n00by: Python: "second argument must be callable"?

Question

I'm building a GTK application with Python that makes an equation that always equals 42. I'm launching it from a linux command line. It says my function isn't callable or something. I think it's callable and I really don't know what this means, sorry for posting such a nooby question. Here's the code:

#!/usr/bin/env python

import random
import gtk

def makePlus():
    num2 = random.randint(0,41)
    num = 42 - num2
    return str(num) + " + " + str(num2)

def makeMinus():
    num2 = random.randint(0,41)
    num = 42 + num2
    return str(num) + " - " + str(num2)

def makeTimes():
    num2 = random.randint(0,41)
    num2 = float(num2)
    num = 42.0 / num2
    return str(num) + " x " + str(num2)

def makeDivideBy():
    num2 = random.randint(0,41) 
    num = 42 * num2
    return str(num) + " / " + str(num2)

def setResultAs(operation):
    if operation == "plus":
        result = makePlus()
    if operation == "minus":
        result = makeMinus()
    if operation == "times":
        result = makeTimes()
    if operation == "divide":
        result = makeDivideBy()
    textbox.set_text(result)

win = gtk.Window()
win.set_title("OSFT Equation Generator")
win.connect('delete-event', gtk.main_quit)

mainbox = gtk.VBox()
win.add(mainbox)

textbox = gtk.Entry()
mainbox.pack_start(textbox)

btnbox = gtk.HBox()
mainbox.pack_start(btnbox)

plusBtn = gtk.Button("Plus")
btnbox.pack_start(plusBtn)
plusBtn.connect("clicked", setResultAs("plus"))

minusBtn = gtk.Button("Minus")
btnbox.pack_start(minusBtn)
minusBtn.connect("clicked", setResultAs("minus"))

timesBtn = gtk.Button("Times")
btnbox.pack_start(timesBtn)
timesBtn.connect("clicked", setResultAs("times"))

divideBtn = gtk.Button("Divide by")
btnbox.pack_start(plusBtn)
divideBtn.connect("clicked", setResultAs("divide"))

win.show_all()
gtk.main()

And this is the error i get:

Traceback (most recent call last):
  File "./osftgen.py", line 53, in <module>
    plusBtn.connect("clicked", setResultAs("plus"))
TypeError: second argument must be callable

Please help.

Answer 1

def clickedPlus(widget):
    setResultAs("plus")

plusBtn.connect("clicked", clickedPlus)

You when connecting events to action, you need to pass a string event name and then a callable function, meaning "when the event named 'clicked' happens to the plusBtn, run the clickedPlus function.

Answer 2

The code calls the setResultAs(..) and use the return value of the function as the callback (handler).

Use following form (gobject.GObject.connect):

object.connnect("signla_name", handler, arg1, arg2, ...)

For example, replace following line

plusBtn.connect("clicked", setResultAs("plus"))

with:

plusBtn.connect("clicked", setResultAs, "plus")

UPDATE

In addition to the above, the callback function setResultAs should receive widget (the widget that emit the signal) as the first parameter.

Replace following line:

def setResultAs(operation):

with:

def setResultAs(widget, operation):

And there's a typo:

divideBtn = gtk.Button("Divide by")
btnbox.pack_start(plusBtn) # <-------- `plusBtn` should be `divideBtn`
divideBtn.connect("clicked", setResultAs("divide"))

Answer 3

Callable is Python's abstract way of saying "is it some kind of function?"

You haven't passed a function to connect; you've passed None (because setResultAs doesn't return a value). Have something there that gtk can call back.