built in dict is returning names of attributes with class names as prefix

Question

class Enrollment(object):

    def __init__(self,enrollmentId=None, enrollmentReference=None):
        self.enrollmentId = enrollmentId
        self.enrollmentReference = enrollmentReference

    @property
    def enrollmentId(self):
        return self.__enrollmentId

    @enrollmentId.setter
    def enrollmentId(self, enrollmentId):
        self.__enrollmentId = enrollmentId

    @property
    def enrollmentReference(self):
        return self.__enrollmentReference

    @enrollmentReference.setter
    def enrollmentReference(self, enrollmentReference):
        self.__enrollmentReference = enrollmentReference

If i now try to print the attributes of the above class:

print(Enrollment().__dict__)

It prints the attributes prefixed with class name as below:

{'_Enrollment__enrollmentId': None, '_Enrollment__enrollmentReference': None}

Note: If I remove object as the super class, all works well and it prints the attributes correctly as below:

{'enrollmentId': None, 'enrollmentReference': None}

I have been been wrapping my head around this for 2 days now with no luck. Not able to understand why the class name is prefixed to attributes. I need to serialize the Enrollment object to JSON.

I don't this is a duplicate because it asks "what causes this effect?" — Raymond Hettinger, Mar 03 '18 at 08:04
I am puzzled why it does not happen with old style class and how can i get around with it in new style classes — aby, Mar 03 '18 at 08:08

Raymond Hettinger · Accepted Answer · 2018-03-03T08:06:28.807

1

In a class definition, Python transforms __x into _classname__x. This is called name mangling. Its purpose is to support class local references so that subclasses don't unintentionally break the internal logic of the parent class.

edited Mar 03 '18 at 08:06

answered Mar 03 '18 at 08:01

Raymond Hettinger

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2

Thanks for compact/ordered dicts. – Alex Mar 03 '18 at 08:12
@Raymond - Thanks for the short and precise reply. I am puzzled why it does not happen with old style class and how can i get around with it in new style classes.I need a dictionary of attributes to pass on to json.dumps serializer – aby Mar 03 '18 at 08:17
The single underscore worked for me. – aby Mar 06 '18 at 23:04

score 0 · Answer 2 · answered Mar 03 '18 at 08:02

Cause

This is due to Python's name mangling of class attribute names that start with __ and are suffixed with at most a single _. This suggests a stricter privacy for these attributes.

Note: This is still a heuristic and shouldn't be counted on for access prevention.

From the docs:

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.

class A:
    def __foo_(self): pass

print(vars(A))

outputs

{'__module__': '__main__', '_A__foo_': <function A.__foo_ at 0x1118c2488>, '__dict__': <attribute '__dict__' of 'A' objects>, '__weakref__': <attribute '__weakref__' of 'A' objects>, '__doc__': None}

Notice that __foo_ has been mangled into _A__foo_.

Uses

Why might this be useful? Well the example from the docs is pretty compelling:

Name mangling is helpful for letting subclasses override methods without breaking intraclass method calls. For example:

class Mapping:
    def __init__(self, iterable):
        self.items_list = []
        self.__update(iterable)

    def update(self, iterable):
        for item in iterable:
            self.items_list.append(item)

    __update = update   # private copy of original update() method

class MappingSubclass(Mapping):

    def update(self, keys, values):
        # provides new signature for update()
        # but does not break __init__()
        for item in zip(keys, values):
            self.items_list.append(item)

tldr

Read the docs on name mangling.

built in __dict__ is returning names of attributes with class names as prefix

2 Answers2

Cause

Uses

tldr

built in dict is returning names of attributes with class names as prefix