ch06动态类型简介（python）

>>> a = 3


#python的动态类型
>>> a = 3          # It's an integer
>>> a = 'spam'     # Now it's a string
>>> a = 1.23       # Now it's a floating point



>>> a = 3
>>> a = 'spam'


#python的对象有引用计数，当该计数为0的时候，垃圾回收机制收回该对象占用的内存空间
>>> x = 42
>>> x = 'shrubbery'      # Reclaim 42 now (unless referenced elsewhere)
>>> x = 3.1415           # Reclaim 'shrubbery' now
>>> x = [1, 2, 3]        # Reclaim 3.1415 now


#python的引用
>>> a = 3
>>> b = a
>>> a = 'spam'


#int类型是不可变的
>>> a = 3
>>> b = a
>>> a = a + 2


#list是可变的
>>> L1 = [2, 3, 4]
>>> L2 = L1
>>> L1 = 24


#L1和L2指向同一个内存对象
>>> L1 = [2, 3, 4]    # A mutable object
>>> L2 = L1           # Make a reference to the same object
>>> L1[0] = 24        # An in-place change
>>> L1                # L1 is different
[24, 3, 4]
>>> L2                # But so is L2!
[24, 3, 4]


#L2只是L1的一个副本
>>> L1 = [2, 3, 4]
>>> L2 = L1[:]        # Make a copy of L1
>>> L1[0] = 24
>>> L1
[24, 3, 4]
>>> L2                # L2 is not changed
[2, 3, 4]


#python的复制
import copy
X = copy.copy(Y)         # Make top-level "shallow" copy of any object Y
X = copy.deepcopy(Y)     # Make deep copy of any object Y: copy all nested parts



>>> x = 42
>>> x = 'shrubbery'      # Reclaim 42 now



>>> L = [1, 2, 3]
>>> M = L                # M and L reference the same object
>>> L == M               # Same value，比较的是内容
True
>>> L is M               # Same object，比较是否为同一个对象
True


is判断是否指向同一个对象



>>> L = [1, 2, 3]
>>> M = [1, 2, 3]        # M and L reference different objects
>>> L == M               # Same values
True
>>> L is M               # Different objects
False


#python的int和str有优化，有缓存
>>> X = 42
>>> Y = 42               # Should be two different objects
>>> X == Y
True
>>> X is Y               # Same object anyhow: caching at work!
True


#获取引用次数
>>> import sys
>>> sys.getrefcount(1)   # 837 pointers to this shared piece of memory
837



A = "spam"
B = A
B = "shrubbery"

A = ["spam"]
B = A
B[0] = "shrubbery"

A = ["spam"]
B = A[:]
B[0] = "shrubbery"


python类型属于对象，不属于变量


动态类型也是python动态的根本


如果使用正确的话，动态类型和多态产生的代码，
可以自动的适应系统的新需求


python只有这一种赋值模型，引用