How to append a file in Python

This article shows how to append a text file in Python.

Table of contents

• 1. Append to a file
• 2. Difference between a and a+
• 3. Download Source Code
• 4. References

P.S Tested with Python 3.8

1. Append to a file

The a creates a new file or opens an existing file for writing; the file pointer position at the end of the file, aka append mode.

test 1
test 2
test 3

with open('file-new.txt', 'a') as f:
    f.write("a new line")   # append this string to the file

Output

test 1
test 2
test 3
a new line

2. Difference between a and a+

The a+ adds reading to the a mode, aka read append mode.

• The a creates a new file or opens an existing file for writing; the file pointer position at the end of the file.
• The a+ creates a new file or opens an existing file for reading and writing, and the file pointer position at the end of the file.

2.1 In open mode a, if we read the file, Python throws io.UnsupportedOperation: not readable.

with open('file.txt', 'a+') as f:
    f.readlines()           # read it? `a` mode is only for writing
    f.write("a new line")   # append this string to the file

Output

io.UnsupportedOperation: not readable  

2.2 The below example uses a+ to open a file for reading and writing; it read a file, counts the number of lines, and append the result to the file.

test 1
test 2
test 3

with open('file.txt', 'a+') as f:
    f.seek(0)                       # a+ file pointer at end, move to beginning
    lines = f.readlines()
    f.write("\n" + str(len(lines))) # append number of lines to a file

Output

test 1
test 2
test 3
3

3. Download Source Code

4. References

• Python doc – open
• How to read a file in Python
• How to write a file in Python
• Python difference between r+, w+ and a+ in open()
• Stackoverflow – Confused by python file mode "w+"