Why Python files are not written until the program stops

In this article, we’ll explain why, when writing to a file in a Python program, the file contents aren’t immediately written to disk. Instead, they’re written only after the program stops.

Read more: Python Tutorial

How File Writing Works

In Python, when writing to a file, you typically use the write method to write data to the file and the close method to close the file. However, this doesn’t mean the data is immediately written to disk. In fact, Python uses a buffering mechanism to improve the efficiency of file reading and writing.

File Buffering

File buffering temporarily stores part of a file’s contents in memory and then writes the buffered data to disk all at once. This mechanism reduces frequent disk operations, thereby improving I/O performance.

Python has two file buffering modes: full buffering mode and line buffering mode. Full buffering mode stores data in a memory buffer until it is full, and then writes it to disk. Line buffering mode stores data in a memory buffer until a newline character is encountered or the buffer is full.

File Object Flushing

To control the timing of buffer flushing, we can use the file object’s flush method to manually flush the buffer. When the flush method is called, Python immediately writes the buffered data to disk.

The following code demonstrates the file flushing process:

f = open("test.txt", "w")
f.write("Hello, World!")
f.flush() # Flushes the buffer and writes immediately to disk
f.close() # Closes the file

In the above code, we open a file “test.txt” and write the contents to the buffer. Then, we call the flush method to flush the buffer and close the file. By manually flushing the buffer, we ensure that the data is written immediately to disk.

Automatic Flushing of File Objects

In addition to manually flushing the buffer, we can also enable automatic buffer flushing by setting the buffering parameter of the write method of the file object. When the buffering parameter is set to 1, line buffering is used, and data is flushed to disk when a newline character is encountered or the buffer is full. When the buffering parameter is set to 0, unbuffered mode is used, and data is written immediately to disk. When the buffering parameter is set to greater than 1, full buffering is used, and data is flushed to disk when the buffer is full.

The following code example demonstrates how to use the buffering parameter to automatically flush the buffer:

f = open("test.txt", "w", buffering=1)
f.write("Hello, World!")
f.close() # The file automatically flushes the buffer before closing.

In the above code, we open a file “test.txt” and set the buffering parameter to 1. We then write data directly to the buffer and close the file. Because the auto-flush buffer parameter is set, the file buffer is automatically flushed before closing, ensuring that the data is written to disk.

How to Ensure Immediate File Writing

In some cases, we need to ensure that the file contents are written to disk immediately without waiting for the program to terminate. In addition to the previously described methods of manually flushing the buffer and setting the auto-flush buffer, we can also achieve this by using the with statement. The with statement automatically closes the file and flushes the buffer after the code block executes.

The following code demonstrates how to use the with statement to ensure that a file is written to disk immediately:

with open("test.txt", "w") as f:
f.write("Hello, World!") # The file automatically flushes the buffer after the code block completes.

In the above code, we use the with statement to open the file and write data to the buffer. After the code block completes, the file is automatically closed and the buffer is flushed, ensuring that the data is written to disk immediately.

Summary

This article explains why, in Python programs, files are written only after the program stops. This is because Python uses a file buffering mechanism to improve I/O performance. To control the flushing of the buffer, you can manually call the flush method or set the automatic buffer flushing parameter. Alternatively, using the with statement ensures that files are written to disk immediately. By understanding the file buffering mechanism, we can better handle file writing operations and improve program performance and stability.