Head First Python Chapter3: file read and exception handling

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

Start by creating a file directory: HeadFirstPython\chapter3 Download the file you need to use at head first Pythong official website: sketch.txt, and put it in the previously built directory.

Related syntax Read file

the_file = open(‘sketch.txt)  # 打开文件，获取到文件对象# 对文件中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# 类似于Java中的catch块exceptas# 可以指定待捕获的异常    print("OS error: {0}".format(err))except ValueError:    pass# 不做任何操作，直接跳过except:    print("Unexpected error:", sys.exc_info()[0])

Get files in an interactive environment

In the Python interactive environment (Pyhton idle), you can use some of the following commands to do some file manipulation:

>>>  Import OS # importing OS from standard library>>> os.getcwd () # Gets the current working directory, similar toLinuxUnder the PWD 'D: \\program\\Python34 '>>> Os.chdir ("D:\code\python\HeadFirstPython\chapter3") # Switch working directory >>> 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 ":", so consider optimizing at the output. When the file is processed, there will be a corresponding problem, such as some lines in the file does 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. There are two ways to deal with these anomalies:
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, captured and then related operations.

The first way of thinking:

import osif os.path.exists(‘sketch.txt‘):    data = open(‘sketch.txt‘)    forin data:        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(‘文件不存在！‘)

Second idea: 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 # 不做任何操作，直接跳过 data.close()except: print(‘文件不存在‘)

Related knowledge points

1, split() : The string is split, the function prototype is str.split(sep=None, maxsplit=-1) , it contains two parameters, the first is to use the separator, the second is the maximum number of splits. 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, while creating an iterator, you can read the file by line
3. readline() : Read a line of the file
4 seek() : Re-point the iterator to the beginning of the file (first line)
5. close() : Close Open File
6 find() : Finds the position of the substring in the string, does not exist then returns-1

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

Head First Python Chapter3: file read and exception handling