变量、常量、input、if语句

chapter2 python基础

2.1 变量命名规则

• 字母、数字、下划线组成
• 不能数字开头
• 不能使用python中的关键字
• 变量名具有意义
• 推荐驼峰（arvingood）、或者下划线方式 (arvin_good) 命名

标准示例：

your_name = 'jack ma'

2.2 常量

• 全部大写的变量名就是常量 arvin = 99

示例：

arvin_age = 18

2.3 注释

• 三种方式：
  1. “#”号单行注释
  2. ‘’‘三个单引号和三个双引号”“”进行多行注释

例：

# the idle boy is almost invariably poor and anxious. 单行注释

'''
这是多行注释示范
'''

"""
多行注释示范
"""

2.4 输入输出

• 用户输入 input
  1. 注意：input获取到的内容都是字符串类型
• 用户输出 print
  1. print打印出内容

your_id = input("input your id identifier: ")
your_pwd = input("input your password: ")
print("your id is {}, your password is {}".format(your_id, your_pwd))

运行结果：
input your id identifier: jack
input your password: default
your id is jack, your password is default

2.5 五种if条件判断语句

• 单if条件判断

  if 条件： #条件判断 > < ==

  ​ print(‘真实’) #条件成立则执行

  if 10 > 8:
      print("you are right.")

• if else条件判断

  if 10 > 20:
    print("10 is bigger than 2.")
  else:
    print("10 is smaller than 2.")

• if elif elif else 条件判断

  a = "jack"
  if 10 > 50:
    print("10 is bigger than 5.")
  elif type(a) == "str":
    print("a is a string.")
  elif 20 == 20:
    print("20 is equal 20.")
  else:
      print("all of them are false.")
  
  运行结果：
  20 is equal 20.

• 多个同级if语句

  a = "jack"
  if 10 == 20:
    print("10 is equal 20.")
  if a == "jack":
    print(a ,"is the same as jack.")
  if 10 == 10:
    print("10 is equal 10.")
  
  运行结果：
  jack is the same as jack.
  10 is equal 10.

• if与if语句之间的嵌套

  id = int(input("input your id number: "))
  pwd = input("input your passwrd: ")
  if id == 9:
    print("go on.")
    if pwd == "default":
        print("you logged in successfully.")
    else:
        print("your password is incorrect.")
  else:
    print("your id was typed incorrectly.")