Call a classmethod on a variable class name

[Matthew Keene]

I would like to be able to call a specific classmethod on a class name
that is going to be passed from another parameter. In other words, I
have a call that looks something like:

x = Foo.bar()

and I would like to generalise this so that I can make this call on any
particular class which provides the bar classmethod.

I have implemented this using exec, like so:

className = parameters.clas sName
exec "x = " + className + ".bar()"

but this feels somewhat clumsy. (I do have the requisite exception
handling to cope with the supplied class not existing or not
implementing the bar method, by the way).

Is there any more Pythonesque way of doing this ? I guess what I'm
probably looking for is something like the way I understand the send
function works in Ruby

[Arnaud Delobelle]

Re: Call a classmethod on a variable class name

On Apr 13, 9:51 am, Matthew Keene <dfg...@yahoo.c om.auwrote:

I would like to be able to call a specific classmethod on a class name
that is going to be passed from another parameter.  In other words, I
have a call that looks something like:
>
   x = Foo.bar()
>
and I would like to generalise this so that I can make this call on any
particular class which provides the bar classmethod.
>
I have implemented this using exec, like so:
>
  className = parameters.clas sName
  exec "x = " + className + ".bar()"
>
but this feels somewhat clumsy.  (I do have the requisite exception
handling to cope with the supplied class not existing or not
implementing the bar method, by the way).
>
Is there any more Pythonesque way of doing this ?  I guess what I'm
probably looking for is something like the way I understand the send
function works in Ruby

If your class lives in the current global namespace, you can get it
with

>>cls = globals()[classname]

Then you can access its .bar() method directly:

>>cls.bar()

Example:

>>class Foo(object):

... @classmethod
... def bar(cls): print 'Oh my Baz!'
...

>>globals()['Foo']

<class '__main__.Foo'>

>>globals()['Foo'].bar()

Oh my Baz!

If your class lives in a module, just do getattr(module, classname)
instead to get the class object.

HTH

--
Arnaud

[Matthew Keene]

Re: Call a classmethod on a variable class name

Arnaud Delobelle wrote:

>
If your class lives in the current global namespace, you can get it
with
>

>>cls = globals()[classname]

>
Then you can access its .bar() method directly:
>

>>cls.bar()

>
Example:
>

>class Foo(object):

... @classmethod
... def bar(cls): print 'Oh my Baz!'
...

>globals()['Foo']

<class ' main .Foo'>

>globals()['Foo'].bar()

Oh my Baz!
>
If your class lives in a module, just do getattr(module, classname)
instead to get the class object.
>
HTH
>
>

Yes, that feels much nicer and works well.

Thanks for the prompt reply !

[Gary Herron]

Re: Call a classmethod on a variable class name

Matthew Keene wrote:

I would like to be able to call a specific classmethod on a class name
that is going to be passed from another parameter. In other words, I
have a call that looks something like:
>
x = Foo.bar()
>
and I would like to generalise this so that I can make this call on any
particular class which provides the bar classmethod.
>
I have implemented this using exec, like so:
>
className = parameters.clas sName
exec "x = " + className + ".bar()"
>

Yuck. No. Don't do that. As a general rule, anything you could build
and put into an exec is a functionality that is exposed more directly by
Python.

but this feels somewhat clumsy. (I do have the requisite exception
handling to cope with the supplied class not existing or not
implementing the bar method, by the way).
>
Is there any more Pythonesque way of doing this ? I guess what I'm
probably looking for is something like the way I understand the send
function works in Ruby
>

First off, if possible, don't pass the class name, but instead pass the
class itself:

class SomeClass:
def foo():
... whatever...

...
parameters.theC lass = SomeClass
...
parameters.theC lass.bar()

If you can't do that, then look up that class from the class name and
make your call:

class SomeClass:
def foo():
... whatever...

...
parameters.clas sName = 'SomeClass'
...
theClass = globals()[parameters.clas sName]
parameters.theC lass.bar()


(Hint: It matters not whether foo is a classmethod, saticmathod or
normal method.)

Gary Herron