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Further Procedural Programming
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Annotations are a very new feature of Python, and because Python does not impose any prede ned meaning on them, the uses they can be put to are limited only by our imagination Further ideas for possible uses, and some useful links, are available from PEP 3107 Function Annotations , wwwpythonorg/ dev/peps/pep-3107
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Further Object-Oriented Programming
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In this section we will look more deeply into Python s support for object orientation, learning many techniques that can reduce the amount of code we must write, and that expand the power and capabilities of the programming features that are available to us But we will begin with one very small and simple new feature Here is the start of the de nition of a Point class that has exactly the same behavior as the versions we created in 6:
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class Point: __slots__ = ("x", "y") def __init__(self, x=0, y=0): selfx = x selfy = y
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Attribute access functions 349
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When a class is created without the use of __slots__, behind the scenes Python creates a private dictionary called __dict__ for each instance, and this dictionary holds the instance s data attributes This is why we can add or remove attributes from objects (For example, we added a cache attribute to the get_function() function earlier in this chapter) If we only need objects where we access the original attributes and don t need to add or remove attributes, we can create classes that don t have a __dict__ This is achieved simply by de ning a class attribute called __slots__ whose value is a tuple of attribute names Each object of such a class will have attributes of the speci ed names and no __dict__; no attributes can be added or removed from such classes These objects consume less memory and are faster than conventional objects, although this is unlikely to make much difference unless large numbers of objects are created If we inherit from a class that uses __slots__ we must declare slots in our subclass, even if empty, such as __slots__ = (); or the memory and speed savings will be lost
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Controlling Attribute Access
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It is sometimes convenient to have a class where attribute values are computed on the y rather than stored Here s the complete implementation of such a class:
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class Ord:
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def __getattr__(self, char): return ord(char)
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With the Ord class available, we can create an instance, ord = Ord(), and then have an alternative to the built-in ord() function that works for any character that is a valid identi er For example, orda returns 97, ordZ returns 90, and ord returns 229 (But ord! and similar are syntax errors) Note that if we typed the Ord class into IDLE it would not work if we then typed ord = Ord() This is because the instance has the same name as the built-in ord() function that the Ord class uses, so the ord() call would actually become a call to the ord instance and result in a TypeError exception The problem would not arise if we imported a module containing the Ord class because the interactively created ord object and the built-in ord() function used by the Ord class would be in two separate modules, so one would not displace the other If we really need to create a class interactively and to reuse the name of a built-in we can do so by ensuring that the class calls the built-in in this case by importing the builtins module which provides unambiguous access to all the built-in functions, and calling builtinsord() rather than plain ord() Here s another tiny yet complete class This one allows us to create constants It isn t dif cult to change the values behind the class s back, but it can at least prevent simple mistakes
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class Const: def __setattr__(self, name, value): if name in self__dict__: raise ValueError("cannot change a const attribute") self__dict__[name] = value def __delattr__(self, name): if name in self__dict__: raise ValueError("cannot delete a const attribute") raise AttributeError("'{0}' object has no attribute '{1}'" format(self__class____name__, name))
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With this class we can create a constant object, say, const = Const(), and set any attributes we like on it, for example, constlimit = 591 But once an attribute s value has been set, although it can be read as often as we like, any attempt to change or delete it will result in a ValueError exception being raised We have not reimplemented __getattr__() because the base class object__getattr__() method does what we want returns the given attribute s value or raises an AttributeError exception if there is no such attribute In the __delattr__() method we mimic the __getattr__() method s error message for nonexistent attributes, and to do this we must get the name of the class we are in as well as
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