python文档：控制流（if,for,函数,lambda等)
4.1. if 语句可能最为人所熟知的编程语句就是 if 语句了。例如>>> x = int(input("Please enter an integer: "))Please enter an integer: 42>>> if x < 0:... x = 0... print('Negative changed to zero')... elif x == 0
