I'm getting KeyError: ' ' in my code and I don't see why

Question

It runs well but when I close the window a keyError: ' ' appears. It is getting the values from the key selected but I don´t see why is giving me that. I would appreciate some help here. I clarify that the dictionaries are string:string and I'm concatenating to get a string variable to then do some operations. I add that combo1 and combo2 are strings fixed. The issue is that it concatenate in var2 and works but after that appear this error.

from tkinter import *
from tkinter import ttk
import sqlite3    

class dd:
   def __init__(self, window):
       self.wind = window

       def setvalue(event):
             var = str(self.combo1.get()) + "-" + str(self.combo2.get()) + "-"
             var2 = self.get_eln()[self.combo3.get()] + "-" + self.get_esr([self.combo4.get()]
             self.var3 = var + var2

       frame = LabelFrame(self.wind, text='Data', height = 50, width = 1000, bd=4)
       frame.grid(row=0, column=0, sticky=W+E)
       frame.grid_propagate(0)


       #Combo box 3
       self.combo3 = ttk.Combobox(frame, value=list(self.get_eln().keys()), width=60)
       self.combo3.bind("<<ComboboxSelected>>", setvalue)
       self.combo3.grid(row=1, column=4, pady=4, sticky=W)

       #Combo box 4
       self.combo4 = ttk.Combobox(frame, value=list(self.get_esr().keys()),width=20)
       self.combo4.bind("<<ComboboxSelected>>", setvalue)
       self.combo4.grid(row=1, column=6, pady=4, sticky=W)

   def run_query(self, query, parameters = ()):
       with sqlite3.connect(self.db) as conn:
           cursor = conn.cursor()
           result = cursor.execute(query, parameters)
           conn.commit()
       return result

   def get_eln(self):
       query = 'SELECT Eln, E_a FROM Elns'
       db_rows = self.run_query(query)
       return {Eln:E_a for Eln, E_a in db_rows}

   def get_esr(self):
       query = 'SELECT Esr, Est_a FROM Ess'
       db_rows = self.run_query(query)
       return {Esr:Est_a for Esr,Est_a in db_rows}


if __name__ == '__main__':
   window = Tk()
   application = a(window)
   window.mainloop()

When I close the window appears this:

Exception in Tkinter callback
Traceback (most recent call last):
  File "Python\Python38-32\lib\tkinter\__init__.py", line 1883, in __call__
    return self.func(*args)
  File "code.py", line 6, in setvalue
    var2 = self.get_eln()[self.combo3.get()] + "_" + self.get_esr()[self.combo4.get()]
KeyError: ''
Exception in Tkinter callback
Traceback (most recent call last):
  File "C:Python\Python38-32\lib\tkinter\__init__.py", line 1883, in __call__
    return self.func(*args)
  File "code.py", line 6, in setvalue
    var2 = self.get_eln()[self.combo3.get()] + "_" + self.get_esr()[self.combo4.get()]
KeyError: ''

I believe the error is in the concatenation of var 2.

Answer 1

Like I said in the comment, the problem seems to be from trying to get the value from the dict returned from self.get_eln() and self.get_esr() when the self.combo3 or self.combo4 has no value.

I guess you could either check if both have values before trying to do so like this:

if self.combo3.get() and self.combo4.get():
    var2 = self.get_eln()[self.combo3.get()] + "-" + self.get_esr([self.combo4.get()]
else:
    var2 = '' # not sure which value you want for this var

and this way you ensure you don't get a KeyError .

Otherwise, if you wish to follow another approach with try ... except you could always do something like

try:
    var2 = self.get_eln()[self.combo3.get()] + "-" + self.get_esr([self.combo4.get()]
except KeyError:
    # make sure you don't want to 
    var2 = ''

I don't want to give you a simple copy-paste answer, so if you don't understand something, just comment and I'll try to help!