Tkinter ValueError: could not convert string to float: ''

Question

It's the whole day I'm trying to solve this ridiculous problem, but without success; in this forum there is a lot of material, but usually they are enormous amounts of code and I cannot understand what's going on. On the internet they generally suggest casting but it doesn't work. To make things easy I wrote a sample code to illustrate the issues I'm having in a bigger code

import tkinter as tk
from tkinter import *
#from tkinter import ttk

my_window = tk.Tk()

my_label = tk.Label(master=my_window,text="N")
my_label.pack()

my_entry = tk.Entry(master=my_window)
my_entry.pack()
N = my_entry.get()

print(float(N))

my_window.mainloop()

Very nice and simple, but I get the following message

ValueError: could not convert string to float: ''

I tried many possibilities, but anything worked. Any little suggestion is very much appreciated. Thanks

Edit:

I want to understand how to assign float(N) to a global variable, say a, so I can use it later on in the code, and I just took the code of @Cool Cloud and modified it a little

import tkinter as tk
from tkinter import *

my_window = tk.Tk()

a=1. 

def cast():
    N = my_entry.get()
    try: # Try to execute this
        print(float(N))
        a=float(N)
    except ValueError: # If ValueError(meaning it is not a float) is triggered then print...
        print('Not a number!')

my_label = tk.Label(master=my_window,text="N")
my_label.pack()

my_entry = tk.Entry(master=my_window)
my_entry.pack()
# N = my_entry.get() N will be empty as my_entry is empty when code is executed

Button(my_window,text='Click me to convert to 
float',command=cast).pack() # A button to trigger event

print(a)

my_window.mainloop()

The output of this is in the following image

As you can see it directly prints 1.0 without waiting for the assignment a=float(N), so my doubt is how can I actually do this assignment, to use it later in my code. Thanks

P.S.: I understand that print(a) inside the definition of cast() would give correctly 123.0 in this case, but my problem is more general: I'm trying to understand how to entry an N value, making it float and "propagate" it to the rest of the program. My doubt is given by the fact that print(a) almost at the very last line of the program, and still doesn't wait for cast() to come in.

Answer 1

GUI programming is event driven, which means you will have to code based on events triggered. Every python code runs from top to bottom, and in this case all the code from top to bottom, outside functions, is executed. That means as soon as:

my_entry = tk.Entry(master=my_window)
my_entry.pack()

...is executed, the next line to be executed is N = my_entry.get(), and at the time of execution there is NOTHING inside the entry widget and hence N becomes empty string, and then you are trying to convert that empty string to a float, which will obviously give an error.

What you should be doing is, make a button and when you click it(event is triggered) and connect it to a function that will get the input and convert to float. So that when you click the button you have entered something onto the entry and now it is no longer empty.

import tkinter as tk
from tkinter import *

my_window = tk.Tk()

def cast():
    N = my_entry.get()
    try: # Try to execute this
        print(float(N))
    except ValueError: # If ValueError(meaning it is not a float) is triggered then print...
        print('Not a number!')

my_label = tk.Label(master=my_window,text="N")
my_label.pack()

my_entry = tk.Entry(master=my_window)
my_entry.pack()
# N = my_entry.get() N will be empty as my_entry is empty when code is executed

Button(my_window,text='Click me to convert to float',command=cast).pack() # A button to trigger event

my_window.mainloop()

Another way, and an unpopular way, is to use DoubleVar() which will get() the text for you, in this case, your entry will accept any value but when you get() the value from the DoubleVar() it will raise a TclError if it is not a float.

def cast():
    # Unlike before, error will be generated at get() and you dont need float()
    # because if there is no error then it is guaranteed to be a float anyway
    try:
        N = var.get()
        print(N)
    except TclError: # If TclError is triggered then print...
        print('Not a number!')
    
var = DoubleVar() # tkinter control variable
my_entry = tk.Entry(master=my_window,textvariable=var)
my_entry.pack()

Note: You can do my_entry.get() and it wont give any error even if the input is not a float, error comes only in the case of var.get().