Python：语句表达式（code structure）

Python：语句表达式（code structure）

1. 赋值

普通赋值

多重赋值

a, b = b, a

‘,’

2. 输入语句

raw_input()

：

input()

：

3. 输出语句

python2：

print a,b

print>>file,a,b

print '%d,%d,%s'%(a,b,c)

print "{0} like {1}".format('we','python')

print "{a} like {b}".format(a='we',b='python')

python3：

print([obj, ...] [,sep=''] [,end='\n'] [,file=sys.stdout])

4. 条件语句

语法：

#---------语句中的xxx代表逻辑表达式
if xxx:
statements1
elif xxx:
statements2
else:
statements3

其他

- 逻辑表达式
- `not`
- `and`：返回第一个`false`或者最后一个`true`
- `or`：返回第一个`true`或者最后一个`false`
- 三元表达式
- `a=y if x>o else z`

5. 循环语句

while/else

语句

while xxx:
statements1
if xxx:
break/continue
else:
statements2   #如果上面的循环体是自然结束而不是break跳出的话就会执行这条else里的内容

for

语句

for x in objects:
statements1
if xxx:break/continue
else:
statements2

一些循环体运用的例子

#!/usr/bin/env python
# coding: utf-8
#copyRight by heibanke

# 打印［10-0]之间的偶数
x = 10
while x:
if x % 2 == 0: # Even?
print x
x = x-1

# 判断是否素数(质数)：大于1的自然数，除1和它本身外没有整数能够整除
y=input()
x = y // 2
while x > 1:
if y % x == 0:
print y, 'has factor', x
break
x -= 1
else: # Normal exit
print y, 'is prime number'

####################### for ###########################
sum = 0
for x in [1, 2, 3, 4]:
sum += x

S = u"你在远方"
T = (u"我", u"在", u"故乡")
for x in S: print x,
for x in T: print x,

file = open('test.txt','w')
D = {'a': 1, 'b': 2, 'c': 3}
for key in D:
print >>file, key, '=>', D[key]

file.close()

#####################################################

items = [u"小方", u"小白", u"小三", u"小王"] # A set of objects
key = u"小李" # Keys to search for

for item in items: # For all items
if item == key: # Check for match
print key, "was found"
break
else:
print key, "not found!"

#####################################################
file = open('test.txt')
while True:
line = file.readline() # Read line by line
if not line: break
print line.strip() # Line already has a \n

file.close()

for line in open('test.txt'):
print line.strip()

6. 列表解析

List Comprehensioins

[ expression for item in iterable ]

[ expression for item in iterable if condition]

rows = range(1,4)
cols = range(1,3)
cells = [(row, col) for row in rows for col in cols]
for cell in cells:
print(cell)

#最后的输出结果：
#(1,1)
#(1,2)
#(2,1)
#...就不全列出来了

for

if

Dictionary Compresion

{key_expression: value_expression for expression in iterable}

if

word = 'letters'
letter_counts = {letter :  word.count(letter) for letter in word}
print letter_counts
#结果是：{'l':1, 'e':2, 't':2, 'r':1, 's':1}

'letters'

e

t

count()

letter_counts = {letter :  word.count(letter) for letter in set(word)}
#这时再打印letter_counts得到的结果是：{'t':2, 'l':1, 'e':2, 'r':1, 's':1}

set()

Key

set()

Set Comprehensions

{expression for expression in iterable}

Generator Comprehensions

number_thing = (number for number in range(1, 6))

type(number_thing)

<class 'generator'>

7. 异常处理

基本形式为：

try:
statements 1
except:
statements 2

expect

expect expectiontype as name

8. 作业

9. 作业答案

#!/usr/bin/python
#coding:utf-8

import random

num = random.randint(0, 100)
count = 6
flag = True

while count:
flag = True
while flag:
try:
input_num = input("0-100,please choose a number[q to quit]\n")
flag = False
except:
print 'something wrong,you should input a number'
continue
if input_num > num:
print 'you num is too big,', 'chances left: ', count-1
elif input_num < num:
print 'you num is too small,', 'chances left: ', count-1
elif str(input_num) == r'num':
print 'please do not cheat, ', 'chances left: ', count-1
else:
print 'you did a good job'
break
count -= 1
else:
print 'it a pity! game over~~ and secret number is ', num

#!/usr/bin/python
#coding:utf-8
#copyRight by TristanHuang

import random

num = random.randint(0, 100)
count = 6

while count:
try:
input_num_str = raw_input("0-100,please choose a number[q to quit]\n")
if input_num_str == 'q':
break
input_num=int(input_num_str)
except:
print 'something wrong,you should input a number'
continue

if input_num > num:
print 'you num is too big,', 'chances left: ', count-1
elif input_num < num:
print 'you num is too small,', 'chances left: ', count-1
else:
print 'you did a good job'
break
count -= 1
else:
print 'it a pity! game over~~ and secret number is ', num

#!/usr/bin/env python
#coding: utf-8
#__author__ = 'Tristan'

#初始化账号数据缓存
data = list()

#先读取账号文件的所有记录
f = open('account.txt', 'r')
for line in f:
a = line.split(',')   #读取一行的账号和密码
data.append(a[0])     #记录账号到缓存中
print data

character = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_'
first_c = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'

while True:
print u"--------注册--------\n"\
+ u"请输入用户名（首字母必须为字母，最小长度为4, 大小写字母,数字,_,-,的组合）\n"
name =raw_input()

accountValid = True

#核查用户名的有效性
if len(name) < 4:
print u"你的输入%s不够4位, 请重新输入!" % (name)
accountValid = False
elif not name[0] in first_c:
print u"你的输入%s首字符必须是字母, 请重新输入!" % (name)
accountValid = False
else:
#检查是否已存在
if name in data:
print u"这个用户名%s已经存在，请重新输入!"%(name)
valid = False

flag = True
# 密码是否有效
while flag and accountValid:
print u"该用户名可以使用，请输入密码(最小长度为6, 大小写字母,数字,_,-,的组合):\n"
password = raw_input()

#检查是否有效
if len(password)<6:
print u"你的输入密码%s不够6位, 请重新输入!"%(password)
continue
for a in password:
if not a in character:
print u"你的输入密码%s有非法字符, 请重新输入!"%(password)
flag=True
break
else:
flag = False

#写入账号信息
if accountValid and (not flag):
file = open('account.txt', 'a')
#md5数据加密
import hashlib
password_md5 = hashlib.md5(password).hexdigest()
file.write(name+','+password_md5+'\n')
file.close()
print u"恭喜你，注册成功"
break