Not possible to hide local variables

Grant Edwards invalid at invalid.invalid
Wed Apr 29 16:16:03 CEST 2015


On 2015-04-28, Cecil Westerhof <Cecil at 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:
          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

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.

-- 
Grant Edwards               grant.b.edwards        Yow! If Robert Di Niro
                                  at               assassinates Walter Slezak,
                              gmail.com            will Jodie Foster marry
                                                   Bonzo??



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