Recap: Interface Segregation

Shape each interface by the code that calls it, and stop splitting when no client uses a piece alone.

The interface segregation principle keeps interfaces as small as their clients need.

No class should depend on methods it does not use. That covers implementers forced to write methods and callers that depend on methods they never call.
The signs are stubs and defensive callers. A method that raises or does nothing, and a try around a call that should be safe.
Split by client, not by method. List the callers first, give each group of calls an interface, and combine pieces where one caller needs several.
Splitting changes declarations, not logic. One class can still implement every piece, as the Xerox Job did.
Small interfaces are in the standard library. Python's Sized, Iterable and Container combine into Collection.
One method per interface is usually too far. Split only where some client uses a piece on its own.

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