Object-Oriented Programming in Java

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6 Object-Oriented Programming
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def pop(self, key, *args): if key not in self: if len(args) == 0: raise KeyError(key) return args[0] self__keysremove(key) return super()pop(key, args)
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If the given key is in the dictionary this method returns the corresponding value and removes the key value item from the dictionary The key must also be removed from the keys list The implementation is quite subtle because the pop() method must support two different behaviors to match dictpop() The rst is dpop(k); here the value for key k is returned, or if there is no key k, a KeyError is raised The second is dpop(k, value); here the value for key k is returned, or if there is no key k, value (which could be None) is returned In all cases, if key k exists, the corresponding item is removed
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def popitem(self): item = super()popitem() self__keysremove(item[0]) return item
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The dictpopitem() method removes and returns a random key value item from the dictionary We must call the base class version rst since we don t know in advance which item will be removed We remove the item s key from the keys list, and then return the item
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def clear(self): super()clear() self__keysclear()
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Here we clear all the dictionary s items and all the keys list s items
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def values(self): for key in self__keys: yield self[key] def items(self): for key in self__keys: yield (key, self[key]) def __iter__(self): return iter(self__keys) keys = __iter__
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Dictionaries have four methods that return iterators: dictvalues() for the dictionary s values, dictitems() for the dictionary s key value items, dictkeys() for the keys, and the __iter__() special method that provides support for the iter(d) syntax, and operates on the keys (Actually, the base class versions of these methods return dictionary views, but for most purposes the behavior of the iterators implemented here is the same) Since the __iter__() method and the keys() method have identical behavior, instead of implementing keys(), we simply create an object reference called keys and set it to refer to the __iter__() method With this in place, users of SortedDict can call dkeys() or iter(d) to get an iterator over a dictionary s keys, just the same as they can call dvalues() to get an iterator over the dictionary s values The values() and items() methods are generator methods see the sidebar Generator Functions (279 ) for a brief explanation of generator methods In both cases they iterate over the sorted keys list, so they always return iterators that iterate in key order (with the key order depending on the key function given to the initializer) For the items() and values() methods, the values are looked up using the d[k] syntax (which uses dict__getitem__() under the hood), since we can treat self as a dict
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def __repr__(self): return object__repr__(self) def __str__(self): return ("{" + ", "join(["{0!r}: {1!r}"format(k, v) for k, v in selfitems()]) + "}")
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We cannot provide an eval()-able representation of a SortedDict because we can t produce an eval()-able representation of the key function So for the __repr__() reimplementation we bypass dict__repr__(), and instead call the ultimate base class version, object__repr__() This produces a string of the kind used for non-eval()-able representations, for example, '<SortedDictSortedDict object at 0xb71fff5c>' We have implemented the SortedDict__str__() method ourselves because we want the output to show the items in key sorted order The method could have been written like this instead:
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items = [] for key, value in selfitems(): itemsappend("{0!r}: {1!r}"format(key, value)) return "{" + ", "join(items) + "}"
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Using a list comprehension is shorter and avoids the need for the temporary items variable
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6 Object-Oriented Programming
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The base class methods dictget(), dict__getitem__() (for the v = d[k] syntax), dict__len__() (for len(d)), and dict__contains__() (for x in d) all work ne as they are and don t affect the key ordering, so we have not needed to reimplement them The last dict method that we must reimplement is copy()
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def copy(self): d = SortedDict() super(SortedDict, d)update(self) d__keys = self__keyscopy() return d
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The easiest reimplementation is simply def copy(self): return SortedDict( self) We ve chosen a slightly more complicated solution that avoids re-sorting the already sorted keys We create an empty sorted dictionary, then update it with the items in the original sorted dictionary using the base class dictupdate() to avoid the SortedDictupdate() reimplementation, and replace the dictionary s self__keys SortedList with a shallow copy of the original one When super() is called with no arguments it works on the base class and the self object But we can make it work on any class and any object by passing in a class and an object explicitly Using this syntax, the super() call works on the immediate base class of the class it is given, so in this case the code has the same effect as (and could be written as) dictupdate(d, self) In view of the fact that Python s sort algorithm is very fast, and is particularly well optimized for partially sorted lists, the ef ciency gain is likely to be little or nothing except for huge dictionaries However, the implementation shows that at least in principle, a custom copy() method can be more ef cient than using the copy_of_x = ClassOfX(x) idiom that Python s built-in types support And just as we did for SortedList, we have set __copy__ = copy so that the copycopy() function uses our custom copy method rather than its own code
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def value_at(self, index): return self[self__keys[index]] def set_value_at(self, index, value): self[self__keys[index]] = value
These two methods represent an extension to the dict API Since, unlike a plain dict, a SortedDict is ordered, it follows that the concept of key index positions is applicable For example, the rst item in the dictionary is at index position 0, and the last at position len(d) - 1 Both of these methods operate on the dictionary item whose key is at the index-th position in the sorted keys list Thanks to inheritance, we can look up values in the SortedDict using the item access op-