Python Cookbook 2Nd Edition Jun 1002005 [Electronic resources] نسخه متنی

Recipe 11.2. Avoiding lambda in Writing Callback Functions

Credit: Danny Yoo, Martin Sjogren

Problem

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

Solution

Between the classic lambda approach and a powerful
general-purpose currying mechanism is a third, extremely simple way
for doing callbacks that can come in handy in many practical cases:

def command(callback, *args, **kwargs):
def do_call( ):
return callback(*args, **kwargs)
# 2.4 only: do_call._ _name_ _ = callback._ _name_ _
return do_call

Discussion

I remember a utility class (to perform the same task handled by a
closure in this recipe) quite a while back, but I
don't remember who to attribute it to. Perhaps I saw
it in John E. Grayson, Python and Tkinter
Programming (Manning).

Writing a lot of callbacks that give customized arguments can look a
little awkward with lambda, so this
command closure provides alternative syntax that is
easier to read. 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
enables you to fix some of the arguments when you bind the callback,
while others may be given at call time (see Recipe 16.4). However,
"doing the simplest thing that can possibly
work" is a good programming principle (this wording
of the principle is due, I believe, to Kent Beck). If your
application needs 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 that you do need the generality.

See Also

Recipe 16.4; information
about Tkinter can be obtained from a variety of sources, such as
Fredrik Lundh, An Introduction to Tkinter
(PythonWare: http://www.pythonware.com/library), New
Mexico Tech's Tkinter
Reference (http://www.nmt.edu/tcc/help/lang/python/docsl),
Python in a Nutshell, and various other books.