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Python Cookbook by David Ascher, Alex Martelli

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Avoiding lambda in Writing Callback Functions

Credit: Danny Yoo

Problem

You need to use many callbacks without arguments, typically while writing a Tkinter-based GUI, and would rather avoid using lambda.

Solution

Between the classic lambda approach and a powerful general-purpose currying mechanism, there’s a third, extremely simple way that can come in handy in many practical cases:

class Command:
    def _ _init_ _(self, callback, *args, **kwargs):
        self.callback = callback
        self.args = args
        self.kwargs = kwargs

    def _ _call_ _(self):
        return apply(self.callback, self.args, self.kwargs)

Discussion

I remember seeing this utility class a while back, but don’t remember who to attribute it to. Perhaps I saw this in John E. Grayson’s book, Python and Tkinter Programming (Manning).

Writing a lot of callbacks that give customized arguments can look a little awkward with lambda, so this Command class gives an alternative syntax that looks nicer. For example:

import Tkinter
def hello(name): print "Hello", name
root = Tk(  )

# the lambda way of doing it:
Button(root, text="Guido", command=lambda name="Guido": hello(name)).pack(  )

# using the Command class:
Button(root, text="Guido", command=Command(hello, "Guido")).pack(  )

Of course, you can also use a more general currying approach, which lets you fix some of the arguments when you bind the callback, while others may be given at call time (see Recipe 15.8). However, “doing the simplest thing that can possibly work” is a good programming principle. If your application needs some callbacks that fix all arguments at currying time and others that leave some arguments to be determined at callback time, it’s probably simpler to use the more general currying approach for all the callbacks. But if all the callbacks you need must fix all arguments at currying time, it may be simpler to forego unneeded generality and use the simpler, less-general approach in this recipe exclusively. You can always refactor later if it turns out you need the generality.

See Also

Recipe 15.8; information about Tkinter can be obtained from a variety of sources, such as Pythonware’s An Introduction to Tkinter, by Fredrik Lundh (http://www.pythonware.com/library), New Mexico Tech’s Tkinter reference (http://www.nmt.edu/tcc/help/lang/python/docs.html), and various books.

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