Editor for the chest alt-items table of A Link to the Past (no header). The table can also be edited with an hex editor but two drop-down menus should be far easier to use…

Upon opening a chest, the game checks whether the player has the main item, if not they receive the main item, if yes they receive the alt item.

In the regular game, this is for instance used for the Red Boomerang, since it can be obtained from a fairy fountain or a chest. In case, the player got it from the fairy, the chest rewards 300 rupees instead.

This is a Python (2 or 3) script using Tkinter (which is part of the Python standard library, so no additional GUI toolkit packages are needed).

The address of the table has been provided by qwertymodo. The item names have been taken from Hyrule Magic. See also here and here.