python中循环删除list和dict类型注意事项

列表和字典在循环操作（增删）时，其长度会改变

# 删除 li = [11, 22, 33, 44, 'rock']中索引为单数的元素
# 方法一
del li[1::2]
print(li)   # [11, 33, 'rock']

# 方法二
new_li = []
for index in range(len(li)):
    print(index)
    if index % 2 == 0:
        new_li.append(li[index])
li = new_li
print(li)

# 方法三
# 取首不取尾
for index in range(len(li)-1, 0-1, -1):
    print(index)
    if index % 2 != 0:
        li.pop(index)
print(li)

# 错误示例
for index in range(len(li)):
    print(index)
    if index % 2 != 0:
        li.pop(index)
print(li)

# 删除 dic = {'k1': 1, 'k2': 1, 'k3': 1, 'name': 'rock'}中所有包含k字母的键值对

# dict字典在循环时不能改变字典的长度，即不能增删改查字典中的元素
dic = {'k1': 1, 'k2': 1, 'k3': 1, 'name': 'rock'}
li = []
for key in dic.keys():
    if 'k' in key:
        li.append(key)
for i in li:
    dic.pop(i)
print(dic)  # {'name': 'rock'}

# 错误示例 RuntimeError: dictionary changed size during iteration
for key in dic.keys():
    if 'k' in key:
        dic.pop(key)
print(dic)

for key in dic.keys():
    print(key[0])
    if key[0] == 'k':
        dic.pop(key)
print(dic)