深入理解Python之序列切片

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前言

本文着重介绍

  • 基本序列协议: __len__和__getitem__
  • 带有多个条目对象的安全展示
  • 合理的切片支持
  • 哈希处理
  • 自定义格式化语言拓展

Vector: 自定义序列类型

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class Vector:
    typecode = 'd'

    def __init__(self, components):
        self._components = array(self.typecode, components)

    def __iter__(self):
        return iter(self._components)

    def __repr__(self):
        components = reprlib.repr(self._components)
        components = components[components.find('['):-1]
        return 'Vector({})'.format(components)

    def __str__(self):
        return str(tuple(self))

    def __bytes__(self):
        return bytes([ord(self.typecode)]) + bytes(self._components)

    def __eq__(self, other):
        return tuple(self) == tuple(other)

    def __abs__(self):
        return math.sqrt(sum(x * x for x in self))

    def __bool__(self):
        return bool(abs(self))

    @classmethod
    def frombytes(cls, octets):
        typecode = chr(octets[0])
        memv = memoryview(octets[1:]).cast(typecode)
        return cls(memv)

  • 使用reprlib来进行安全展示

协议和鸭子类型

为Vector实现__len__和__getitem__协议

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def __len__(self):
    return len(self._components)

def __getitem__(self, item):
    return self._components[item]

增强切片功能

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def __getitem__(self, item):
    cls = type(self)
    if isinstance(item, slice):
        return cls(self._components[item])
    elif isinstance(item, numbers.Integral):
        return self._components[item]
    else:
        msg = '{cls.__name__} indices must be integers'
        raise TypeError(msg.format(cls=cls))

动态属性访问

__getattr__当属性查找失败被调用。简单来说就是，给定表达式my_obj.x, Python检查对象是否含有x属性。如果没有，继续寻找类(my_obj.__class__)，然后继承链向上查找。如果依然没有找到x，那么__getattr__就会被调用

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def __getattr__(self, name):
    cls = type(self)
    if len(name) == 1:
        pos = cls.shortcut_names.find(name)
        if 0 <= pos < len(self._components):
            return self._components[pos]
    msg = '{.__name__!r} object has no attribute {!r}'
    raise AttributeError(msg.format(cls, name))

def __setattr__(self, name, value):
    cls = type(self)
    if len(name) == 1:
        if name in cls.shortcut_names:
            error = 'readonly attribute {attr_name!r}'
        elif name.islower():
            error = "can't set attributes 'a' to 'z' in {cls_name!r}"

        else:
            error = ''

        if error:
            msg = error.format(cls_name=cls.__name__, attr_name=name)
            raise AttributeError(msg)

    super().__setattr__(name, value)

哈希和更快的==

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def __hash__(self):
    hashes = map(hash, self._components)
    return functools.reduce(operator.xor, hashes, 0)

def __eq__(self, other):
    return len(self) == len(other) and all(a == b for a, b in zip(self, other))

格式化

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def __format__(self, fmt_spec):
    if fmt_spec.endswith('h'):
        fmt_spec = fmt_spec[:-1]
        coords = itertools.chain([abs(self), self.angles()])
        outer_fmt = '<{}>'

    else:
        coords = self
        outer_fmt = '({})'
    components = (format(c, fmt_spec) for c in coords)
    return outer_fmt.format(', '.join(components))