Raymond Hettinger added the comment:

IIRC, the original motivation for ABCs was to differentiate distinct uses of 
__getitem__ (we forever struggled with differentiating sequences from mapping). 
 It seems to me that this proposal is a step backwards.  Other than a feeling 
of lightness, I don't think this proposal does anything for us.  What is point 
of knowing an object is Subscriptable without knowing how it is to be used.

The OP has a sense that Mapping and Sequence are "too heavy" but I think the 
reality that useful classes almost never use __getitem__ in isolation; rather, 
it is part of a small constellation of methods that are typically used 
together.  I would prefer that collections.abc continue to reflect that reality.

Also, I worry that collections.abc is becoming cluttered.  The existence of use 
ABCs like MutableMapping is being drowned-out by one-trick-ponies.  We're 
developing an unfavorable ratio of theoretical building blocks versus the 
practical tools.

----------
nosy: +rhettinger

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<http://bugs.python.org/issue25988>
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