Python_Crawler_Foundation2_Key points

Python基础复习_Key Points 

 

1. 弱类型：不需要为变量指定类型。（C语言是强类型，必须给变量指定类型。如：int，string等。强类型的好处：方便做语法检查） 

a = 1234
print(a)
a = ‘abcd’
print(a)

#outcomes:

1234
abcd

2. 变量必须先赋值。Python中变量要先赋值再使用。而C语言中，定义一个变量（int i)，i是有一个初始值。

3. Python是通过引用传递变量的。引用变量的地址空间。

4. Python2 和Python3的常见差异

• xrange　　在Python2中，xrange主要是解决python循环中的效率问题。与range相比，xrange使用generator来解决空间效率问题。而在python3中，xrange和range的效率是一样的。
• print　　    在python2中，无需加括号，但在python3中，要加括号。
• 数据类型统一，取消unicode和long
• utf8：python3代码默认utf8

  

1. none是True和False之外的一种状态。可以等价为false。

2. del是从容器里删除对象。

3. python中只有三种操作：与、或、非。没有&&，||等。

4. 函数是没有返回值的，要返回值，需加return

默认参数：

def hello(who = 'world'):
    print('hello %s!' %(who))

hello()
hello('sea')

#Outcome:

hello world!
hello sea!

Lambda VS Normal defunction:

def g(x):
    return x*5
def f(gf,x):                 # gf has got the memory address of g(x), no matter what it is called.
    return gf(x)+100    # gf(x) call g(x) and pass the argument x to g(x).
print(f(g,100))            # g -> pass the memory address of g(x) to the argument.        
print(f(lambda x:x*110,100))   
# process: 1. lambda generate a memory address for itself (x*110)
#2. call f(g,100) and pass the memory address of lambda to the argument gf.
#3. gf(x) call lambda x*110 and pass 100 to x, so the result of lambda equals 11100 

#Outcome:

600
11100

def f(gf,x,y):
    return gf(x,y)+100
print(f(lambda x,y:x*y,100,200))

#Outcome:

20100

 5. while & for 

totoal = 0
i = 1
while i <= 100:
    total += i
    i += 1     #没有++i或者--i
print（total）

#outcome：

5050

for循环只作用用于容器！！

没有这种写法： 

for (i = 0; i<100; ++i)
       pass

 上面这种循环只能用while实现。

i = 0
while i < 3:
    j = 0 
    while j <=3:
        if j == 2:
            break / continue       #只退出当前循环，循环继续执行j=3.。。
        print(i,j)
        j += 1
    i +=1

#outcomes: 

Break:
0    0
0    1
1    0    
1    1
2    0
2    1　　

Continue：
0    0
0    1

1. List

   

用lambda对第一元素做排序

切片

2. String

  

 

Mode

‘r’ – Read mode which is used when the file is only being read 
‘w’ – Write mode which is used to edit and write new information to the file (any existing files with the same name will be erased when this mode is activated) 
'b' - 'b' appended to the mode ('rb','rb+','wb','wb+') opens the file in binary mode
‘a’ – Appending mode, which is used to add new data to the end of the file; that is new information is automatically amended to the end 
‘r+’ – Special read and write mode, which is used to handle both actions when working with a file 

Open a file 

>>> f = open('workfile', 'w')

Open a file with 'with'　

>>> with open('workfile') as f:　　#If you’re not using the with keyword, then you should call f.close() to close the file and immediately free up any system resources used by it.
...     read_data = f.read()
>>> f.closed　　　　#If you don’t explicitly close a file, Python’s garbage collector will eventually destroy the object and close the open file for you, but the file may stay open for a while.
#with as equals:
try:
　　f = open('workfile','r')
　　for line in f.readlines():
　　　　print(line)
except:
　　...
finnally:
　　f.close()

Read a file:

>>> f.read()                #Read a whole file.
#Outcome:
'This is the entire file.
'


>>> f.readline()           #Read a line of the file
#Outcome:
'This is the first line of the file.
'
>>> f.readline()
#Outcome:
'Second line of the file
'               


>>> for line in f:          #Read a file line-by-line
...     print(line, end='')
#outcomes:
This is the first line of the file.
Second line of the file

Write a file:

f.write('This is a test
')

#错误处理

import logging    #在生产环境中，最有效的调试方式。

''' 4 作业，自己实现将不同的等级的信息写到不同日志文件。
logging.info()            
logging.debug()
''' 8 
try:
    r=10/0
except ZeroDivisionError as e:        #捕捉异常
    print(type(e))
    print(e)
finally:                #主要是防止服务端资源泄漏！
    print('always come here.')