1.if 的简单事例
part_a = ["affair","affect","affection","afterword"]
for part in part_a:
if part == "affect": #简单if语句，注意：
print(part.upper())
else:
print(part.title())
parts = "agony"
if parts not in part_a:
print("There is no this word:"+ parts.title()) #否定if
2.if-elif-else 语句
age =23
if age<12:
print("\nyou are children.")
elif age>=18 : #嵌套if
print("\nyou are adult.")
else:
print("\nyou art teenager.")
如果你想执行一个代码块，就用if-elif-else 结果，如果运行多个代码块，就用一系列if语句
