In PyCharm, there is a vertical line as highlighted on the screenshot.
When I copy a long line of code into the editor, it wraps automatically if longer than this line.
What is this line and what's its purpose?
That’s the wrap guide. It is a visual indicator of when your lines of code are getting too long. It defaults to the column that your Preferred Line Length is set to.
To deactivate the line:
Settings->Editor->Appearance then deselect "Show right margin".