title: TypeError: can only concatenate str (not "set") to str
body: <p>Basically i am trying to use the <code>Entry box</code> i made in tkinter as <code>input</code>, to pass a <code>value</code> to my Signal Generator. But i get the error mentioned in the title. If i pass the <code>value</code> through the terminal though, it works without a problem, so it's probably an issue with tkinter, and not with the Instrument ( Rohde and Schwarz SMB100A).</p>
  
  <p>I tried passing the value as <code>string</code>, as the error suggests, but no luck.</p>
  
  <pre class="lang-py prettyprint-override"><code>import visa
  import tkinter as tk
  
  
  rm = visa.ResourceManager()
  print(rm.list_resources())
  inst = rm.open_resource('TCPIP::192.168.100.200::INSTR')
  
  #This one would work, after i bind the function to the button,
  #i just press it and it passes the preset value. 
  #All i want to do is pass a value through the Entry widget, 
  #instead of having a set one.
  #freq = str(250000) 
  #def freqset_smb100a():
      #inst.write("SOUR:FREQ:CW " + freq)
  
  inst.write("OUTP ON")
  
  def freqset_smb100a():
      inst.write(f"SOUR:FREQ:CW " + {str(input_var.get())})
  
  
  
  HEIGHT = 400
  WIDTH = 600
  
  root = tk.Tk()
  
  input_var = tk.StringVar()
  
  canvas = tk.Canvas(root, height=HEIGHT, width=WIDTH)
  canvas.pack()
  
  frame = tk.Frame(root, bg='#80c1ff', bd=5)
  frame.place(relx=0.5, rely=0.1, relwidth=0.75, relheight=0.1, anchor='n')
  
  button = tk.Button(frame, text="Set Freq", font=40, command=freqset_smb100a)
  button.place(relx=0.7, relheight=1, relwidth=0.3)
  
  entry = tk.Entry(frame, font=15, textvariable=str(input_var.get))
  entry.place(relx=0.35, relheight=1, relwidth=0.3)
  
  
  root.mainloop()
  </code></pre>
  
  <p>That's the error i get when i press the <code>button</code> to pass the value.</p>
  
  <pre><code>Exception in Tkinter callback
  Traceback (most recent call last):
    File "C:\Users\vrozakos\AppData\Local\Programs\Python\Python37-32\lib\tkinter\__init__.py", line 1705, in __call__
      return self.func(*args)
    File "C:/PyTests/Signal_gen_v2.py", line 19, in freqset_smb100a
      inst.write(f"SOUR:FREQ:CW " + {str(input_var.get())})
  TypeError: can only concatenate str (not "set") to str
  </code></pre>