object-oriented programming if coordinate' color is deleted the effect is to make that coordinate' color the background color again we use dict pop(to remove the item since it will work correctly whether or not an item with the given coordinate is in the dictionary both __setitem__(and __delitem__(have the potential to make the set of colors contain more colors than the image actually uses for exampleif unique nonbackground color is deleted at certain pixelthe color remains in the color set even though it is no longer used similarlyif pixel has unique nonbackground color and is set to the background colorthe unique color is no longer usedbut remains in the color set this means thatat worstthe color set could contain more colors than are actually used by the image (but never lesswe have chosen to accept the trade-off of potentially having more colors in the color set than are actually used for the sake of better performancethat isto make setting and deleting color as fast as possible--especially since storing few more colors isn' usually problem of courseif we wanted to ensure that the set of colors was in sync we could either create an additional method that could be called whenever we wantedor accept the overhead and do the computation automatically when it was needed in either casethe code is very simple (and is used when new image is loaded)self __colors (set(self __data values(){self __background}this simply overwrites the set of colors with the set of colors actually used in the image unioned with the background color we have not provided __len__(implementation since it does not make sense for two-dimensional object alsowe cannot provide representational form since an image cannot be created fully formed just by calling image()so we do not provide __repr__((or __str__()implementations either if user calls repr(or str(on an image objectthe object __repr__(base class implementation will return suitable stringfor example'<image image object at ac>this is standard format used for non-eval()-able objects the hexadecimal number is the object' id--this is unique (normally it is the object' address in memory)but transient we want users of the image class to be able to save and load their image dataso we have provided two methodssave(and load()to carry out these tasks we have chosen to save the data by pickling it in python-speak pickling is way of serializing (converting into sequence of bytesor into stringa python object what is so powerful about pickling is that the pickled object can be collection data typesuch as list or dictionaryand even if the pickled object has other objects inside it (including other collectionswhich