Assigning to self.__class__

2006-01-26 Thread Paul McGuire 
I have some places in pyparsing where I've found that the most
straightforward way to adjust an instance's behavior is to change its class.
I do this by assigning to self.__class__, and things all work fine.

(Converting to use of __new__ is not an option - in one case, the change is
temporary, and before the end of the function, I change it back again.)

Any comments on this practice?  Is this intended capability for Python
objects, or am I taking advantage of a fortuitous accident, which may get
undone at a future time?

-- Paul


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Re: Assigning to self.__class__

2006-01-26 Thread bruno at modulix 
Paul McGuire wrote:
 I have some places in pyparsing where I've found that the most
 straightforward way to adjust an instance's behavior is to change its class.

Hooray ! You've just (re)discovered the state pattern... for which the
most stupid simple implementation in Python is to :

(snip) assigning to self.__class__, (snip)

!-)

 
 Any comments on this practice?  

It can be very confusing for newbies and peoples having no experience
with *dynamic* languages, and I guess control-freaks and
static-typing-addicts would runaway screaming.  But I like it anyway !-)

 Is this intended capability for Python
 objects, 

AFAIK, yes.

 or am I taking advantage of a fortuitous accident, which may get
 undone at a future time?

It's certainly not  a fortuitous accident.


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Re: Assigning to self.__class__

2006-01-26 Thread Heiko Wundram 
bruno at modulix wrote:
 Paul McGuire wrote:
 or am I taking advantage of a fortuitous accident, which may get
 undone at a future time?
 
 It's certainly not  a fortuitous accident.

And even the (printed) cookbook has examples which assign to
self.__class__... I guess this means this feature isn't going to go away
soo. ;-)

--- Heiko.
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Re: Assigning to self.__class__

2006-01-26 Thread Alex Martelli 
Heiko Wundram [EMAIL PROTECTED] wrote:

 bruno at modulix wrote:
  Paul McGuire wrote:
  or am I taking advantage of a fortuitous accident, which may get
  undone at a future time?
  
  It's certainly not  a fortuitous accident.
 
 And even the (printed) cookbook has examples which assign to
 self.__class__... I guess this means this feature isn't going to go away

Hmmm, I'm the main decision-maker for what goes in the printed cookbook,
but it's Guido who decides what stays or goes in the language, and it's
not as if we always agree (though I will admit that when we disagree it
later usually turns out he's right, but that's another issue;-).  Still,
in this case I don't think we disagree!-)


Alex
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Re: Assigning to self.__class__

2006-01-26 Thread Terry Reedy 

Paul McGuire [EMAIL PROTECTED] wrote in message 
news:[EMAIL PROTECTED]
I have some places in pyparsing where I've found that the most
 straightforward way to adjust an instance's behavior is to change its 
 class.
 I do this by assigning to self.__class__, and things all work fine.

 (Converting to use of __new__ is not an option - in one case, the change 
 is
 temporary, and before the end of the function, I change it back again.)

 Any comments on this practice?  Is this intended capability for Python
 objects, or am I taking advantage of a fortuitous accident, which may get
 undone at a future time?

Very little if any of the new class stuff is accidental.  As I remember, 
.__class__ was originally read-only and was intentionally changed when 
practical use cases for rewriting it were developed and presented with a 
request for change.  Though I could be wrong, I would comfortably use this 
feature myself.

Terry J. Reedy



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