Accessing class attributes from parents in instance methods

Question

I would like to read class attributes from each class all the way up the class inheritance chain.

Something like the following:

class Base(object):
    def smart_word_reader(self):
        for word in self.words:
            print(word)

class A(Base):
    words = ['foo', 'bar']

class B(A):
    words = ['baz']

if __name__ == '__main__':
    a = A()
    a.smart_word_reader()  # prints foo, bar as expected
    b = B()
    b.smart_word_reader()  # prints baz - how I can I make it print foo, bar, baz?

Obviously, each words attribute is overriding the others, as it should. How can I do something similar that will let me read the words attribute from each class in the inheritance chain?

Is there a better way I can approach this problem?

Bonus points for something that will work with multiple inheritance chains (in a diamond shape with everything inheriting from Base in the end).

score 3 · Accepted Answer · answered Sep 27 '18 at 08:51

I guess you could introspect the mro manually, something to the effect of:

In [8]: class Base(object):
   ...:     def smart_word_reader(self):
   ...:         for cls in type(self).mro():
   ...:             for word in getattr(cls, 'words', ()):
   ...:                 print(word)
   ...:
   ...: class A(Base):
   ...:     words = ['foo', 'bar']
   ...:
   ...: class B(A):
   ...:     words = ['baz']
   ...:

In [9]: a = A()

In [10]: a.smart_word_reader()
foo
bar

In [11]: b = B()

In [12]: b.smart_word_reader()
baz
foo
bar

Or perhaps in reversed order:

In [13]: class Base(object):
    ...:     def smart_word_reader(self):
    ...:         for cls in reversed(type(self).mro()):
    ...:             for word in getattr(cls, 'words', ()):
    ...:                 print(word)
    ...:
    ...: class A(Base):
    ...:     words = ['foo', 'bar']
    ...:
    ...: class B(A):
    ...:     words = ['baz']
    ...:

In [14]: a = A()

In [15]: a.smart_word_reader()
foo
bar

In [16]: b = B()

In [17]: b.smart_word_reader()
foo
bar
baz

Or a more complicated pattern:

In [21]: class Base(object):
    ...:     def smart_word_reader(self):
    ...:         for cls in reversed(type(self).mro()):
    ...:             for word in getattr(cls, 'words', ()):
    ...:                 print(word)
    ...:
    ...: class A(Base):
    ...:     words = ['foo', 'bar']
    ...:
    ...: class B(Base):
    ...:     words = ['baz']
    ...:
    ...: class C(A,B):
    ...:     words = ['fizz','pop']
    ...:

In [22]: c = C()

In [23]: c.smart_word_reader()
baz
foo
bar
fizz
pop

In [24]: C.mro()
Out[24]: [__main__.C, __main__.A, __main__.B, __main__.Base, object]

timgeb · Answer 2 · 2018-09-27T09:08:11.480

I would make smart_word_reader a classmethod and iterate over reversed(cls.__mro__).

class Base:
    @classmethod
    def smart_word_reader(cls):
        the_words = (word for X in reversed(cls.__mro__)
                          for word in vars(X).get('words', []))
        print('\n'.join(the_words))

Demo:

>>> a = A()
>>> b = B()
>>> a.smart_word_reader()
foo
bar
>>> b.smart_word_reader()
foo
bar
baz

edit

subtle bug: We don't want getattr (consider class C(B): pass) to look for a missing attribute in child classes. Changed the getattr call to vars(X).get('words', []).

Accessing class attributes from parents in instance methods

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