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笔记：序列的修改、散列和切片
from array import array
import reprlib, math, numbers
from functools import reduce
from operator import xor
from itertools import chain
# Vector_v1
class Vector:
typecode = 'd'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
def __repr__(self):
components = reprlib.repr(self._componeents) # array('d', [1, 2, 3, 4, ...])
components = components[components.find('['):-1] # [1, 2, 3, 4, ...]
return 'Vector({})'.format(components) # Vector([1, 2, 3, 4])
def __str__(self):
return str(tuple(self))
def __bytes__(self):
return (bytes([ord(self.typecode)]) + bytes(self._componeents))
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))
def __len__(self):
return len(self._componeents)
def __getitem__(self, index):
return self._componeents[index]
@classmethod
def frombytes(cls, octets):
typecode = chr(octets[0])
memv = memoryview(octets[1:]).cast(typecode)
return cls(memv)
reprlib.repr() 获取对象的有限长度表现形式, 多的用 ... 表示, eg. array('d', [0, 1, 2, 3, 4, ...])
# Test
v1 = Vector([3, 4, 5])
print(len(v1))
print(v1[0], v1[-1])
v7 = Vector(range(7))
print(v7[1:4])
3
3.0 5.0
array('d', [1.0, 2.0, 3.0])
切片原理
class Seq:
def __getitem__(self, index):
return index
s = Seq()
print(1, s[1])
print(2, s[1:4])
print(3, s[1:4:2])
print(4, s[1:4:2, 9])
print(5, s[1:4:2, 7:9])
1 1
2 slice(1, 4, None)
3 slice(1, 4, 2)
4 (slice(1, 4, 2), 9)
5 (slice(1, 4, 2), slice(7, 9, None))
print(slice)
print(dir(slice))
print(help(slice.indices))
<class 'slice'>
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'indices', 'start', 'step', 'stop']
Help on method_descriptor:
indices(...)
S.indices(len) -> (start, stop, stride)
Assuming a sequence of length len, calculate the start and stop
indices, and the stride length of the extended slice described by
S. Out of bounds indices are clipped in a manner consistent with the
handling of normal slices.
None
S.indices(len) -> (start, stop, stride)
自动将 slice 适配到 长度为 len 的对象上
print(slice(None, 10, 2).indices(5))
print(slice(-3, None, None).indices(5))
(0, 5, 2)
(2, 5, 1)
print('ABCDE'[:10:2]) # 等价于
print('ABCDE'[0:5:2])
print('ABCDE'[-3:]) # 等价于
print('ABCDE'[2:5:1])
ACE
ACE
CDE
CDE
# Vector_v2
class Vector:
typecode = 'd'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
################################# NEW ######################################
def __getitem__(self, index):
cls = type(self)
if isinstance(index, slice): # slice 切片
return cls(self._componeents[index])
elif isinstance(index, numbers.Integral): # int 索引
return self._componeents[index]
else: # 抛出异常
msg = '{cls.__name__} indices must be integers'
raise TypeError(msg.format(cls=cls))
def __repr__(self):
components = reprlib.repr(self._componeents) # array('d', [1, 2, 3, 4, ...])
components = components[components.find('['):-1] # [1, 2, 3, 4, ...]
