Bugs item #1539847, was opened at 2006-08-14 07:47
Message generated for change (Comment added) made by gbrandl
You can respond by visiting:
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1539847&group_id=5470
Please note that this message will contain a full copy of the comment thread,
including the initial issue submission, for this request,
not just the latest update.
Category: Python Interpreter Core
Group: Python 2.4
Status: Closed
Resolution: Invalid
Priority: 5
Submitted By: W Barnes (w_barnes)
Assigned to: Nobody/Anonymous (nobody)
Summary: Identifiers begining with __ renamed
Initial Comment:
Identifiers used in a class function that start with
__ are renamed to _classname__identifer even if the
identifier is owned by some other object.
Code snippet:
size = len(Data.__entry_dates)
Here I'm trying to access the identifier
__entry_dates in the module Data from the function
DataTestCase.testEntryDates() but I get the following:
Traceback (most recent call last):
File "DataTest.py", line 247, in testEntryDates
size = len(Data.__entry_dates)
AttributeError: 'module' object has no
attribute '_DataTestCase__entry_dates'
I'm using Python 2.4.3 on Windows XP sp2
Thanks,
Walter
----------------------------------------------------------------------
>Comment By: Georg Brandl (gbrandl)
Date: 2006-08-16 06:34
Message:
Logged In: YES
user_id=849994
> The error message is a bit misleading though. If I define
> __widget in foo then access it from bar as foo.__widget
> why does it rename it as _bar__widget if it was not
> defined there? If it needs to do this internally why not
> use the unmangled name for the error message?
This is how name mangling works. All __x attribute access in
a class is mangled with *this* class's name -- it happens in
the compiling stage, where the compiler doesn't know
anything but this name (this also rules out changing the
error message). In case of other classes' attributes this is
fine since the attribute is *supposed* to be private to this
other class.
With global __x variables, the advice is "don't do that".
Still, I have no problem accessing them through the module's
__dict__, unmangled of course.
----------------------------------------------------------------------
Comment By: W Barnes (w_barnes)
Date: 2006-08-16 04:29
Message:
Logged In: YES
user_id=1541460
I just noticed some more weirdness in regards to name
mangling...
Apparently there is no way for a class to access global
attributes that are declared private. That is if a
module 'foo' has a global attribute '__widget' a
class 'bar' also defined in foo will get the following
error if it attempts to access __widget:
NameError: global name '_bar__widget' is not defined
Using either _foo__widget directly or __dict__ doesn't
work.
For now, I can get around this by using a non-private
global helper function as a middle man.
I'm a Python newbie but, for what it's worth, here are
some suggestions:
First, if an attribute is accessed as a child of some
object other than 'self' or the name of the enclosing
class no name mangling should be done. It seems that name
mangling isn't necessary here as an attribute error will
all ways be raised whether or not the object has that
attribute.
Second, when searching for a global attribute that begins
with __ it should be renamed to reflect the name of the
module not the enclosing class. In the example above, if
_bar__widget cannot be found in bar then it should look
for _foo__widget in the global namespace.
Finally, as I mentioned before, for clarity if no match is
found the error message should include the unmangled name.
----------------------------------------------------------------------
Comment By: W Barnes (w_barnes)
Date: 2006-08-16 02:10
Message:
Logged In: YES
user_id=1541460
Thanks!
I was unaware that name mangling applied to global
attributes as well as class attributes.
The error message is a bit misleading though. If I define
__widget in foo then access it from bar as foo.__widget
why does it rename it as _bar__widget if it was not
defined there? If it needs to do this internally why not
use the unmangled name for the error message?
----------------------------------------------------------------------
Comment By: Georg Brandl (gbrandl)
Date: 2006-08-14 08:07
Message:
Logged In: YES
user_id=849994
This is exactly how __ name mangling is supposed to work.
These are meant to be private, and thus shouldn't be
accessed from another class.
If the attribute was in another class, you could do the
mangling (with the correct class name!) yourself, as it's on
a module in this case, use __dict__:
size = len(Data.__dict__['__entry_dates'])
----------------------------------------------------------------------
You can respond by visiting:
https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1539847&group_id=5470
_______________________________________________
Python-bugs-list mailing list
Unsubscribe:
http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
