> For the complete documentation index, see [llms.txt](https://drlilianblot.gitbook.io/introduction-to-programming-with-python/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://drlilianblot.gitbook.io/introduction-to-programming-with-python/file-i-o.md).

# 11 - File I/O

## Persistence

Most of the programs we have seen so far are transient in the sense that they run for a short time and produce some output, but when they end, their data disappears. If you run the program again, it starts with a clean slate.

Other programs are **persistent**: they run for a long time (or all the time); they keep at least some of their data in permanent storage (a hard drive, for example); and if they shut down and restart, they pick up where they left off. 

Examples of persistent programs are operating systems, which run pretty much whenever a computer is on, and web servers, which run all the time, waiting for requests to come in on the network. 

One of the simplest ways for programs to maintain their data is by reading and writing text files. Text files are a fundamental data storage and communication medium in computer science. These files contain human-readable text in the form of characters and are widely used for various purposes, ranging from data storage and configuration to data interchange.

{% embed url="<https://youtu.be/OJu9Ecdz_HE>" %}