return 'Vector({})'.format(components) # Vector([1, 2, 3, 4])
def __str__(self):
return str(tuple(self))
def __bytes__(self):
return (bytes([ord(self.typecode)]) + bytes(self._componeents))
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))
def __len__(self):
return len(self._componeents)
@classmethod
def frombytes(cls, octets):
typecode = chr(octets[0])
memv = memoryview(octets[1:]).cast(typecode)
return cls(memv)
v7 = Vector(range(7))
print(1, v7[-1])
print(2, v7[1:4])
print(3, v7[-1:])
print(4, v7[1, 2]) # 报误，不支持多维切片
1 6.0
2 (1.0, 2.0, 3.0)
3 (6.0,)
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In [28], line 5
3 print(2, v7[1:4])
4 print(3, v7[-1:])
----> 5 print(4, v7[1, 2])
Cell In [22], line 19, in Vector.__getitem__(self, index)
17 else: # 抛出异常
18 msg = '{cls.__name__} indices must be integers'
---> 19 raise TypeError(msg.format(cls=cls))
TypeError: Vector indices must be integers
# Vector_v3 动态存取属性
class Vector:
typecode = 'd'
shortcut_numes = 'xyzt'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
def __getitem__(self, index):
cls = type(self)
if isinstance(index, slice): # slice 切片
return cls(self._componeents[index])
elif isinstance(index, numbers.Integral): # int 索引
return self._componeents[index]
else: # 抛出异常
msg = '{cls.__name__} indices must be integers'
raise TypeError(msg.format(cls=cls))
################################# NEW ######################################
def __getattr__(self, name):
cls = type(self)
if len(name) == 1:
pos = cls.shortcut_numes.find(name)
if 0 <= pos < len(self._componeents):
return self._componeents[pos]
msg = '{.__name__!r} object has no attribute {!r}'
raise AttributeError(msg.format(cls, name))
v5 = Vector(range(5))
print(1, v5)
print(2, v5.x)
v5.x = 10
print(3, v5.x) # v5[0] 变了？
print(4, v5) # v5 实际上并没有变
1 (0.0, 1.0, 2.0, 3.0, 4.0)
2 0.0
3 10
4 (0.0, 1.0, 2.0, 3.0, 4.0)
解释:
当且仅当对象中没有 指定名称 的属性时, 才会调用 __getattr__
当执行 v5.x = 10 会给 v5 创建 x 这个属性, 这个属性也称为 虚拟属性
之后访问 v5.x 便是该属性的值, 而不通过 __getattr__ 获取
# 改进
# Vector_v3 动态存取属性
class Vector:
typecode = 'd'
shortcut_numes = 'xyzt'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __getattr__(self, name):
cls = type(self)
if len(name) == 1:
pos = cls.shortcut_numes.find(name)
if 0 <= pos < len(self._componeents):
return self._componeents[pos]
msg = '{.__name__!r} object has no attribute {!r}'
raise AttributeError(msg.format(cls, name))
################################# NEW ######################################
def __setattr__(self, name, value) -> None:
cls = type(self)
if len(name) == 1:
if name in cls.shortcut_numes:
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)
通过 __setattr__ 方法防止其修改部分属性
v6 = Vector(range(6))
print(1, v6)
print(2, v6.x)
v6.x = 10
1 <__main__.Vector object at 0x000001BD0AD009A0>
2 0.0
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In [35], line 5
2 print(1, v6)
3 print(2, v6.x)
----> 5 v6.x = 10
Cell In [34], line 35, in Vector.__setattr__(self, name, value)
33 if error:
34 msg = error.format(cls_name=cls.__name__, attr_name=name)
---> 35 raise AttributeError(msg)
36 super().__setattr__(name, value)
AttributeError: readonly attribute 'x'
散列 & 快速等值测试
__hash__
需要依次计算 v[0] ^ v[1] ^ v[2] ...
reduce()
__eq__
# 复习一下 reduce
print(reduce(lambda a, b: a * b, range(1, 6)))
120
# 计算多个数异或与
print(reduce(lambda a, b: a ^ b, range(233)))
print(reduce(xor, range(233)))
n = 0
for i in range(1, 233):
n ^= i
print(n)
232
232
232
# Vector_v3 加入 __hash__
class Vector:
typecode = 'd'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
################################# OLD ######################################
def __eq__(self, other):
return tuple(self) == tuple(other)
################################# NEW ######################################
def __hash__(self):
hashes = (hash(x) for x in self._componeents)
return reduce(xor, hashes, 0)
注：
使用 reduce() 最好提供三个参数 reduce(function, iterable, initializer)
通常, 对于 +, |, ^ initializer = 0
通常, 对于 *, & initializer = 1
