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Date: Wed, 05 Dec 2012 14:21:21 -0500
From: Dave Angel <d@davea.name>
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To: Olivier Scalbert <olivier.scalbert@algosyn.com>
Subject: Re: Secretly passing parameter to function
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On 12/05/2012 01:50 PM, Olivier Scalbert wrote:
> Hi all !
>
> I have a problem that is not easy to explained, so I have tried to
> reduce it a lot.
>
> We are using a framework, that we can not modify.
>
> in framework.py:
> def do(something):
>    '''
>    Here we are in a framework that can not be modified ...
>    It does a lot of things
>
>    and finally:
>    '''
>    something()
>
> in test.py:
> from framework import *
>
> def step1():
>    print "Do step1"
>
> def step2():
>    print "Do step2"
>
>
> # We ask the framework to do some work.
> do(step1)
> do(step2)
> do(step3)
>
>
> We are writing step1, step2, ... and asking the framework to process
> them.
> Everything is ok, until we want to add a parameter to some steps.
> We want to be able to do that:
>
> in test.py:
> from framework import *
>
> def step1(param):
>    print "Do step1 with param"
>
> def step2():
>    print "Do step2"
>
>
> # We ask the framework to do some work.
>
> do(step1, param = None)
> do(step1, param = [0, 1, 5]) # again
> do(step2)
>
> Of course it does not work ...
> TypeError: do() takes exactly 1 argument (2 given)
>
> And we can not modify the framework (in which "do" is defined.
>
> One solution would be to use a global variable that can be set before
> each step. But it is not very elegant ...
>
> One other approach would be to add dynamically an attribute the the
> step1 function, and retrieve it inside the function, but it is perhaps
> overkill.
>
> Do you have some ideas ?
>
Other approaches are  lamba, default-argument trick, a function closure,
a callable class instance, and functools.partial.

The real question you have to ask is what is the scope AND LIFETIME of
this parameter.  Suppose you want to want to have five of these same
calls, with five different parameters?  (Example, a GUI where you have a
single function which might be called on an event of any of five buttons
-- you want to pass the button-object to the function)


import functools

def step1(param):
    print "Do step1 with param", param

def step2():
    print "Do step2"

class Framework:
    def __init__(self):
        self.pending = []
    def do(self, func):
        print "current", self.pending
        self.pending.append(func)
    def flush(self):
        for func in self.pending:
            func()


frame = Framework()
frame.do(step2)
frame.do(step2)
frame.do(step2)
frame.do(functools.partial(step1, 45))

frame.flush()



-- 

DaveA