def commandComposer(input1, action, input2):
    composer1 = {"first":"A", "second":"B", "third":"C"}
    composer2 = {"first":"A", "second":"B"}
    commandDict = {"action1":f"TEST{composer2[input1]}{input2:02X}", "action2":f"TEST{composer1[input1]}{input2:04X}"}

    if action == "action1":
        print(commandDict["action1"].format())

    elif action =="action2":
        print(commandDict["action2"].format())

commandComposer("first", "action2", 0x0)

commandComposer("second", "action2", 0x0)

# the line below return key error for third
# commandComposer("third", "action2", 0x0)

# if i modify  "composer2 = {"first":"A", "second":"B", "third":0}" 
# in thiss way work correctly