First, I use echo 'hello,' >> a.txt to create a new file with one line looks like that. And I know \n is at the last of the line.

Then I get some data from python, for example "world", I wish to append "world" at the first line, so I use the python code below:

f = open('a.txt','a')
f.write("world\n")
f.flush()
f.close()

And, here is the result. I know the start point for python to write is at the next line, but I don't know how to fix it.

To overwrite the previous file contents you need to open it in 'r+' mode, as explained in this table. And to be able to seek to arbitrary positions in the file you need to open it in binary mode. Here's a short demo.

qtest.py

with open('a.txt', 'rb+') as f:# Move pointer to the last char of the filef.seek(-1, 2)f.write(' world!\n'.encode())

test

$ echo 'hello,' >a.txt
$ hd a.txt 
00000000  68 65 6c 6c 6f 2c 0a                              |hello,.|
00000007
$ ./qtest.py
$ hd a.txt 
00000000  68 65 6c 6c 6f 2c 20 77  6f 72 6c 64 21 0a        |hello, world!.|
0000000e