Python常见的数据结构的遍历

s = "hello"
for char in s:  # 'h' → 'e' → 'l' → 'l' → 'o'
    print(char)

lst = [1, 2, 3]
for item in lst:  # 1 → 2 → 3
    print(item)

tup = (1, 2, 3)
for item in tup:  # 1 → 2 → 3
    print(item)

d = {'a': 1, 'b': 2}
# 默认遍历键
for key in d:  # 'a' → 'b'
    print(key)
# 遍历值
for value in d.values():  # 1 → 2
    print(value)
# 遍历键值对
for key, value in d.items():  # ('a', 1) → ('b', 2)
    print(key, value)

s = {1, 2, 3}
for item in s:  # 顺序不确定
    print(item)

for i in range(5):  # 0 → 1 → 2 → 3 → 4
    print(i)

with open('file.txt') as f:
    for line in f:  # 一行一行读取
        print(line)

s = "abc"
for i, char in enumerate(s):  # (0,'a') → (1,'b') → (2,'c')
    print(i, char)

names = ["Alice", "Bob"]
ages = [25, 30]
for name, age in zip(names, ages):  # ("Alice",25) → ("Bob",30)
    print(name, age)

# 生成器表达式
gen = (x*2 for x in range(3))  # 0 → 2 → 4
for num in gen:
    print(num)

num = 123
for digit in num:  # ❌ TypeError: 'int' object is not iterable
    print(digit)

num = 123
for digit in str(num):  # '1' → '2' → '3'
    print(digit)

flag = True
for item in flag:  # ❌ TypeError: 'bool' object is not iterable
    print(item)

value = None
for item in value:  # ❌ TypeError: 'NoneType' object is not iterable
    print(item)

obj = "I'm a string, but..."
for char in obj:  # ✅ 可以，因为字符串是可迭代的
    print(char)
num = 42
for item in num:  # ❌ 不行
    print(item)
# 需要包装成容器
for item in [num]:  # ✅ 可以
    print(item)

from collections.abc import Iterable
print(isinstance("hello", Iterable))  # ✅ True
print(isinstance([1,2,3], Iterable))  # ✅ True
print(isinstance(123, Iterable))      # ❌ False
print(isinstance(True, Iterable))     # ❌ False
print(isinstance(None, Iterable))     # ❌ False