r+ mode in files

[grkiran2011]

Hi Viewer,

I know r+ mode is used for reading and writing to a file. What I find strange is as soon as I open the file and then write to the file instead of reading, it writes from the first position. If I read first and then write to the file, it writes at the end of the file. Please see the code below:

test.data (created the file before running the code):

Welcome to Python line 1
Welcome to Python line 2
Welcome to Python line 3
Welcome to Python line 4

with open(file = 'test.data', mode = 'r+', encoding = 'utf-8') as fp:
    fp.write('new line\n')

The file gets modified as follows,

Output:
new line
 Python 1
Welcome to Python 2
Welcome to Python 3
Welcome to Python 4

Now I re-create the original text file test.data. And now when I read just first line and then write to it, it writes to the end of the file.

with open(file = 'test.data', mode = 'r+', encoding = 'utf-8') as fp:
    print(fp.readline())
    fp.write('new line\n')
    print(fp.readline())

The text file after the program executes:

Output:
Welcome to Python 1
Welcome to Python 2
Welcome to Python 3
Welcome to Python 4
new line

The following is the output from the above line of code:

Output:
Welcome to Python 1

I am unable to understand why Python does write to the file at the end of the file instead of at the location of the file pointer.

Please help.

Thanks.