How to call a callback function using lambda expression in button widget's command attribute in tkinter python

Question

I have created a gui using tkinter in which I created a new window with Toplevel widget and added the widgets to this window such as a entry box , label and button widget . The command attribute of the button widget calls a callback function using the lambda expression such that the gui must not proceed before executing this callback function . But , due to some problem when I press the button widget the callback function is not called .

def toplevel_widget(obj):
    def suppress():
        try:
            if obj.fillna_value.get():
                fill_na_value_window.withdraw()
            else:
                raise Empty_na_value_entry_Error("The textbox can't be left empty!!\nFill any data in it.")
        except Empty_na_value_entry_Error as e:
            msgbox.showerror("Error",e.msg)

    fill_na_value_window=tk.Toplevel(obj)
    obj.fillna_value=tk.StringVar()
    tk.ttk.Label(fill_na_value_window,text="Enter the value to be inserted").grid(row=1,column=0,padx=10,pady=10)
    tk.ttk.Entry(fill_na_value_window,textvariable=obj.fillna_value,width=15).grid(row=1,column=1,padx=10,pady=10)
    tk.ttk.Button(fill_na_value_window,text="Ok",command=lambda : suppress,width=10).grid(row=2,columnspan=2)


    Cleanser_manual.fillna_toplevel_widget(obj)
    val=obj.fillna_value.get()
    print("Value : ",val)  #here **Value :    is printed on screen**

Here the suppress() function is not called even after the button is clicked

Because the callback you passed simply returns the suppress function object . Don't use lambda, just pass suppress itself (you could use lambda: suppress() but that's just redundant) — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jul 23, 2019 at 19:30

angeldeluz777 · Accepted Answer · 2019-07-23 19:37:41Z

1

You are not calling supress inside the lambda function, you missed the parenthesis

(..., command=lambda: supress(), ...)

At any case you don't need lambda at all, you could just pass the supress function as parameter

(..., command=supress, ...)

answered Jul 23, 2019 at 19:37

angeldeluz777

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Comments

Kyle McCormick · Accepted Answer · 2019-07-23 19:39:47Z

To expand on @angeldeluz777's answer:

What you are trying to do is pass a reference to a callback function as the command argument to Button. However, what command=lambda : suppress is actually doing is passing in a lambda function that does nothing except return the reference to the callback function, which is why the button click is having no effect.

Change your code from:

 tk.ttk.Button(fill_na_value_window,text="Ok",command=lambda : suppress,width=10).grid(row=2,columnspan=2)

to:

 tk.ttk.Button(fill_na_value_window,text="Ok",command=suppress,width=10).grid(row=2,columnspan=2)

This might help illustrate what's going on:

Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def f():
...     print("Hello")
...
>>> callback = f
>>> callback()
Hello
>>> callback2 = lambda: f
>>> callback2()
<function f at 0x7fafacc10c80>
>>>

Finomnis · Accepted Answer · 2019-07-23 19:36:02Z

0

As @juanpa.arrivillaga said.

tk.ttk.Button(fill_na_value_window,text="Ok",command=suppress,width=10).grid(row=2,columnspan=2)

answered Jul 23, 2019 at 19:36

Finomnis

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