python, tkinter, command is executed when program is run not when button clicked

deathalele · 06-13-2009, 01:50 PM

Quote:

from Tkinter import *

class Application(Frame):
def __init__(self, master=None):
Frame.__init__(self, master)
self.grid()
self.createWidgets()

def createWidgets(self):
def hello_world(cnvs):
id = cnvs.create_line(10, 10, 90, 10)

self.cnvs = Canvas (bg='red', height=100, width=100)
self.cnvs.grid(row=4, column=1, sticky=N)

self.newbtn = Button ( self, text="Draw Line", command=hello_world(self.cnvs))
self.newbtn.grid(row=3, column=1)

app = Application()
app.master.title("Sample application")
app.mainloop()

when i run this program the line is draw in the canvas as soon as the program is run, not when the button is clicked. Also when i click the button nothing happens,

help please

thanks in advance

pgpython · 06-14-2009, 03:40 AM

First of all your indentation makes it difficult to see which commands belongs to which methods. I am going to assume that the indentation in your program is mostly correct or it wouldnt run. Without seeing the correct indentation you have used. I would say that the command to draw the line isn't indented propely.

deathalele · 06-15-2009, 05:02 AM

sorry i was using tabs for indentation which got lost when i posted,

here it is using spaces

Code:

from Tkinter import *

class Application(Frame):
    def __init__(self, master=None):
        Frame.__init__(self, master)
        self.grid()
        self.createWidgets()

    def createWidgets(self):
        def hello_world(cnvs):
            id = cnvs.create_line(10, 10, 90, 10)

        self.cnvs = Canvas (bg='red', height=100, width=100)
        self.cnvs.grid(row=4, column=1, sticky=N)

        self.newbtn = Button ( self, text="Draw Line", command=hello_world(self.cnvs))

        self.newbtn.grid(row=3, column=1)


app = Application()
app.master.title("Sample application")
app.mainloop()

I'm pretty sure it's not indentation but i could be wrong

pgpython · 06-15-2009, 06:26 AM

Ok I see your problem here and you need to regoranise you code.

First of all you have a function inside a function. This isn't a good thing to do in python unless you need a closure which you don't. You could do any of the following to improve it:

a) You could move hello_world outside the Application class
b) make hello_world a method of the Application class
c) Create a new class for the Canvas and make hello_world a method of that
d) Get rid of hello_world entirely and use lambda instead

Any of which would be better and which you chose depends on how and where you want the function to be called.

Your going to have to do this this anyway because what you have told python here:

Code:

self.newbtn = Button ( self, text="Draw Line", command=hello_world(self.cnvs))

is create a new Button called self.newbutn with the parameter of self, the value of the text "Draw Line" and the value of command to be whatever hello_world(self.cnvs) evaluates as which is None. Which is not what you wanted. What you wanted to say is:

Code:

self.newbtn = Button ( self, text="Draw Line", command=hello_world)

Note you cannot pass self.cnvs to the command because what You are passing is a function object to be called later. Hence why you need to reorganise the code a bit.

deathalele · 06-15-2009, 02:12 PM

Code:

from Tkinter import *


class Application(Frame):
        def __init__(self, master=None):
                Frame.__init__(self, master)
                self.grid()
                self.createWidgets()

        def createWidgets(self):

                self.cnvs =  Canvas (bg='red', height=100, width=100)
                self.cnvs.grid(row=4, column=1, sticky=N)
                self.newbtn = Button ( self, text="Draw Line", command=hello_world)
                self.newbtn.grid(row=3, column=1)

def hello_world():
        print 'HELLO WORLD'



app = Application()
app.master.title("Sample application")
app.mainloop()

I've got this to work, where it prints HELLO WORLD into stdout but how would i pass arguments to the function which is what i need now

thanks

pgpython · 06-15-2009, 03:13 PM

if you really need to. There are 2 things you can do

1 create a function class and pass the attributes to the function:

In python a function is basically just an object with the method __call__ note the double underscore signifying its a special method. When a function gets called its __call__ which will get called so you could write something like:

Code:

class Command(object):

    def __init__(self, func, *args, **keywds):
          self.func = func
          self.args = args
          self.keywds = keywds

    def __call__(self);
           return self.func(*self.args, **self.keywds)

you can then create a Command object passing it the functions and any arguments and keyword arguments ready to be called later and it will behave as any other function

2. Using a closure, in python nested scopes are allowed. Hence why you can write hello_world inside of createWidgets using this technique you can write a function which will write a function. Particulary useful if the inner function functionality remains the same but what it operates on doesnt.

Code:

def factoryMethod(self, *args, **keywds):

      def myfunc(): pass #myfunc can see args and keywds
     
      return myfunc

you can call factoryMethod and myfunc functioned will be returned operating on the parameters passed initally to factoryMethod