Detailed explanation of the usage of the open() function in Python to specify the file opening method

When we use the open() function to open a file, there are several opening modes.

'r'->Read only

'w'->Write only, clear the file if it already exists, create it if it does not exist.

'a'->Append, write to the end of the file

'b'->Binary mode, such as opening images, audio, and word files.

'+'->Update (readable and writable)

This one with the '+' sign is a bit difficult to understand, go to the code to get a feel for it.

with open('foo.txt', 'w+') as f: 
    f.write('bar\n') 
    f.seek(0)  
    data = f.read()

You can see that the above code can not only be written, but also read. Note that you must first locate the beginning, f.seek(0), otherwise you will read empty data.

Some people may be confused. Since the '+' sign is readable and writable, what is the difference between 'w+' and 'r+'.

That is,

'w+' will clear and create (the file will be cleared if it already exists, and created if it does not exist.)

'r+' will not clear or create

Do not open the text file in binary mode

First Look at the "weird" phenomenon in the code below.

Suppose under Windows, I have a f.txt file with the following content.

hello

world

code one,

with open('f.txt', 'r') as f: 
    print f.readlines() 
with open('f.txt', 'rb') as f: 
    print f.readlines()

output

['hellon', 'worldn']

['hellorn', 'worldrn']

code two,

with open('f.txt', 'rb') as f: 
    data = f.read() 
with open('f.txt', 'w') as f: 
    f.write(data)

open The file becomes like this,

hello^M

world^M

First, understand the concepts of line feed character 'n' and carriage return character 'r'.

'n', line feed character (LF, Line-Feed), refers to a new line.

'r', carriage return (CR, Carriage-Return), refers to returning to the beginning of the line.

Because the line break flags under different systems are different.

windows->'rn'

unix->'n'

mac->'r'

This is why txt under windows has '^M' at the end of the line when it is opened in linux.

This is why I ran a script under Linux to export game data and opened it in local windows and it became one line.

In fact, text files are also binary files, which are text-encoded binary files. The text files process some invisible characters to increase readability.

In python, you can get the newline identifier of the current system through os.linesep. For example, under Windows, os.linesep is 'rn'.

When operating the newline flag in python, it doesn’t matter what platform it is on, just use 'n'. Python will automatically convert it to different flags according to different systems.

With the above theoretical basis, we can analyze the "weird" phenomenon of the code at the beginning of this article.

In code 1, if the file is opened in text mode, the newline flag will be processed by python into 'n', while if it is opened in binary mode, it will remain unchanged.

In code two, open in binary mode and write in text mode. When opened in binary, it is still 'rn', and when writing in text mode, python will convert 'n' into 'rn', so it is actually equivalent to writing 'rrn', so there is an extra '^M '.