What Does Python\'s File.flush() Method Do Under the Hood?

What is Python's File.flush() Method Actually Doing?

Python's documentation states that flush() doesn't necessarily write file data to disk. This may seem contradictory to its intended purpose, as one would expect flush() to force data to the disk.

Buffering Levels

To understand flush(), we need to consider two buffering levels:

1. Internal buffers: Created by Python's runtime and used to speed up write operations by temporarily storing data before writing it to the file.
2. Operating system buffers: Created by the operating system to buffer data before sending it to the actual storage device.

flush() and Internal Buffers

When calling flush(), it only writes data from the internal buffers to the file object managed by the operating system. This means that the data may still not be written to the disk.

os.fsync() and Operating System Buffers

To ensure data is written to the disk, we need to use the os.fsync() method after calling flush(). os.fsync() writes data from the operating system buffers to the storage device, ensuring durability.

When to Use flush() and fsync()

In most cases, flush() and fsync() are not necessary, as Python's default behavior is to write data to disk when the buffer is full or when the file is closed. However, they can be useful in scenarios where it's crucial that data is immediately written to the disk for reliability and data integrity purposes.

Additional Note

With the advent of cached disks, there may be even more layers of buffers involved. It's assumed that flush() and fsync() handle these buffers as well, but further research may be necessary for confirmation.

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