Clear text from tkinter entry widget

Question

I am using Tkinter entry widgets to allow users to input text to a GUI. The entry widgets have default text which I would like to clear with a single button press. I have the following code:

from Tkinter import *

def delete_entries(fields):
  for field in fields:
    field.delete(0,END)

def UserInput(status,name):
  optionFrame = Frame(root)
  optionLabel = Label(optionFrame)
  optionLabel["text"] = name
  optionLabel.pack(side=LEFT)
  var = StringVar(root)
  var.set(status)
  w = Entry(optionFrame, textvariable= var)
  w.pack(side = LEFT)
  optionFrame.pack()

if __name__ == '__main__':
  root = Tk()
  fields = 'ExperimentNumber', 'OutputFileName', 'npts1', 'block1'    
  ExperimentNumber = UserInput("1", "Experiment number")
  OutputFileName = UserInput("output.txt", "Output file name")
  npts1 = UserInput("1000", "Number of points")
  block1 = UserInput("8", "Block size")

  Delete_button = Button(root, text = 'Clear all', command = delete_entries(fields))
  Delete_button.pack()

I have tried creating fields with the list of variables that I want to delete (as above) and iterating over this in the function delete_entries(), however this returns an error because the entries in fields are strings. I have tried replacing fields with fields = ExperimentNumber for example, but this returns an error because ExperimentNumber hasn't yet been defined. Finally I tried putting ExperimentNumber within the delete function i.e.

def delete_entries():
      ExperimentNumber.delete(0,End)

but this doesn't work because ExperimentNumber has the attribute NoneType (I don't understand why this is, because the delete_entries() function isn't called until after the ExperimentNumber Entry widget is created via the function UserInput). How can I go about deleting all the text in the Entry widgets? I have about 20 of these in my actual code and would like the user to be able to clear all the fields with one button press.

Solution

You are on the right track but you missed two little things. I added those two in your code and tried to explain with comments.

from Tkinter import *

def delete_entries():
  for field in fields:
    field.delete(0,END)

def UserInput(status,name):
  optionFrame = Frame(root)
  optionLabel = Label(optionFrame)
  optionLabel["text"] = name
  optionLabel.pack(side=LEFT)
  var = StringVar(root)
  var.set(status)
  w = Entry(optionFrame, textvariable= var)
  w.pack(side = LEFT)
  optionFrame.pack()
  return w
  #this return is crucial because if you don't return your widget's identity,
  #you can not use them in another function

if __name__ == '__main__':
  root = Tk()    
  ExperimentNumber = UserInput("1", "Experiment number")
  OutputFileName = UserInput("output.txt", "Output file name")
  npts1 = UserInput("1000", "Number of points")
  block1 = UserInput("8", "Block size")
  #you are saying it does not work because of they are strings.
  #then don't assign strings to fields. assign variables.
  fields = ExperimentNumber, OutputFileName, npts1, block1
  #since fields is defined in global scope, you don't need to use it as parameter

  Delete_button = Button(root, text = 'Clear all', command = delete_entries)
  Delete_button.pack()

  root.mainloop()