Python file operation differences in read, write, append

The common patterns for opening files are:

• R, read-only mode "default"
• W, write-only mode "unreadable; not exist" created; empty content;
• A, append mode "readable; not present, create; append content only;"

"+" means you can read and write a file at the same time

• r+, read and write "readable, can write" "can be understood as read before writing, do not erase the original file content, the pointer in 0"
• w+, write read "Readable, can write" "can be understood as read first, erase the original file content, the pointer in 0"
• A +, write read "Readable, can write" "does not erase the original file content, but the pointer directly to the last, read the original content first reset the pointer"

Mode	can do operation	If the file does not exist	whether to overwrite	pointer position
R	Read Only	Error	-	0
r+	Readable and writable	Error	Whether	0
W	can only write	Create	Is	0
w+	can be written and readable	Create	Is	0
A	can only write	Create	No, append write	At last
A +	Readable and writable	Create	No, append write	At last

You can make a test file, modify the next open mode, and then output look at the pointer difference

f=open(‘I:\\python\\test\\text.txt‘,‘r+‘)print(‘指针在:‘,f.tell())lines=f.read()if f.writable():    f.write(‘nono\n‘)else:    print("此模式不可写")print(‘指针在:‘,f.tell())f.close()

A + mode, although can read, but the pointer is to the end, direct read, will not be content, you can use Seek () to reset the pointer

f=open(‘I:\\python\\test\\text.txt‘,‘a+‘)print(‘指针在:‘,f.tell())lines=f.read()print(‘文件内容是：‘,lines)  #输出为空print(‘seek 0‘)f.seek(0)print(‘指针在:‘,f.tell())lines=f.read()print(‘文件内容是：‘,lines)if f.writable():    f.write(‘nono\n‘)else:    print("此模式不可写")print(‘指针在:‘,f.tell())f.close()

Python file operation differences in read, write, append