表达式
表达式（Expression）是 运算符（operator）和 操作数（operand）所构成的 序列
>>> 1 + 1
2
>>> a = [1,2,3]
>>> 1 + 1 + 1 + 1
4
>>> 1 + 2 * 3
7
>>> 1 * 2 + 3
5
>>> a = 1 + 2 * 3
>>> a = 1
>>> b = 2
>>> c = a and b or c
>>> c = int('1') + 2运算符优先级
同级的运算符的优先级还是有区别的 比如逻辑运算符里的 and的优先级大于or
>>> a = 1
>>> b = 2
>>> c = 3
>>> a + b * c
7
>>> 1 or 2
1
>>> 1 and 3
3
>>> a or b and c
1
>>> a or (b and c)
1
>>> a or 3
1
>>> (a or b) and c
3
>>> (a or b) and (c + 1) //两个括号同级，左结合
4
>>> a = 1
>>> b = 2
>>> c = a + b //出现赋值符号时，右结合
>>> print(c)
3
>>> c = a or b
>>> print(c)
1
>>> a = 1
>>> b = 2
>>> c = 2
>>> not a or b + 2 == c
False
>>> ((not a) or ((b + 2) == c)) //优先级：not > and > or
False在文本文件中编写Python代码
python脚本是后缀名为.py的文件，通过命令行“python filename.py”执行
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注释
单行注释用#
多行注释用```
流程控制语句
主要有 条件控制（if else）、 循环控制（for while）、 分支
条件控制（if else)
# encoding: utf-8
mood = False
if mood :
print('go to left')
# print('back away')
# print('back away')
else :
print('go to right')
a = 1
b = 2
c = 2
# if后面不仅可以是布尔值，还可以是表达式
if a or b + 1 == c :
print('go to left')
# print('back away')
# print('back away')
else :
print('go to right')# encoding: utf-8
"""
一段小程序
"""
# constant 常量 建议全大写
ACCOUNT = 'hughie'
PASSWORD = '123456'
# python变量建议都用小写，用下划线分隔单词，不用驼峰命名
print('please input account')
user_account = input()
print('please input password')
user_password = input()
if ACCOUNT == user_account and PASSWORD == user_password:
print('success')
else:
print('fail')# encoding: utf-8
# snippet 片段
if condition:
pass
else:
pass
a = True
if a:
# pass 空语句/占位语句
pass
else:
print('')
if True:
pass
if False:
pass
# 嵌套分支
if condition:
if condition:
pass
else:
pass
else:
if condition:
pass
else:
pass
# 代码块
if condition:
code1
code11
code22
code333
code444
code5555
code6666
code2
code3
else:
code1
code2
code3
改写为elif
# encoding: utf-8
"""
a = x
a = 1 print('apple')
a = 2 print('orange')
a = 3 print('banana')
print('shopping')
"""
a = input()
print('a is' + a)
if a == 1:
print('apple')
else:
if a == 2:
print('orange')
else:
if a == 3:
print('banana')
else:
print('shopping')
# 改写为elif
a = input()
print(type(a))
print('a is ' + a)
a = int(a)
if a == 1:
print('apple')
elif a == 2:
print('orange')
elif a == 3:
print('banana')
else:
print('shopping')循环（while for）
# encoding: utf-8
# 循环
# 循环语句
# while for
# CONDITION = True
# while CONDITION:
# print('I am while')
counter = 1
# 递归常用while
while counter <= 10:
counter += 1
print(counter)
else:
print('EOF')# encoding: utf-8
# 主要是用来遍历/循环 序列或者集合、字典
# a = ['apple', 'orange', 'banana', 'grape']
# for x in a:
# print(x)
# a = [['apple', 'orange', 'banana', 'grape'], (1, 2, 3)]
# for x in a:
# for y in x:
# # print(y, end='')
# print(y)
# else:
# print('fruit is gone')
# a = [1, 2, 3]
# for x in a:
# if x == 2:
# # break 遇到x==2的时候终止，打印出1
# # break
# # continue 遇到x==2的时候跳过，打印出1，3
# continue
# print(x)
# else:
# print('EOF')
a = [['apple', 'orange', 'banana', 'grape'], (1, 2, 3)]
for x in a:
# if 'banana' in x:
# break
for y in x:
if y == 'orange':
# 内部循环跳出后，外部循环还在执行
break
print(y)
else:
print('fruit is gone')# encoding: utf-8
# for (i=0; i<10; i++){}
# 以上的for循环用python实现
# for x in range(0, 10):
# # range(0,10) 表示从0开始的10个数字，并不包含10
# print(x)
# for x in range(0, 10, 2):
# # range(0,10,2) 2表示步长
# print(x, end=' | ')
# # 打印结果：0 | 2 | 4 | 6 | 8 |
for x in range(10, 0, -2):
print(x, end=' | ')
# 打印结果：10 | 8 | 6 | 4 | 2 |# encoding: utf-8
a = [1, 2, 3, 4, 5, 6, 7, 8]
# for i in range(0, len(a), 2):
# print(a[i], end=' | ')
# 1 | 3 | 5 | 7 |
b = a[0:len(a):2]
print(b)
# [1, 3, 5, 7]