Append text to a file in Python

Open a text file with mode 'a' to add content at the end without truncating existing data. A with statement closes the file when the block ends, including when an exception is raised.

with open('app.log', 'a', encoding='utf-8') as file:
    file.write('A new log entry\n')

Append mode creates the file if it does not exist, but it does not create missing parent directories. write() does not add a newline for you.

Choose the right mode

Use binary modes such as 'ab' when writing bytes, and omit the text encoding argument.

Append several lines

lines = ['First line', 'Second line']
with open('data.txt', 'a', encoding='utf-8') as file:
    file.writelines(line + '\n' for line in lines)

writelines() does not insert separators. Supplying each newline explicitly keeps the resulting file readable.

Flush buffers when timing matters

import os

with open('events.log', 'a', encoding='utf-8') as file:
    file.write('Critical event\n')
    file.flush()             # Move Python-buffered data to the OS
    os.fsync(file.fileno())   # Request filesystem synchronization

Closing the file also flushes Python’s buffers, but flush() alone is not a guarantee that data has reached persistent storage. Synchronization has a cost; choose it according to durability requirements.

FAQ

Why can I not read the content I just appended?

Mode 'a' is write-only. With 'a+', flush pending writes and call file.seek(0) before reading from the beginning. Reading at the end of the file returns an empty string even when the data was written successfully.

Is appending from several processes safe?

Do not assume one Python text write is an atomic logical record across every filesystem. Buffering, write sizes, and network filesystems affect behavior. Use a coordinated writer, a logging queue, or appropriate interprocess locking when records must not interleave.

See Python built-in functions, Python operating-system interfaces.