When a file is opened with 'w' mode and it exists, what occurs?

When a file is opened in 'w' (write) mode and it already exists, its contents are truncated to zero length. This means that any data previously stored in the file is deleted, and the file is now empty. The 'w' mode is specifically designed for writing data to a file, and as a safety measure, it ensures that you're starting with a clean slate rather than inadvertently appending to or modifying existing content.

In contrast, if the file does not exist, opening it in 'w' mode will create a new file. This behavior emphasizes the importance of understanding file modes in Python to avoid unintentional loss of data when handling files.