Not possible to hide local variables

Marko Rauhamaa marko at pacujo.net
Tue Apr 28 11:20:18 CEST 2015


Steven D'Aprano <steve+comp.lang.python at pearwood.info>:

> The convention is, if the caller messes with your private attributes
> or variables, and their code breaks, they have nobody to blame but
> themselves, and we are allowed to laugh at them. We're consenting
> adults here.

I would take it further: as a rule, user code should not think of
modifying the contents of an object unless it is clearly documented as a
supported operation. IOW, you should consider all attributes private, or
at least read-only.

BTW, the principle is actually enforced for some objects:

   >>> "hello".join = lambda *x: "world"
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   AttributeError: 'str' object attribute 'join' is read-only
   >>> "hello".joinn = lambda *x: "world"
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   AttributeError: 'str' object has no attribute 'joinn'
   >>> "hello".__setattr__ = lambda *x: None
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   AttributeError: 'str' object attribute '__setattr__' is read-only


A tougher issue is with encapsulation. A derived class might
accidentally step on the toes of the base class by redefining an
attribute:

========================================================================
class Base:
    def __init__(self):
        self._secret = 17

    def divulge_secret(self):
        return self._secret

class Derived(Base):
    def __init__(self):
        self._secret = 1

print(Derived().divulge_secret())
========================================================================

which outputs 1.


Marko



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