Object-Oriented Programming in Java

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6 Object-Oriented Programming
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# str(letters) == "['A', 'B', 'c', 'e', 'f', 'G', 'G', 'H']" letters[2] # returns: 'c'
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A SortedList object aggregates (is composed of) two private attributes; a function, self__key() (held as object reference self__key), and a list, self__list
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Lambda functions 182
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The key function is passed as the second argument (or using the key keyword argument if no initial sequence is given) If no key function is speci ed the following private module function is used:
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_identity = lambda x: x
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This is the identity function: It simply returns its argument unchanged, so when it is used as a SortedList s key function it means that the sort key for each object in the list is the object itself The SortedList type does not allow the item access operator ([]) to change an item (so it does not implement the __setitem__() special method), nor does it provide the append() or extend() method since these might invalidate the ordering The only way to add items is to pass a sequence when the SortedList is created or to add them later using the SortedListadd() method On the other hand, we can safely use the item access operator for getting or deleting the item at a given index position since neither operation affects the ordering, so both the __getitem__() and __delitem__() special methods are implemented We will now review the class method by method, starting as usual with the class line and the initializer:
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class SortedList: def __init__(self, sequence=None, key=None): self__key = key or _identity assert hasattr(self__key, "__call__") if sequence is None: self__list = [] elif (isinstance(sequence, SortedList) and sequencekey == self__key): self__list = sequence__list[:] else: self__list = sorted(list(sequence), key=self__key)
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Since a function s name is an object reference (to its function), we can hold functions in variables just like any other object reference Here the private self__key variable holds a reference to the key function that was passed in, or to the identity function The method s rst statement relies on the fact that the or operator returns its rst operand if it is True in a Boolean context (which a not-None key function is), or its second operand otherwise A slightly longer but
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Custom Collection Classes
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more obvious alternative would have been self__key = key if key is not None else _identity Once we have the key function, we use an assert to ensure that it is callable The built-in hasattr() function returns True if the object passed as its rst argument has the attribute whose name is passed as its second argument There are corresponding setattr() and delattr() functions these functions are covered in 8 All callable objects, for example, functions and methods, have a __call__ attribute To make the creation of SortedLists as similar as possible to the creation of lists we have an optional sequence argument that corresponds to the single optional argument that list() accepts The SortedList class aggregates a list collection in the private variable self__list and keeps the items in the aggregated list in sorted order using the given key function The elif clause uses type testing to see whether the given sequence is a SortedList and if that is the case whether it has the same key function as this sorted list If these conditions are met we simply shallow-copy the sequence s list without needing to sort it If most key functions are created on the y using lambda, even though two may have the same code they will not compare as equal, so the ef ciency gain may not be realized in practice
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@property def key(self): return self__key
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Once a sorted list is created its key function is xed, so we keep it as a private variable to prevent users from changing it But some users may want to get a reference to the key function (as we will see in the next subsection), and so we have made it accessible by providing the read-only key property
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def add(self, value): index = self__bisect_left(value) if index == len(self__list): self__listappend(value) else: self__listinsert(index, value)
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When this method is called the given value must be inserted into the private self__list in the correct position to preserve the list s order The private SortedList__bisect_left() method returns the required index position as we will see in a moment If the new value is larger than any other value in the list it must go at the end, so the index position will be equal to the list s length (list index positions go from 0 to len(L) - 1) if this is the case we append the new value Otherwise, we insert the new value at the given index position which will be at index position 0 if the new value is smaller than any other value in the list
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