Question about properties

[Frank Millman]

Hi all

I have started experimenting with properties.

The example in the 2.5 docs uses an inconsistent mixture of single and
double underscores for the internal representation of the attribute. I
was going to file a documentation bug, but then I checked the 2.6 docs
online, and I see it has been fixed, by using single underscores
throughout.

IMHO, it would make more sense to use double underscores throughout. I
thought that the main point of using property was to prevent direct
access to the attribute. With a single underscore you can access it if
you prefix the attribute name with a single underscore, thus bypassing
the logic in 'property'.

Is this a valid comment, or does it come under the category of 'we are
all adults here'?

While experimenting, I came across the following curiosity.

I know that prefixing a class attribute with a double-underscore makes
it difficult to access the attribute externally. Here is a simple
example -

>>>class Test(object):

.... def __init__(self,x ):
.... self.x = x
.... self.__y = 123
.... def get_y(self):
.... return self.__y

>>>t = Test(99)
t.x

99

>>>t.get_y()

123

>>>t.__y

AttributeError: 'Test' object has no attribute '__y'

I was surprised that I could do the following -

>>>t.__y = 456
t.__y

456

>>>t.get_y()

123

It's not important, but I am curious to know what is going on
internally here.

Frank Millman

[Fredrik Lundh]

Frank Millman wrote:

I thought that the main point of using property was to prevent direct
access to the attribute.

Not "prevent access to" as much as "add behaviour to".

Is this a valid comment, or does it come under the category of 'we are
all adults here'?

The latter. And the "__" doesn't provide much protection, really (as
we'll see below).

While experimenting, I came across the following curiosity.

I know that prefixing a class attribute with a double-underscore makes
it difficult to access the attribute externally. Here is a simple
example -

>>>class Test(object):

... def __init__(self,x ):
... self.x = x
... self.__y = 123
... def get_y(self):
... return self.__y

>>>t = Test(99)
t.x

99

>>>t.get_y()

123

>>>t.__y

AttributeError: 'Test' object has no attribute '__y'

I was surprised that I could do the following -

>>>t.__y = 456
t.__y

456

>>>t.get_y()

123

It's not important, but I am curious to know what is going on
internally here.

hint:

>>dir(t)

['_Test__y', ..., '__y', 'get_y', 'x']

>>t._Test__y

123

>>t.__y

456

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