How can I get TK buttons, generated by a for loop, to pass input to their command? (Python)

Question

I have a program which dynamically generates a GUI. I don't know how many buttons I will have, if any at all.

The specific problem is something like this:

for varname in self.filetextboxes:
    if self.varDict[varname]=='':
        self.varDict[varname] = (StringVar(),)
        self.varDict[varname][0].set('')

    fileButton = Button(self, text=" ", command = lambda:self.varDict[varname][0].set(tkFileDialog.askopenfilename()), image=self.filephoto)

    ftb = Entry(self, textvariable = self.varDict[varname][0],width=40,background='white')

I have a for loop which creates the textboxes and the buttons. The StringVar() is stored in a dictionary with key varname.

because I can't pass arguments in the callback function of the button, I instead define a lambda in each button. This sets the StringVar() associated with the textbox created in this loop to the output of a filedialog box.

The problem is, the varname passed to the lambda isn't passing the value, but the name of the variable only. So while the textboxes are associated with the variable they were created with in the for loop, lambdas in the buttons uses the current value of varname at all times.

In other words, each textbox links to only one variable, but all the buttons only set the text of the final textbox created, i.e. the textbox with the final value of varname.

Is there another way to approach this? Can I make the lambda somehow only use the value of varname as it is defined, and not use future values of varname?

Answer 1

Generally, you would associate the button with the varname and StringVar() via a dictionary, the key being the button's id or number, pointing to whatever. Since you are using a StringVar I assume you are doing this with Tkinter. An simple example follows that passes the button's number to a function that accesses the dictionary.

from Tkinter import *
from functools import partial

class ButtonsTest:
   def __init__(self):
      self.top = Tk()
      self.top.title('Buttons Test')
      self.top_frame = Frame(self.top, width =400, height=400)
      self.button_dic = {}
      self.buttons()
      self.top_frame.grid(row=0, column=1)

      Button(self.top_frame, text='Exit', 
              command=self.top.quit).grid(row=10,column=1, columnspan=5)

      self.top.mainloop()

   ##-------------------------------------------------------------------         
   def buttons(self):
      b_row=1
      b_col=0
      for but_num in range(1, 11):
         ## note that the correct "but_num" is stored
         self.button_dic[but_num] = "self.cb_button_%d()" % (but_num)
         b = Button(self.top_frame, text = str(but_num), 
                    command=partial(self.cb_handler, but_num))
         b.grid(row=b_row, column=b_col)

         b_col += 1
         if b_col > 4:
            b_col = 0
            b_row += 1

   ##----------------------------------------------------------------
   def cb_button_1(self):
      print "push button 1 and this code is executed"

   ##----------------------------------------------------------------
   def cb_button_2(self):
      print "push button 2 and this code is executed"

   ##----------------------------------------------------------------
   def cb_button_3(self):
      print "push button 3 and this code is executed"

   ##----------------------------------------------------------------
   def cb_handler( self, cb_number ):
      print "cb_handler", cb_number, self.button_dic[cb_number]                
      if cb_number < 4:
         exec(self.button_dic[cb_number])

##===================================================================
BT=ButtonsTest() 

Answer 2

I have to admit -- this one has me a little bit stumped. Since you're pairing a Button with an Entry and stringvar, one workaround is to wrap those together in a class. (I could probably argue that this is a little more elegant anyway...)

import Tkinter as tk

class ButtonEntry(tk.Frame):
    def __init__(self,master,ss):
        tk.Frame.__init__(self)
        self.var=tk.StringVar()
        self.var.set(ss)
        self.Button=tk.Button(self,text='Button',command=lambda :self.var.set("foo!"))
        self.Entry=tk.Entry(self,textvariable=self.var)
        self.Button.grid(row=0,column=0)
        self.Entry.grid(row=0,column=1)


class App(tk.Frame):
    def __init__(self,master=None):
        tk.Frame.__init__(self,master)
        self.BEs=[]
        for i in range(10):
            b=ButtonEntry(self,'Button %d'%i)
            b.grid(row=i,column=0)
            self.BEs.append(b)


if __name__ == '__main__':
    root=tk.Tk()
    f=App(root)
    f.grid(row=0,column=0)
    root.mainloop()

However, I would really love to know why the lambda behaves the way it does. I thought for sure what you had should work.

EDIT

I've tracked down the behavior of the lambda function. https://stackoverflow.com/a/10452819/748858 gives a great example of how to do this properly. The problem is that the variables in the lambda function are still bound to the scope where the lambda was declared. In order to disassociate them with that scope, you need to set them as a keyword argument to the function. Nice! I learned something new this morning.