inheritance - Trouple accessing python superclass attributes -

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• python method name double-underscore overridden? 2 answers

i have 2 classes loosely take form below:

class foo:      def __init__(self, foo):         self.__foo = foo   class bar(foo):      def bar(self):         print self.__foo 

when try invoke bar method on instance of bar, fails.

b = bar('foobar')     b.bar() 

result:

traceback (most recent call last):   file "foobar.py", line 14, in <module>     b.bar()   file "foobar.py", line 10, in bar     print self.__foo attributeerror: bar instance has no attribute '_bar__foo' 

my understanding code should work based on two other questions, why doesn't it?

simple. __foo contains 2 underscores in beginning, it's assumed class-private method , it's transformed _classname__method.

when request access attribute named such on bar object asks bar class if has method (not foo class), self.__foo same self._bar__foo.

from documentation:

when identifier textually occurs in class definition begins 2 or more underscore characters , not end in 2 or more underscores, considered private name of class. private names transformed longer form before code generated them. transformation inserts class name, leading underscores removed , single underscore inserted, in front of name. example, identifier __spam occurring in class named ham transformed _ham__spam.

if modify code slightly

class foo:     def __init__(self, foo):         self.__foo = foo         assert hasattr(self, '_foo__foo'), 'attribute has been created'   class bar(foo):     def bar(self):         assert hasattr(self, '_foo__foo'), 'no errors, inheritance' 

assert statements not cause assertionerrors.

add __getattribute__ method bar class capture requests bar objects:

class bar(foo):      def bar(self):         print('accessing __foo bar')         print(self.__foo)      def __getattribute__(self, name):         print('requested', name)         return super().__getattribute__(name)  b = bar('foobar') b.bar() 

there 3 lines (apart attributeerror) in output:

requested bar accessing __foo bar requested _bar__foo # attributeerror follows 

as can see, if attribute requesting has 2 leading underscores, python renaming on fly.