为什么在 Tkinter 中尝试将标签小部件添加到屏幕时出现“错误的窗口路径名”错误？

Question

I'm creating a homework planner program using Tkinter, and I've run into a problem when trying to add an error label to the screen to show the user that they might have entered a duplicate assignment. Here's the constructor for the label (entFrame is where I put the user entries):

duplicateLabel = Label(entFrame, text="Warning: the entered assignment is a duplicate!", bg=bgColor, font=regularFont)

When a user submits an assignment, a function runs that creates a new assignment object. In that function, I call a method of the assignment to check if it is a duplicate. In that method, I create a list of all the lines of a csv file where I store the assignments. Then, I check the new assignment's attributes to some of the stored attributes of previous assignments. If the conditional is true, I add the warning label to the screen. Otherwise, I try to destroy the label to remove it from the screen to get rid of a previous warning from an old assignment.

    def checkDuplicates(self):
        contents = []
        file = open("homeworkAssets/assignmentsFile", "r")
        for content in csv.reader(file):
            contents.append(content)
        file.close()
        for assignment in contents:
            if self.course == assignment[1] and self.dueDate == assignment[2] and self.details == assignment[3]:
                duplicateLabel.grid(row=0, column=0, columnspan=2)
            else:
                duplicateLabel.destroy()

My first thought was that the .destroy() method was deleting the Label object, although when I added a print statement after that last line, the console read: .!labelframe.!labelframe.!label6 which I assume means it hasn't been deleted. When I run the program with an empty csv file and I add one assignment, and then a duplicate assignment, I receive this error message (sorry for the bad formatting, I couldn't figure out how to do it properly):

File "/Users/max/Desktop/Coding/Python/Misc./homeworkPlannerRevised.py", line 212, in checkDuplicates duplicateLabel.grid(row=0, column=0, columnspan=2) File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/tkinter/__init__.py", line 2493, in grid_configure self.tk.call( _tkinter.TclError: bad window path name ".!labelframe.!labelframe.!label6"

Why is this error occurring and how do I fix it? Thanks in advance!

Answer 1

.destroy() removes widget from window (and memory) but it doesn't remove path from variable.
print(duplicateLabel) shows path to object even if it is destroyed.

You run destroy() in loop - so one loop can destroy label and next loop may try to destroy it again - and this can make problem.

You could assign None to variable after destroying and check None before destroying.

if duplicateLabel is not None:
    duplicateLabel.destroy()
    duplicateLabel = None

but if you destroy object then later you have to create it again

if duplicateLabel is None:
   duplicateLabel = Label(...)

But frankly you don't need destroy() but grid_forget() to hide widget without destroying.

if self.course == assignment[1] and self.dueDate == assignment[2] and self.details == assignment[3]:
    duplicateLabel.grid(row=0, column=0, columnspan=2)
else:
    duplicateLabel.grid_forget()