Python基础之变量的相关知识总结

>>> a = [1, 2, 3]
>>> b = a
>>> a.append(4)
>>> b
[1, 2, 3, 4]
>>> b is a
True

>>> a = [1, 2, 3]
>>> c = [1, 2, 3]
>>> c == a
True
>>> c is a
False

>>> x1 = [3, [66, 55, 44], (7, 8, 9)]
# x2是x1的浅复制
>>> x2 = list(x1)

# 不可变元素没有影响
>>> x1.append(100)
>>> x1
[3, [66, 55, 44], (7, 8, 9), 100]
>>> x2
[3, [66, 55, 44], (7, 8, 9)]  

# x1[1]是列表，可变元素会影响x2
# 因为它们引用的是同一个对象
>>> x1[1].remove(55)
>>> x1
[3, [66, 44], (7, 8, 9), 100]
>>> x2
[3, [66, 44], (7, 8, 9)]  

# x2[1]也会反过来影响x1
>>> x2[1] += [33, 22]
>>> x1
[3, [66, 44, 33, 22], (7, 8, 9), 100]  
>>> x2
[3, [66, 44, 33, 22], (7, 8, 9)]

# 不可变元组也不会有影响
# +=运算符创建了一个新元组
>>> x2[2] += (10, 11)
>>> x1
[3, [66, 44, 33, 22], (7, 8, 9), 100]  
>>> x2
[3, [66, 44, 33, 22], (7, 8, 9, 10, 11)]

>>> a = [10, 20]
>>> b = [a, 30]
>>> a.append(b)
>>> a
[10, 20, [[...], 30]]
>>> from copy import deepcopy
>>> c = deepcopy(a)
>>> c
[10, 20, [[...], 30]]

>>> def f(a, b):
...     a += b
...     return a
... 

# 数字不变
>>> x = 1
>>> y = 2
>>> f(x, y)
3
>>> x, y
(1, 2)

# 列表变了
>>> a = [1, 2]
>>> b = [3, 4]
>>> f(a, b)
[1, 2, 3, 4]
>>> a, b
([1, 2, 3, 4], [3, 4])

# 元组不变
>>> t = (10, 20)
>>> u = (30, 40)
>>> f(t, u)
(10, 20, 30, 40)
>>> t, u
((10, 20), (30, 40))

class TwilightBus:
    """A bus model that makes passengers vanish"""

    def __init__(self, passengers=None):
        if passengers is None:
            self.passengers = []
        else:
            self.passengers = passengers

    def pick(self, name):
        self.passengers.append(name)

    def drop(self, name):
        self.passengers.remove(name)

>>> basketball_team = ['Sue', 'Tina', 'Maya', 'Diana', 'Pat']
>>> bus = TwilightBus(basketball_team)
>>> bus.drop('Tina')
>>> bus.drop('Pat')
>>> basketball_team
['Sue', 'Maya', 'Diana']

 def __init__(self, passengers=None):
        if passengers is None:
            self.passengers = []
        else:
            self.passengers = list(passengers)  # 使用构造函数复制副本

>>> import weakref
>>> s1 = {1, 2, 3}
# s2和s1引用同一个对象
>>> s2 = s1
>>> def bye():
...     print("Gone")
...     
# 监控对象和调用回调
>>> ender = weakref.finalize(s1, bye)
>>> ender.alive
True
# 删除s1后还存在s2引用
>>> del s1
>>> ender.alive
True
# s2重新绑定导致{1, 2, 3}引用归零
>>> s2 = "spam"
Gone
# 对象被销毁了
>>> ender.alive
False

class Cheese:

    def __init__(self, kind):
        self.kind = kind

    def __repr__(self):
        return 'Cheese(%r)' % self.kind
>>> import weakref
>>> stock = weakref.WeakValueDictionary()
>>> catalog = [Cheese('Red Leicester'), Cheese('Tilsit'),
...                 Cheese('Brie'), Cheese('Parmesan')]
...
>>> for cheese in catalog:
        # 用作缓存
        # key是cheese.kind
        # value是cheese的弱引用
...     stock[cheese.kind] = cheese
...
>>> sorted(stock.keys())
['Brie', 'Parmesan', 'Red Leicester', 'Tilsit']

# 删除catalog引用，stock弱引用不影响垃圾回收
# WeakValueDictionary的值引用的对象被销毁后，对应的键也会自动删除
>>> del catalog
>>> sorted(stock.keys())  # 还存在一个cheese临时变量的引用
['Parmesan']

# 删除cheese临时变量的引用，stock就完全清空了
>>> del cheese
>>> sorted(stock.keys())
[]

class MyList(list):
    pass
a_list = MyList(range(10))
weakref_to_a_list = weakref.ref(a_list)