Python_Crawler_Foundation2_Key points

Python基礎複習_Key Points 

 

1. 弱類型：不須要爲變量指定類型。（C語言是強類型，必須給變量指定類型。如：int，string等。強類型的好處：方便作語法檢查） 

1 a = 1234
2 print(a)
3 a = ‘abcd’
4 print(a)

View Code

#outcomes:

1234
abcd

2. 變量必須先賦值。Python中變量要先賦值再使用。而C語言中，定義一個變量（int i)，i是有一個初始值。

3. Python是經過引用傳遞變量的。引用變量的地址空間。

4. Python2 和Python3的常見差別

• xrange　　在Python2中，xrange主要是解決python循環中的效率問題。與range相比，xrange使用generator來解決空間效率問題。而在python3中，xrange和range的效率是同樣的。
• print　　    在python2中，無需加括號，但在python3中，要加括號。
• 數據類型統一，取消unicode和long
• utf8：python3代碼默認utf8

  

1. none是True和False以外的一種狀態。能夠等價爲false。

2. del是從容器裏刪除對象。

3. python中只有三種操做：與、或、非。沒有&&，||等。

4. 函數是沒有返回值的，要返回值，需加return

默認參數：

1 def hello(who = 'world'):
2     print('hello %s!' %(who))
3 
4 hello()
5 hello('sea')

View Code

#Outcome:

hello world!
hello sea!

Lambda VS Normal defunction:

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

View Code

#Outcome:

600
11100

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

View Code

#Outcome:

20100

 5. while & for 

1 totoal = 0
2 i = 1
3 while i <= 100:
4     total += i
5     i += 1     #沒有++i或者--i
6 print（total）

View Code

#outcome：

5050

for循環只做用用於容器！！

沒有這種寫法： 

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

 上面這種循環只能用while實現。

1 i = 0
2 while i < 3:
3     j = 0 
4     while j <=3:
5         if j == 2:
6             break / continue       #只退出當前循環，循環繼續執行j=3.。。
7         print(i,j)
8         j += 1
9     i +=1

View Code

#outcomes: 

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

Continue：
0    0
0    1

1. List

   

用lambda對第一元素作排序

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' -  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 

'b'

Open a file 

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

Open a file with 'with'　

>>> with open('workfile') as f:　　#If you’re not using the  keyword, then you should call  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()withf.close()

Read a file:

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


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


>>> 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\n')

 

 

#錯誤處理

 1 import logging    #在生產環境中，最有效的調試方式。
 2 
 3 ''' 4 做業，本身實現將不一樣的等級的信息寫到不一樣日誌文件。
 5 logging.info()            
 6 logging.debug()
 7 ''' 8 
 9 try:
10     r=10/0
11 except ZeroDivisionError as e:        #捕捉異常
12     print(type(e))
13     print(e)
14 finally:                #主要是防止服務端資源泄漏！
15     print('always come here.')