简化数据结构的初始化过程

简化数据结构的初始化过程

我们编写了许多类， 把它们当作数据结构用。但是我们厌倦了编写高度重复且样式相同的 init () 函数。

解决方案

通常我们可以将初始化数据结构的步骤归纳到一个单独的 init()函数中，并将其定义在一个公共的基类中。示例如下：

class Structure:
    # Class variable that speicifies expected fields

    _fields = []
    def __init__(self, *args):
        if len(args) != len(self._fields):
            raise TypeError('Expected {} arguments'.format(len(self._fields)))

        # Set the arguments
        for name, value in zip(self._fields, args):
            setattr(self, name, value)


        # Example class definitions
import math
if __name__ == '__main__':
    class Stock(Structure):
        _fields = ['name', 'shares', 'price']

    class Point(Structure):
        _fields = ['x', 'y']

    class Circle(Structure):
        _fields = ['radius']
        def area(self):
            return math.pi * self.radius ** 2

    s = Stock('ACME', 50, 91.1)
    p = Point(2, 3)
    c = Circle(4.5)
    print('圆c的面积是：{:.2f}'.format(c.area()))

    s2 = Stock('ACME', 50)

如果使用这些类， 就会发现它们非常易于构建。结果如下：

D:\Python39\python.exe D:/My_Project/example1.py
Traceback (most recent call last):
  File "D:\My_Project\example1.py", line 33, in <module>
    s2 = Stock('ACME', 50)
  File "D:\My_Project\example1.py", line 7, in __init__
    raise TypeError('Expected {} arguments'.format(len(self._fields)))
TypeError: Expected 3 arguments
圆c的面积是：63.62

Process finished with exit code 1

我们应该提供对关键字参数的支持，这里有几种设计上的选择。一种选择是对关键字参数做映射，这样它们就只对应与定义在_fields中的属性名。示例如下：

class Structure:
    _fields = []

    def __init__(self, *args, **kwargs):
        if len(args) > len(self._fields):
            raise TypeError('Expected {} arguments'.format(len(self._fields)))

        # Set all of the positional arguments
        for name, value in zip(self._fields, args):
            setattr(self, name, value)

        # Set the remaining keyword arguments
        for name in self._fields[len(args):]:
            setattr(self, name, kwargs.pop(name))

        # Check for any remaining unknown arguments
        if kwargs:
            raise TypeError('Invalid argumen(s): {}'.foramt(','.join(kwargs)))


# Example use
if __name__ == '__main__':
    class Stock(Structure):
        _fields = ['name', 'shares', 'price']
    s1 = Stock('ACME', 50, 91.1)
    s2 = Stock('ACME', 50, price=91.1)
    s3 = Stock('ACME', shares=50, price=91.1)
    s4 = Stock('ACME', 40, 81.1, 88)  # 多一个参数，会产生报错