Python init() error, although i am not using init()

Question

I am new to Python. I try to use it to create a GUI, in order to control my arduino.

Since i don't fully understand python, much less Tkinter or pySerial, i found a snippet of code online.

The snippet is supposed to create a GUI, and also there will be an option to select the COM port, to communicate with the arduino.

This is the code:

import serial.tools.list_ports
from tkinter import *

def on_select(selection):
    # open the port and command it to start the LED blinking here
    print(selection)

root = Tk()
ports = serial.tools.list_ports.comports()
default = StringVar(root, "Please Select Port")
OptionMenu(root, default, *ports, command=on_select).pack()
root.mainloop()

I run this (i have Python 3.8.2) and this is the error i get:

Traceback (most recent call last):
  File "code.py", line 11, in <module>
    OptionMenu(root, default, *ports, command=on_select).pack()
TypeError: __init__() missing 1 required positional argument: 'value'

However, i do not have an init() in order to pass an argument.

you're calling it implicitly, with wrong / not enough positional parameters — Jean-François Fabre
– Jean-François Fabre ♦, Commented Apr 5, 2020 at 20:14
'__ init __' is the initalization function of a class. It is automatically called when you create an object of a class. Are you creating an object on line 11? — Mace
– Mace, Commented Apr 5, 2020 at 20:15

Jean-François Fabre · Accepted Answer · 2020-04-05 20:17:56Z

1

you're calling it implicitly, with wrong / not enough positional parameters here:

OptionMenu(root, default, *ports, command=on_select)

When you invoke a class with parameters, __init__ is called.

help(OptionMenu) shows the initializer signature:

class OptionMenu(Menubutton)
 |  OptionMenu(master, variable, value, *values, **kwargs)

You have to set at least one value. Other values are optional.

usage example:

w = OptionMenu(master, variable, "one", "two", "three")

Here your ports list is empty, so unpacking doesn't yield any argument. Which explains the error. One "fix" would be:

OptionMenu(root, default, *(ports or ["<empty>"]), command=on_select)

If the list is empty, you have only one option (empty). Or check if not empty and raise a user-level message if list is empty

answered Apr 5, 2020 at 20:17

Jean-François Fabre♦

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2 Comments

user1584421 Over a year ago

Thanks! I tried your code and it works in creating a blank window. However, there are no COM porta available, even though when i connect the arduino. Is there something wrong in my port declaration?

Jean-François Fabre Over a year ago

This is a different question.I can't answer that, since I don't know anything about arduino :)

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