newbie-name mangling?

[bobueland]

I'm reading van Rossum's tutorial. Mostly it is well written and
examples are given. However sometimes I get lost in a text, when it
doesn't give any examples and no clues. There are several examples of
this in chapter 9 about classes. Here's one from 9.6 (Private
Variables). I quote

"There is limited support for class-private identifiers. Any identifier
of the form __spam (at least two leading underscores, at most one
trailing underscore) is textually replaced with _classname__spam, where
classname is the current class name with leading underscore(s)
stripped.
This mangling is done without regard to the syntactic position of the
identifier, so it can be used to define class-private instance and
class variables, methods, variables stored in globals, and even
variables stored in instances. private to this class on instances of
other classes. ...
Outside classes, or when the class name consists of only underscores,
no mangling occurs. Name mangling is intended to give classes an easy
way to define "private" instance variables and methods, without
having to worry about instance variables defined by derived classes, or
mucking with instance variables by code outside the class. Note that
the mangling rules are designed mostly to avoid accidents;"

Could someone provide an example of the above or direct me to a page
where it is used.

Bob

 

[Diez B. Roggisch]

Could someone provide an example of the above or direct me to a page
where it is used.

class MangledMembers:

def __init__(self):
self.__mangled_propertyname = "fooo"


mm = MangledMembers()

try:
print mm.__mangled_propertyname
except AttributeError:
print "It's private!"

print "But I can get it:", mm._MangledMembers__mangled_propertyname


Look at the code, and carefully reread the paragraph in the tutorial.

Regards,

Diez

 

[Brian van den Broek]

(e-mail address removed) said unto the world upon 2005-12-13 15:05:

I'm reading van Rossum's tutorial. Mostly it is well written and
examples are given. However sometimes I get lost in a text, when it
doesn't give any examples and no clues. There are several examples of
this in chapter 9 about classes. Here's one from 9.6 (Private
Variables). I quote

"There is limited support for class-private identifiers. Any identifier
of the form __spam (at least two leading underscores, at most one
trailing underscore) is textually replaced with _classname__spam, where
classname is the current class name with leading underscore(s)
stripped.
This mangling is done without regard to the syntactic position of the
identifier, so it can be used to define class-private instance and
class variables, methods, variables stored in globals, and even
variables stored in instances. private to this class on instances of
other classes. ...
Outside classes, or when the class name consists of only underscores,
no mangling occurs. Name mangling is intended to give classes an easy
way to define "private" instance variables and methods, without
having to worry about instance variables defined by derived classes, or
mucking with instance variables by code outside the class. Note that
the mangling rules are designed mostly to avoid accidents;"

Could someone provide an example of the above or direct me to a page
where it is used.

Bob

Hi Bob,

as a post of mine on the weekend shows, I don't have full mastery of
the issues here, either. But, on the theory that sometimes the best
explanations can come from those only a bit further on, here's an
example. (Do watch for more expert postings and take their word over
mine def __init__(self, a, b, c):
self.a = a
self._b = b
self.__c = c


Traceback (most recent call last):
File "<pyshell#94>", line 1, in -toplevel-
m.b
AttributeError: 'Mangled' object has no attribute 'b'
Traceback (most recent call last):
File "<pyshell#96>", line 1, in -toplevel-
m.c
AttributeError: 'Mangled' object has no attribute 'c'
Traceback (most recent call last):
File "<pyshell#97>", line 1, in -toplevel-
m.__c
AttributeError: 'Mangled' object has no attribute '__c'

The __ mangling is, as I understand it, a way of saying "I don't think
you, as a user of the code, should mess with this attribute unless
you really know what you are doing. Proceed with caution."

It isn't really private as Python lets you get to pretty much
anything. It is a strong signal, though.

HTH,

Brian vdB

 

[bobueland]

Thanks Diez,

Everything becomes very clear from this example
Bob

 

[bobueland]

Good example Brian

It shows clearly the special role that two underscores play in a class
definition.

Thanks Bob