Difference between 'function' and 'method'

Paul Boddie paul at boddie.org.uk
Tue Mar 4 11:38:26 CET 2008

On 4 Mar, 09:22, "甜瓜" <littlesweetme... at gmail.com> wrote:
>      This is a big problem puzzles me for a long time. The core question is:
> How to dynamically create methods on a class or an instance?
> Let me state it step by step.
> 1.
> def gunc(self):
>     pass
> class A(object):
>     def func(self):
>         pass
> a = A()
> a.func   # gives "bound method", type is "instancemethod"
> A.func  # gives "unbound method", type is "instancemethod"
> gunc    # gives "function", type if "function"
> # ?? Does this line attach a method to instance?  ... I don't think so.
> a.gunc = gunc

No, unfortunately not. You might get the detailed explanation from
someone else, but generally, you have to assign functions to classes;
these are then exposed as methods via instances of such classes.

  A.gunc = gunc

> I found stardard library 'new' may help. Is that right?

Yes, it can help:

  import new
  a.gunc = new.instancemethod(gunc, a, A)

> 2.
> a = A()  # instance of old class A
> # Do attach a new method to class A...
> b = A()  # instance of new class A
> Does "a" can get the new method automatically?

Can "a" get the new method automatically? Apparently, yes:

  class A:

  a = A()

  def f(self, x):
      print x

  A.f = f
  a.f(123) # works, printing 123

> Does new method have the *same* concept level with old methods?
> Especially, if there
> are classes inherit from class A, how does name resolution work on this case?

As far as I'm aware, after you've added a method to a class, instances
will regard the method like all the previously existing methods, since
method lookup is a dynamic operation.

I'll not address your other questions in this message since I'm not a
heavy user of decorators and don't want to provide a quick answer
without fully testing it first. However, it is important to remember
that the "magic" occurs for class attributes, not instance attributes.
So, for example, it's quite possible that you'd want to assign a
function to an instance attribute, but you wouldn't want the instance
to suddenly "own" that function...

  def somefunc(x, y):
      # Do something with x and y...
      return x + y

  a = A()
  a.somefunc = somefunc # store the function somewhere

  # Later...

  adder = a.somefunc

  # Later still...

  result = adder(p, q) # wouldn't work if somefunc became a method

Python seems to do the most intuitive thing, I think, but it's quite
tricky to contemplate all the implications.


P.S. I can never really remember all the different situations and
outcomes with method assignment, but then it's something I hardly ever
do. I'd be interested to know whether my situation is unusual, however.

More information about the Python-list mailing list