How to turn on line numbers in IDLE?
Version 3.8 or newer:
To show line numbers in the current window, go to Options and click Show Line Numbers.
To show them automatically, go to Options > Configure IDLE > General and check the Show line numbers in new windows box.
Version 3.7 or older:
Unfortunately there is not an option to display line numbers in IDLE although there is an enhancement request open for this.
However, there are a couple of ways to work around this:
Under the edit menu there is a go to line option (there is a default shortcut of Alt+G for this).
There is a display at the bottom right which tells you your current line number / position on the line:
There's a set of useful extensions to IDLE called IDLEX that works with MacOS and Windows http://idlex.sourceforge.net/
It includes line numbering and I find it quite handy & free.
Otherwise there are a bunch of other IDEs some of which are free: https://wiki.python.org/moin/IntegratedDevelopmentEnvironments