Python: Function Problem

[JRaz]

I'm new to python so go easy :P

Anyway I have the following code


from Tkinter import *

root=Tk()

def function1(name):
print 'function1'

def function2(name):
print 'function2'

lbl1=Label(root,text='Function1')
lbl1.bind('<1>',function1)
lbl1.pack()

lbl2=Label(root,text='Function2')
lbl2.bind('<1>',function2(0))
lbl2.pack()

root.mainloop()

Basically the problem is this the second function gets run on program execution and then cant be run on mouse click whereas the first function is only run on mouse click (the way i want it ).

The only difference is that function2 gets a variable passed to it.

[duff]

"function1" is a function object.
"function2(0)" is a function call which returns None.

[JRaz]

"function1" is a function object.
"function2(0)" is a function call which returns None.
which means?

[skymt]

which means?

It means you have to replace "function2(0)" with just "function2":
lbl2.bind('<1>',function2(0))

[JRaz]

But the point is i WANT to pass a value (in this case 0) to the function, I just don't want it to run on program execution.

[skymt]

But the point is i WANT to pass a value (in this case 0) to the function, I just don't want it to run on program execution.

1. "function()" means "run this function". When used as a function argument, it passes the result, as you would expect. You wouldn't want "newSquare(20, sqrt(36))" to pass a number and a function object called "sqrt", you would want it to do what it does: pass two numbers.

2. "function" means "this is a function". "doSomething(function, function()" passes two values to doSomething. One is the function, the other is the result of the function.

bind expects a function object, so you would use the second form.

That's just basic Python syntax. If always want function2 to act like it was passed "0", code it to do so.

[JRaz]

so how would I achieve the following without using multiple functions and preventing them running on startup.


def function(name):
print name

lbl1=Label(root,text='Function1')
lbl1.bind('<1>',function('bob')
lbl1.pack()

lbl2=Label(root,text='Function2')
lbl2.bind('<1>',function('ben'))
lbl2.pack()

[skymt]

When Tkinter runs a callback function, it passes an event object:


from Tkinter import *

root=Tk()

def function(event):
print event.widget.cget("text")

lbl1=Label(root,text='bob')
lbl1.bind('<1>',function)
lbl1.pack()

lbl2=Label(root,text='ben')
lbl2.bind('<1>',function)
lbl2.pack()

This guide (http://www.pythonware.com/library/tkinter/introduction/events-and-bindings.htm) explains the Tkinter event system in detail.

[duff]

But the point is i WANT to pass a value (in this case 0) to the function, I just don't want it to run on program execution.

You want a lambda function (http://en.wikipedia.org/wiki/Lambda_calculus)...replace "function2(0)" with "lambda: function2(0)".

[skymt]

You want a lambda function (http://en.wikipedia.org/wiki/Lambda_calculus)...replace "function2(0)" with "lambda: function2(0)".

That will also work, but catching the event is cleaner.

[duff]

That wiki page probably has too much theory if you haven't seen it before. Here's a quick example (basically lambda returns a function object allowing you delay a function call and bind parameters)



def func(a,b):
print a,b


f = lambda: func(2,3)
g = lambda x: func(x,6)
h = lambda x: func(3,x)

func(1,2)
f()
g(1)
h(1)

When ran, will print out:


1 2
2 3
1 6
3 1