importing a method

[Flavio]

hi,

I have an object defined with a number of hardcoded methods.

Class soandso:
def __init__(self):
self.this = 0
self.that = 1
def meth1(self):
...
def meth2(self):
...
def custom(self):
pass

I want to allow the user to write a python module that declares a
function so that myprogram can import it and attribute it to the custom
method of the soandso object. So far so good, that is an easy thing to
do in Python.

import usermodule
a=soandso()
a.custom = usermodule.func tion

But, what if the method had to access the self attributes (self.this
and self.that) of the soandso object?

Can it be done? and if so, what is the most Pythonic way of doing it?

thanks in advance,

Flávio

[Ben Finney]

Flavio <fc******@gmail .com> wrote:

Class soandso:
def __init__(self):
self.this = 0
self.that = 1
def meth1(self):
...
def meth2(self):
...
def custom(self):
pass

I want to allow the user to write a python module that declares a
function so that myprogram can import it and attribute it to the custom
method of the soandso object. So far so good, that is an easy thing to
do in Python.

import usermodule
a=soandso()
a.custom = usermodule.func tion
Apparently you haven't tested whether this works. It doesn't:

class Foo(object): ... pass
... def bar(*args): ... print "bar() got arguments:", args
... bar("spam", "eggs") bar() got arguments: ('spam', 'eggs') Foo.bar_method = bar
Foo.bar_method( "spam", "eggs") Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: unbound method bar() must be called with Foo instance as first argument (got str instance instead)
But, what if the method had to access the self attributes (self.this
and self.that) of the soandso object?

To become a method, the function must be bound to an instance, and the
method will then receive the instance as the first argument when
called as a method.

To do this on an already-defined function, use new.instancemet hod.
import new
help(new.instan cemethod)

--
\ "One time a cop pulled me over for running a stop sign. He |
`\ said, 'Didn't you see the stop sign?' I said, 'Yeah, but I |
_o__) don't believe everything I read.'" -- Steven Wright |
Ben Finney

[Chris Curvey]

why not just have your user subclass "soandso" and override the
definition of "custom"?

from soandso import soandso
class MyClass(soandso ):
def custom(self):
self.theother = 3

c = MyClass()

[Flavio]

Because, by the time the user function is imported and attributed to
the custom method, soandso has already been instantiated and contains
the information tha needs to accessed by the user's function.

[Flavio]

If you read my original post, I had no intention of atributing the
user's method to the class, but to the instance.

Anyway I figure it out myself, and its quite a Pythonic solution:

class Foo: name='John'
a=Foo() def p(parent): self=parent
print 'Hi, %s!'%self.name

a.met=p a.met(a)

Hi, John!

This works the same way an object's built-in method would work, since
all methods receive a reference to the parent object through the
required argument "self".

class Foo:
def met(self):
print self

Thanks for all the replies, they helped to catalize my own thoughts!

Flávio

[Flavio]

There only one puzzle left to solve:

altough the solution I proposed works, this variant has problems:

class Foo: name='John'
a=Foo()
def p(): print 'Hi, %s!'%self.name a.met=p
a.met.self = a
a.met() NameError: global name 'self' is not defined

This error is paradoxical since:
a.met.self

<__main__.Foo instance at 0x405ed2ec>

Can anyone explain this?

[Antoon Pardon]

Op 2005-11-27, Flavio schreef <fc******@gmail .com>:

hi,

I have an object defined with a number of hardcoded methods.

Class soandso:
def __init__(self):
self.this = 0
self.that = 1
def meth1(self):
...
def meth2(self):
...
def custom(self):
pass

I want to allow the user to write a python module that declares a
function so that myprogram can import it and attribute it to the custom
method of the soandso object. So far so good, that is an easy thing to
do in Python.

import usermodule
a=soandso()
a.custom = usermodule.func tion

But, what if the method had to access the self attributes (self.this
and self.that) of the soandso object?

Can it be done? and if so, what is the most Pythonic way of doing it?

You mean something like this:

class Foo: .... def __init__(self, v):
.... self.value = v
.... def show(self): .... print self.value
.... import new
f = Foo(17)
f.show = new.instancemet hod(show, f)
f.show()

17

--
Antoon Pardon

[Flavio]

This "new" module sounds pretty cool, too bad its deprecated...

I would not want to add a dependancy to a deprecated module in my code.
But maybe I'll check the code for instancemethod within it and see what
it does.

Flávio

[Flavio]

Addendum to my last reply:

although the New Method is deprecated,

new.instancemet hod (from Antoon's message) can be replaced by

from types import MethodType

f.show = MethodType(show ,f)

and every thing still works.

[Alex Martelli]

Flavio <fc******@gmail .com> wrote:

This "new" module sounds pretty cool, too bad its deprecated...

I would not want to add a dependancy to a deprecated module in my code.
But maybe I'll check the code for instancemethod within it and see what
it does.

If you have a function f and want to make an instancemethod out of it,
you can simply call f.__get__(thein stance, theclass) and that will build
and return the new instancemethod you require.
Alex