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Re: Guarding arithmetic

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On Thu, 23 Aug 2012 02:05:50 -0700 (PDT), Mark Carter
<alt.mcarter@gmail.com> declaimed the following in
gmane.comp.python.general:

> print safe(1/0) # prints 42
>

	1/0 is evaluated before safe() is called... and hence will raise an
exception before getting into the function.

	Python does not defer operations that are arguments of a function
call. You can only defer them by wrapping them into a function...

>>> def safe(arg):
... 	try:
... 		return arg()
... 	except:
... 		return 42
... 	
>>> print safe(lambda : 1/0)
42

	Unfortunately, that means you can't just take an arbitrary equation
and wrap it in a call to safe() as you have to make provision for local
names...

>>> def safe(fnc, *args):
... 	try:
... 		return fnc(*args)
... 	except:
... 		return 42
... 
>>> print safe(lambda a, b : a / b, 1, 0)
42
>>> print safe(lambda a, b : a / b, 1, 1)
1
>>> 
>>> a = 1
>>> b = 0
>>> print safe(lambda a, b : a / b, a, b)
42
>>> b = 2
>>> print safe(lambda a, b : a / b, a, b)
0
>>> 

	By the time you wrap the equation with a lambda all, named, terms,
AND supply the named terms after the lambda, you might as well just wrap
the equation in a plain try/except block.

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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Guarding arithmetic Mark Carter <alt.mcarter@gmail.com> - 2012-08-23 02:05 -0700
  Re: Guarding arithmetic Chris Angelico <rosuav@gmail.com> - 2012-08-23 19:16 +1000
    Re: Guarding arithmetic Mark Carter <alt.mcarter@gmail.com> - 2012-08-23 02:22 -0700
    Re: Guarding arithmetic Chris Angelico <rosuav@gmail.com> - 2012-08-23 19:29 +1000
    Re: Guarding arithmetic Mark Carter <alt.mcarter@gmail.com> - 2012-08-23 02:22 -0700
  Re: Guarding arithmetic Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-23 11:23 +0200
    Re: Guarding arithmetic Mark Carter <alt.mcarter@gmail.com> - 2012-08-23 02:47 -0700
    Re: Guarding arithmetic Mark Carter <alt.mcarter@gmail.com> - 2012-08-23 02:47 -0700
  Re: Guarding arithmetic Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-23 11:28 +0200
  Re: Guarding arithmetic Chris Angelico <rosuav@gmail.com> - 2012-08-23 19:30 +1000
  Re: Guarding arithmetic Peter Otten <__peter__@web.de> - 2012-08-23 12:11 +0200
    Re: Guarding arithmetic rusi <rustompmody@gmail.com> - 2012-08-23 10:15 -0700
  Re: Guarding arithmetic Laszlo Nagy <gandalf@shopzeus.com> - 2012-08-23 13:01 +0200
  Re: Guarding arithmetic MRAB <python@mrabarnett.plus.com> - 2012-08-23 12:21 +0100
  Re: Guarding arithmetic Peter Otten <__peter__@web.de> - 2012-08-23 13:28 +0200
  Re: Guarding arithmetic Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-08-23 15:11 +0100
  Re: Guarding arithmetic Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-08-23 14:49 -0400
  Re: Guarding arithmetic Chris Angelico <rosuav@gmail.com> - 2012-08-24 07:48 +1000

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