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Date: Wed, 29 Apr 2015 22:31:13 -0400
From: Dave Angel <davea@davea.name>
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To: python-list@python.org
Subject: Re: Not possible to hide local variables
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On 04/29/2015 10:16 AM, Grant Edwards wrote:
> On 2015-04-28, Cecil Westerhof <Cecil@decebal.nl> wrote:
>> If I remember correctly you can not hide variables of a class or make
>> them read-only?
>>
>> I want to rewrite my moving average to python. The init is:
>>      def __init__(self, length):
>>          if type(length) != int:
>>              raise ParameterError, 'Parameter has to be an int'
>>          if n < 0:
>>              raise ValueError, 'Parameter should be greater or equal 2'
>>          self.length             = length
>>          self.old_values         = []
>>          self.current_total      = 0
>>
>> But when someone changes length, old_values, or current_total that
>> would wreck havoc with my class instance. What is the best way to
>> handle this?
>
> It's like the punchline to the old doctor joke: if it hurts when you
> do that, then don't _do_ that:
>
>    def __init__(self, length):
>        if type(length) != int:

Better:  if isinstance(length, int):

>            raise ParameterError, 'Parameter has to be an int'
>        if n < 0:

Better:   if length < 0:          (since n is undefined)

>            raise ValueError, 'Parameter should be greater or equal 2'
>        self._length             = length
>        self._old_values         = []
>        self._current_total      = 0
>
> The convention is that properties that start with underscores are
> private.  They're not hidden, but if people touch them and it breaks
> something, it's their fault.  Whether you go all Torvalds on their ass
> for doing so is left as an exercise for the reader.
>


-- 
DaveA