Re: generate methods at runtime, but the wrong one gets called

[Maric Michaud]

Le Monday 25 August 2008 11:37:23 Steven Samuel Cole, vous avez écrit*:

Hello,

I am writing an application that controls robots. Different robots can
do different kinds of movements, such as e.g. open gripper, rotate
gripper, etc. My RobotControl class should support all kinds of
robots. I therefore delegate the actual control work to extra
control-specific classes, one class per movement type, e.g.
OpenGripperControl, RotateGripperControl. These movement control
classes are created by a simple class factory.

i don't get your design, it seems over-complicated to mee at first glance.

This is my class half-way through:

...

>
I tried this in class RobotControl in __init__() in the 'for
movementType ...' loop:

funcName = 'Set' + movementType
function = lambda self, value:
self.__setMovementTypeValue(movementType, value)
method = new.instancemethod(function, self, self.__class__)
setattr(self, funcName, method)

and the code somewhat seems to work, but instead of SetOpenGripper,
SetRotateGripper is called.

The free variable movementType in the lambda is evaluated lately as it werein
its last state once you return from __init__, if you want to early bind it to
its value in each step of a for loop, you must use :

func = lambda s, v, m_t=movementType : s.__setMovementTypeValue(m_t, v)

My questions:

1.) Does this look like a somewhat reasonable approach to someone who
knows more about Python than me ?

At first, what you do is creating instancemethod and bound them to an instance
of your class, I find this confusing. Why not just use __getattr__ special
method ?

(you don't need __setMovementTypeValue in this example)

def __getattr__(self, name) :
if name.startswith('Set') :
movement = name.lstrip('Set')
if movement in self.__controls :
return lambda value : self.__controls[movement].SetValue(value)
raise AttributeError

That said, if I understand you well, your class RobotControl seems to contain
only logic that is not specific to an instance, nor to a class of instances.
In other OOP language I would encourage you to implement this logic in some
sort of singleton, but in python we don't like this construct, module level
variables and function do perfectly the job, simple as they are.

2.) What could I be doing wrong ?

I have a suspicion that my error is not even related to all this fancy
runtime code generation stuff, but something really dumb and I've just
been coding for too long to see it.

http://mail.python.org/pipermail/pyt...ne/446601.html
shows a somewhat comparable constellation and it was a good guideline.
But there, the function is created by the class factory, as well and I
unfortunately can't do that.

Thank you very much,

Steve
--
http://mail.python.org/mailman/listinfo/python-list

--
_____________

Maric Michaud

[Marc 'BlackJack' Rintsch]

On Mon, 25 Aug 2008 12:52:44 +0200, Maric Michaud wrote:

In other OOP language I would encourage you to implement this logic in
some sort of singleton, but in python we don't like this construct,
module level variables and function do perfectly the job, simple as
they are.

Modules are some sort of singletons in Python.

Ciao,
Marc 'BlackJack' Rintsch

[Bruno Desthuilliers]

Maric Michaud a écrit :
(snip)

i don't get your design, it seems over-complicated to mee at first glance.

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