«

The following code will print a message only once:
class PrintOnce:
printOnce = True

def __init__(self):
if PrintOnce.printOnce:
print 'Printing once.'
PrintOnce.printOnce = False

first = PrintOnce()
second = PrintOnce()

The following code will do the same thing for a nested class:
class Outer:
class Inner:
printOnce = True

def __init__(self):
if Outer.Inner.printOnce:
print 'Printing once.'
Outer.Inner.printOnce = False

def __init__(self):
first = Outer.Inner()
second = Outer.Inner()

outer = Outer()


However the following code, which has a private nested class, does not
work:
class Public:
class __Private:
printOnce = True

def __init__(self):
print 'Creating a __Private instance'
if Public.__Private.printOnce:
print 'Printing once.'
Public.__Private.printOnce = False

def __init__(self):
print 'Creating a Public instance'
first = Public.__Private()
second = Public.__Private()

public = Public()

Attempting to run the code will produce this error:
AttributeError: class Public has no attribute '_Private__Private'

What can be done so that this private nested class can have the same
functionality as the public nested class?

<> wrote:
...
> class Outer:
> class Inner:
> printOnce = True
>
> def __init__(self):
> if Outer.Inner.printOnce:
> print 'Printing once.'
> Outer.Inner.printOnce = False
>
> def __init__(self):
> first = Outer.Inner()
> second = Outer.Inner()
>
> outer = Outer()
>
>
> However the following code, which has a private nested class, does not
> work:
> class Public:
> class __Private:
> printOnce = True
>
> def __init__(self):
> print 'Creating a __Private instance'
> if Public.__Private.printOnce:

When, anywhere "immediately inside" a class named X, you use a name
__foo starting with two underscores, that name is mangled to _X__foo.
Here, you're inside class __Private, so the mangling of __Private is to
_Private__Private (I'd actually have expected more stray underscores
hither and thither, but that's the gist of it).

> print 'Printing once.'
> Public.__Private.printOnce = False
>
> def __init__(self):
> print 'Creating a Public instance'
> first = Public.__Private()
> second = Public.__Private()
>
> public = Public()
>
> Attempting to run the code will produce this error:
> AttributeError: class Public has no attribute '_Private__Private'
>
> What can be done so that this private nested class can have the same
> functionality as the public nested class?

Forget all the naming silliness and use self.__class__.printOnce
instead.


Alex

On Mar 27, 10:08 pm, a...@mac.com (Alex Martelli) wrote:
>
> Forget all the naming silliness and use self.__class__.printOnce
> instead.
>
> Alex

I tried self.__class__.printOnce and that worked. Thanks for your
help.