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Classes can support iteration by defining (or inheriting) __getitem__ or __iter__. In all iteration contexts, Python tries to use __iter__ (which returns an object that supports the iteration protocol with a __next__ method) first: if no __iter__ is found by inheritance search, Python falls back on the __getitem__ indexing method (which is called repeatedly, with successively higher indexes).The __str__ and __repr__ methods implement object print displays. The former is called by the print and str built-in functions; the latter is called by print and str if there is no __str__, and always by the repr built-in, interactive echoes, and nested appearances. That is, __repr__ is used everywhere, except by print and str when a __str__ is defined. A __str__ is usually used for user-friendly displays; __repr__ gives extra details or the object’s as-code form.Slicing is caught by the __getitem__ indexing method: it is called with a slice object, instead of a simple index. In Python 2.6, __getslice__ (defunct in 3.0) may be used as well.In-place addition tries __iadd__ first, and __add__ with an assignment second. The same pattern holds true for all binary operators. The __radd__ method is also available for right-side addition.When a class naturally matches, or needs to emulate, a built-in type’s interfaces. For example, collections might imitate sequence or mapping interfaces. You generally shouldn’t implement expression operators if they don’t naturally map to your objects, though—use normally named methods instead.
