[2021 Spring] CS61A 学习笔记 lecture 14 List mutation + Identity + Nonlocal

主要内容：
列表的创建、更新、方法（python基础课程均有讲解，也可以查看菜鸟教程等）
is 和 == 的区别
nonlocal 和 global（可以参考parent frame）

目录

• List creation， List mutation， List methods
  • bonus 1: = 与 +=
  • bonus 2: append 与 extend
• Equality and Identity
  • List
  • strings/numbers
• Scope

List creation， List mutation， List methods

bonus 1: = 与 +=

b = b + [8, 9]改变了b的指向，b += [8, 9]没有改变。（老师只提出了这个现象，没有解释原因，philosophy）

bonus 2: append 与 extend

• append() adds a single element to a list
• extend() adds all the elements in one list to a list
• pop() removes and returns the last element
• remove() removes the first element equal to the argument
  t为列表，t的变化会影响到s.append(t)，但是不会影响到s.extend(t).
  
  

Equality and Identity

• Identity : exp0 is exp1
  • evaluates to True if both exp0 and exp1 evaluate to the same object
• Equality: exp0 == exp1
  • evaluates to True if both exp0 and exp1 evaluate to objects containing equal values
    对于列表和字符串/数字来说不同，可能原因：列表是可变类型，字符串和数字是不可变类型。

List

注意：两个各自定义的列表指向并不相同，所以identity=False，只有值相同。(对于字符串/数字可能不成立，identity=True）

a = ["apples" , "bananas"]
b = ["apples" , "bananas"]
c = a
if a == b == c:
    print("All equal!")
a[1] = "oranges"
if a  is c  and  a == c:
    print("A and C are equal AND identical!")
if a == b:
    print("A and B are equal!") # Nope!
if b == c:
    print("B and C are equal!") # Nope!

strings/numbers

以下全部为True：

a = "orange"
b = "orange"
c = "o"  +  "range"
print(a  is b)
print(a  is c)
a = 100
b = 100
print(a  is b)
print(a  is 10 *  10)
print(a == 10 *  10)
a = 100000000000000000
b = 100000000000000000
print(a  is b)
print(100000000000000000 is 100000000000000000 )

Scope

scope rules