Head First Python Learning Note-chapter3: File read and exception handling

The third chapter mainly introduces simple file reading and simple exception handling operation.

First set up the file folder: HeadFirstPython\chapter3 in the head Pythong official site to download the required files: sketch.txt, and put into the previously built folder.

Related syntax Read file

the_file = open(‘sketch.txt)  # 打开文件，获取到文件对象# 对文件里的数据进行一些处理the_file.close() # 关闭文件

Exception capture

import systry:    # 可能会出现异常的代码    f = open(‘myfile.txt‘)    s = f.readline()    i = int(s.strip())# 对异常进行处理# 相似于Java中的catch块exceptas# 能够指定待捕获的异常    print("OS error: {0}".format(err))except ValueError:    pass# 不做不论什么操作，直接跳过except:    print("Unexpected error:", sys.exc_info()[0])

Get files in an interactive environment

In a python interactive environment (Pyhton idle), you can use some of the following commands for file manipulation:

>>>  Import OS # importing OS from standard library>>> os.getcwd () # Gets the current working folder. Similar toLinuxUnder the PWD 'D: \\program\\Python34 '>>> Os.chdir ("D:\code\python\HeadFirstPython\chapter3") # Switch working folder >>> os.getcwd () 'D: \\code\\python\\Headfirstpython\\chapter3 ' >>> Data = Open('sketch.  TXT ') # Open file, get to file object, equivalent to an iterator iterator>>> Print ( data. ReadLine(), end= "") # reads a line of files,data. ReadLine() Mans: isThis is the right argument?>>> print ( data. ReadLine(), end= "") Other Mans:I ' veTold you Once.>>> data. Seek(0) # to get data back to the beginning of the file0# Use for loop to get each line of the file>>> for Eachlineinch  data:Print (eachline,end="")

Further processing of the data: exception handling

Look at the data in the file and find that each line is delimited with ":". Therefore, the optimization is considered at the output. A corresponding problem occurs when the file is processed. For example, some of the lines in the file do not contain ":", then run throws an ValueError exception, when the file does not exist or read failure, it will be thrown IOError , and so on. Face these anomalies. There are two ways to deal with this:
1. Consider the possible anomalies in the program in advance and deal with them in order to avoid the occurrence of anomalies.
2. Use the exception capture mechanism: Let the exception occur, but the exception is captured. Capture and then do the related operations.

The first way of thinking:

import  osif  os.path.exists (): data = open () for  each_line in  data: Span class= "Hljs-keyword" >if  each_line.find ( ': ' )! =- 1 : (role, Line_spoken) = Each_line.split ( ': ' , 1 ) print (role, End=) print (  ' said: ' , End= ") print (Line_spoken,end=") data.close () else : Print (  file does not exist! 
 
  
‘)

Another way of thinking: catching Exceptions:

try: data = open(‘sketch2.txt‘) for each_line in data: # if each_line.find(‘:‘) != -1: try: (role, line_spoken) = each_line.split(‘:‘, 1) print(role, end=‘‘) print(‘ said: ‘, end=‘‘) print(line_spoken,end=‘‘) except: pass # 不做不论什么操作。直接跳过 data.close()except: print(‘文件不存在‘)

Related knowledge points

1, split() : The string is cut. The function prototype is str.split(sep=None, maxsplit=-1) , it consists of two parameters, the first is the use of the cut, the second is the maximum number of cuts. Such as:

>>>' a '. Split (', ')[' 1 ',' 2 ',' 3 ']>>>' a '. Split (', ', maxsplit=1)[' 1 ',' 2,3 ']>>>' 1,2,,3, '. Split (', ')[' 1 ',' 2 ',"',' 3 ',"']>>>' 1 2 3 '. Split () [' 1 ',' 2 ',' 3 ']>>>' 1 2 3 '. Split (maxsplit=1)[' 1 ',' 2 3 ']>>>' 1 2 3 '. Split () [' 1 ',' 2 ',' 3 ']

2 open() : Used to read the file, create an iterator at the same time. Ability to read files by line
3. readline() : Read a line of the file
4 seek() : The iterator again points to the beginning of the file (the first line)
5. close() : Close Open File
6 find() : Find the position of the substring in the string. Does not exist then returns-1

Head First Python Learning Note-chapter3: File read and exception handling