无法使用 float.get() 从 tkinter 条目中检索浮点数。 我怎样才能解决这个问题？

Question

im trying to retrieve a value from an entry box and retrieve it as a float but it does not work. i have a function that is supposed to retrieve the values from three entry boxes, when a button is pushed the value received are used in 3 different function depending on what has been chosen from a drop down list

def selected():
    length = float(l_input.get())
    width = float(w_input.get())
    height = float(h_input.get())
    selected = var.get()
    if selected == "cube":
        cube
    elif selected == "cuboid":
        cuboid
    elif selected == "cylinder":
        cylinder

this is supposed to run when this button is pressed

size_input = Button(window, text="Enter", command=selected()) 
size_input.place(x=470,y=50,width=50,height=25)

ive tried many times to fix this and either it dosent work or it refuses to run at all and gives me an error like AttributeError: 'NoneType' object has no attribute 'get' or it will give me the error ValueError: could not convert string to float: '' i honestly have no clue for to fix this plz help

l_lb = Label(window, text = "L:")
l_lb.place(x=180,y=50,width=25,height=25) 
l_input = Entry(window)
l_input.place(x=200,y=50,width=57,height=25)
w_lb = Label(window, text = "W:")
w_lb.place(x=280,y=50,width=25,height=25) 
w_input = Entry(window)
w_input.place(x=300,y=50,width=57,height=25)
h_lb = Label(window, text = "H")
h_lb.place(x=380,y=50,width=25,height=25) 
h_input = Entry(window)
h_input.place(x=400,y=50,width=57,height=25)

(above is code for entries )

Answer 1

First, the code you provided does not give errors of the type AttributeError: 'NoneType' object has no attribute 'get' but if you get some, see Tkinter: AttributeError: NoneType object has no attribute <attribute name> .

Second, you get ValueError: could not convert string to float because the function selected() is executed when the button size_input is created and at that moment the entries are empty, therefore l_input.get() returns '' which then triggers an error when trying to convert this empty string to float. To fix that, you need to pass the function to the command option of the button without brackets:

size_input = Button(window, text="Enter", command=selected) 

Side comment: I don't recommend using .place() to display all the widgets because you have to make sure manually that they don't overlap and even then they do not adjust when the window is resized. On the contrary, .grid() and .pack() manage automatically this kind of things