Python Tkinter: Bind function with labels in for loop

Question

I'm creating labels dynamically in a for loop using tkinter. I don't know how many labels will be created, but on clicking of each of the labels, a particular function must be called with a particular parameter.

To do this, I'm using this code:

for link in list_of_links:
    link_label = Label(self.video_window, text="Frame "+str(video_number), fg="blue", cursor="hand2")
    link_label.pack()
    link_label.place(x=xcod2, y=ycod2)
    link_label.bind("<1>", lambda x: self.goto_video_link(link))

Currently, I'm creating 10 labels. The problem is that on clicking any of the ten labels, the goto_video_link function seems to only use the 10th link.

If I click on the 5th label, I want it to use the 5th link.

How do I go about this?

igordsm · Accepted Answer · 2015-04-21 09:16:35Z

6

Lambda expressions are lazily evaluated, which means that self.go_to_link(link) is only evaluated when it is executed. In this moment link contains the value of the last link, so every button will go to the last link.

You need to force the evaluation of link during the for loop. This can be done with a lambda function that returns another lambda function with the value you want. I know it seems confusing, but the code below may make it clearer.

eval_link = lambda x: (lambda p: self.go_to_link(x))
for link in list_of_links:
    link_label = Label(self.video_window, text="Frame "+str(video_number), fg="blue", cursor="hand2")
    link_label.pack()
    link_label.place(x=xcod2, y=ycod2)
    link_label.bind("<1>", eval_link(link))

In this case, to be able to build the inner lambda it is necessary to evaluate link. Since it gets passed as a parameter, the inner most lambda is bound to the local copy x instead of link and since x is a local variable, it is always remade when the function is called.

edited Apr 21, 2015 at 9:16

answered Apr 18, 2015 at 8:22

igordsm

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Randomly Named User Over a year ago

When I did eval_link = lambda x: (lambda: self.go_to_link(x)), I got an error that said TypeError: <lambda>() takes no arguments (1 given). To correct that, I just had to change the eval_link to eval_link = lambda x: (lambda p: self.go_to_link(x)) and then it worked. Thank you!

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