[maemo-developers] New project idea, pygtk questions, and a toolkit

[From: Marius Gedminas]

On Mon, Feb 19, 2007 at 12:41:15PM -0500, Sean Luke wrote:
> - Why is it in python that you can attach a function, and an instance  
> variable, to an instance, but you cannot attach a method?

I am not sure I understand your question.  Are you asking why functions
assigned to instance attributes do not get passed the hidden 'self'
argument?

There's a distinction between functions, unbound methods and bound
methods.  When you assign a function to a class attribute, you get a
unbound method.

  >>> class MyBtn(object):
  ...     pass
  ...
  >>> def foo(*args):
  ...     print args
  >>> foo
  <function foo at 0xb7d5f1ec>
  >>> MyBtn.foo = foo
  >>> MyBtn.foo
  <unbound method MyBtn.foo>

When you access the attribute of the instance, the method gets bound to
the instance

  >>> btn = MyBtn()
  >>> btn.foo
  <bound method MyBtn.foo of <__main__.MyBtn object at 0xb7c1438c>>

When you call a bound method, it calls the underlying function with all
the arguments you passed, plus the 'self' argument stored in the bound
method itself.

  >>> btn.foo(1, 2)
  (<__main__.MyBtn object at 0xb7c1438c>, 1, 2)

When you assign a function to an instance attribute, there's no magic
and you have a function, not a method

  >>> btn.bar = foo
  >>> btn.bar
  <function foo at 0xb7d5f1ec>
  >>> btn.bar(1, 2)
  (1, 2)

If you want to get a 'self' argument, you have to pass it yourself

  >>> btn.bar = lambda *args: foo(btn, *args)
  >>> btn.bar(1, 2)
  (<__main__.MyBtn object at 0xb7c1438c>, 1, 2)

> Or is  
> there a mechanism I'm not aware of?  Second: can you make an  
> anonymous function through any route than a lambda?

If by "anonymous" you mean "a function that does not have a name", then
you can't.  You can define a named function in the middle of another
function.

> Because lambdas,  
> weirdly, don't permit statements inside them.  In NewtonScript (and  
> Self, and JavaScript I think) objects are just dictionaries -- as is  
> the case for Python underneath -- and you can do this:
> 
> myButton := {
> 	_proto: protoButton,
> 	printme: func() begin
> 		print("hi there");
> 		print("I am " & self) end
> 	};
> 
> I'd like to do this in python with the following equivalent (but  
> invalid) syntax:
> 
> myButton= Button()
> myButton.printme = lambda self:
> 			print("hi there")
> 			print("I am " + self)

  myButton = Button()
  def printme(self=myButton):
      print "hi there"
      print "I am", self
  myButton.printme = printme

> Can someone more experienced in Python tell me why this can't be  
> done, or if it can be done, how?

Marius Gedminas
-- 
question = 0xFF;        // optimized Hamlet
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