__eq__ 要和 __hash__ 在一起哦~
# 使用【映射归约】实现 __hash__ (map, reduce)
# Vector_v3 加入 __hash__
class Vector:
typecode = 'd'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
################################# NEW ######################################
def __hash__(self):
hashes = map(hash, self._componeents) # 计算各个元素的hash
return reduce(xor, hashes, 0)
################################# NEW ######################################
def __eq__(self, other):
if len(self) != len(other): # 长度不相等, 直接 False
return False
for a, b in zip(self, other): # 判断每个对应元素是否相等
if a != b:
return False
return True
################################# NEW ######################################
# 另一种方法
def __eq__(self, other):
return len(self) != len(other) and all(a == b for a, b in zip(self, other))
all() 只要有一个是 False , 整个都是 False
上面两种 __eq__ 效果相等
# 回忆一下 zip()
print(1, zip(range(3), 'ABC'))
print(2, list(zip(range(3), 'ABC')))
print(3, list(zip(range(3), 'ABC', [0, 1, 2, 3]))) # 什么？？？一个可迭代对象迭代完了, 就不迭代了
from itertools import zip_longest
print(4, list(zip_longest(range(3), 'ABC', [0, 1, 2, 3], fillvalue=1))) # 按照最长的iter迭代, 空的用 fillvalue 补齐
1 <zip object at 0x000001BD0A82A9C0>
2 [(0, 'A'), (1, 'B'), (2, 'C')]
3 [(0, 'A', 0), (1, 'B', 1), (2, 'C', 2)]
4 [(0, 'A', 0), (1, 'B', 1), (2, 'C', 2), (1, 1, 3)]
zip_longest() 按照最长的 iter 迭代, 空的用 fillvalue 补齐
format 格式化
目标: 得到球面坐标 <r, ɸ1, ɸ2, ɸ3>
# 使用【映射归约】实现 __hash__ (map, reduce)
# Vector_v3 加入 __hash__
class Vector:
typecode = 'd'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
################################# NEW ######################################
def angle(self, n):
r = math.sqrt(sum(x * x for x in self[n:]))
a = math.atan2(r, self[n-1])
if (n == len(self) - 1) and (self[-1] < 0):
return math.pi * 2 - a
else:
return a
def angles(self):
return (self.angle(n) for n in range(1, len(self)))
def __format__(self, fmt_spec=''):
if fmt_spec.endswith('h'): # 获取超球体坐标
fmt_spec = fmt_spec[:-1]
coords = chain([abs(self)], self.angles()) # 生成生成器表达式, 无缝迭代向量的模和各个角坐标
outer_fmt = '<{}>'
else:
coords = self
outer_fmt = '({})'
components = (format(c, fmt_spec) for c in coords)
return outer_fmt.format(', '.join(components))
# Vector Final
class Vector:
typecode = 'd'
def __init__(self, components) -> None:
self._componeents = array(self.typecode, components)
def __iter__(self):
return iter(self._componeents)
def __getitem__(self, index):
cls = type(self)
if isinstance(index, slice): # slice 切片
return cls(self._componeents[index])
elif isinstance(index, numbers.Integral): # int 索引
return self._componeents[index]
else: # 抛出异常
msg = '{cls.__name__} indices must be integers'
raise TypeError(msg.format(cls=cls))
def __repr__(self):
components = reprlib.repr(self._componeents) # array('d', [1, 2, 3, 4, ...])
components = components[components.find('['):-1] # [1, 2, 3, 4, ...]
return 'Vector({})'.format(components) # Vector([1, 2, 3, 4])
def __str__(self):
return str(tuple(self))
def __bytes__(self):
return (bytes([ord(self.typecode)]) + bytes(self._componeents))
def __hash__(self):
hashes = map(hash, self._componeents) # 计算各个元素的hash
return reduce(xor, hashes, 0)
def __eq__(self, other):
return len(self) != len(other) and all(a == b for a, b in zip(self, other))
def __abs__(self):
return math.sqrt(sum(x * x for x in self))
def __bool__(self):
return bool(abs(self))
def __len__(self):
return len(self._componeents)
shortcut_numes = 'xyzt'
def __getattr__(self, name):
cls = type(self)
if len(name) == 1:
pos = cls.shortcut_numes.find(name)
if 0 <= pos < len(self._componeents):
return self._componeents[pos]
msg = '{.__name__!r} object has no attribute {!r}'
raise AttributeError(msg.format(cls, name))
def __setattr__(self, name, value) -> None:
cls = type(self)
if len(name) == 1:
if name in cls.shortcut_numes:
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)
@classmethod
def frombytes(cls, octets):
typecode = chr(octets[0])
memv = memoryview(octets[1:]).cast(typecode)
return cls(memv)
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