pep 8 constants

[MRAB]

Francesco Bochicchio wrote:
> On Wed, 14 Jan 2009 08:13:30 +0000, Steven D'Aprano wrote:
> 
> 
>> Absolutely. It's rather sad that I can do this:
>>
>> import math
>> math.pi = 3.0
>>
>> I like the ability to shoot myself in the foot, thank you very much, but 
>> I should at least get a warning when I'm about to do so:
>>
>> math.PI = 3.0  # use God-like powers to change a constant
>>
>>
> 
> Constants would be a nice addition in python, sure enough.
> But I'm not sure that this can be done without a run-time check every time
> the constant is used, and python is already slow enough. Maybe a check
> that is disabled when running with optimizing flags ?
> 
> But I'm sure this discussion has been already made and the FINAL WORD has
> been already spoken.
> 
 >>> class Constants(object):
	def __setattr__(self, key, value):
		if key in self.__dict__:
			raise ValueError("Can't change constant")
		self.__dict__[key] = value

		
 >>> c = Constants()
 >>> c.PI = 3.0
 >>> c.PI
3.0
 >>> c.PI = 4.0

Traceback (most recent call last):
   File "<pyshell#22>", line 1, in <module>
     c.PI = 4.0
   File "<pyshell#19>", line 4, in __setattr__
     raise ValueError("Can't change constant")
ValueError: Can't change constant