Adding new methods to new-style classes dynamically

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  • Max Derkachev
    #1

    Adding new methods to new-style classes dynamically

    Good day to all.

    Some time ago I'd been playing with a framework which uses dynamic
    class creation havily. Say, I could do:

    class A:
    pass

    # I method name is dynamic
    meth_name = 'foo'

    #method code can also be dynamic - any executable object will be good
    enough
    func = lambda self: self.__class__. __name__

    A.__dict__[meth_name] = func
    a = A()
    print a.foo()[color=blue][color=green]
    >> A[/color][/color]
    # methods could be added/changed at runtime, without re-instantination
    A.__dict__[meth_name] = lambda self: type(self)
    print a.foo()[color=blue][color=green]
    >> <type 'instance'>[/color][/color]

    #well, try this with the new-style class
    class A(object):
    pass

    # the new-style __dict__ is a dictproxy object
    A.__dict__[meth_name] = lambda self: type(self)[color=blue][color=green]
    >>Traceback (most recent call last):
    >> File "<interacti ve input>", line 1, in ?
    >>TypeError: object does not support item assignment[/color][/color]


    Of course, I can do
    A.foo = some_executable _object
    But that does help when a dynamic method name is needed.
    Another option is:
    exec('A.%s = %s'%('foo', 'some_executabl e_option'))
    But I don't like eval'ling things :)

    Is there other way to add/change methods to new-style classes
    dynamically?

    Tags: None
    • Michele Simionato
      #2
      Re: Adding new methods to new-style classes dynamically

      Do you mean setattr?

      setattr(A, meth_name, lambda self: type(self))


      Michele Simionato

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        • John Machin
          #3
          Re: Adding new methods to new-style classes dynamically

          Max Derkachev wrote:
          [snip][color=blue]
          > #well, try this with the new-style class
          > class A(object):
          > pass
          >
          > # the new-style __dict__ is a dictproxy object
          > A.__dict__[meth_name] = lambda self: type(self)[color=green][color=darkred]
          > >>Traceback (most recent call last):
          > >> File "<interacti ve input>", line 1, in ?
          > >>TypeError: object does not support item assignment[/color][/color]
          >
          >
          > Of course, I can do
          > A.foo = some_executable _object
          > But that does help when a dynamic method name is needed.
          > Another option is:
          > exec('A.%s = %s'%('foo', 'some_executabl e_option'))
          > But I don't like eval'ling things :)
          >
          > Is there other way to add/change methods to new-style classes
          > dynamically?[/color]

          Ever considered setattr()? Not only does it appear to do the job but
          also it avoids those __make__your__e yes__bleed__ double underscores.
          [color=blue][color=green][color=darkred]
          >>> class A(object):[/color][/color][/color]
          .... pass
          ....[color=blue][color=green][color=darkred]
          >>> A.__dict__['foo'] = 123[/color][/color][/color]
          Traceback (most recent call last):
          File "<stdin>", line 1, in ?
          TypeError: object does not support item assignment[color=blue][color=green][color=darkred]
          >>> setattr(A, 'foo', 123)
          >>> A.foo[/color][/color][/color]
          123[color=blue][color=green][color=darkred]
          >>>>>> def bar(self):[/color][/color][/color]
          .... print 'bar bar blog ship'
          ....[color=blue][color=green][color=darkred]
          >>> setattr(A, 'bar', bar)
          >>> eh = A()
          >>> eh.bar()[/color][/color][/color]
          bar bar blog ship[color=blue][color=green][color=darkred]
          >>>[/color][/color][/color]

          HTH,
          John

            Comment

            • Max Derkachev
              #4
              Re: Adding new methods to new-style classes dynamically


              John Machin wrote:[color=blue][color=green][color=darkred]
              > >>> A.foo[/color][/color]
              > 123[/color]

              Oh, I've been a little dumb to forget about it :)
              Thanks.

              Max.

                Comment

                • Jack Diederich
                  #5
                  Re: Adding new methods to new-style classes dynamically

                  On Thu, May 05, 2005 at 01:35:09AM -0700, Max Derkachev wrote:[color=blue]
                  > Good day to all.
                  >
                  > Some time ago I'd been playing with a framework which uses dynamic
                  > class creation havily. Say, I could do:[/color]
                  <snip>[color=blue]
                  >
                  > #well, try this with the new-style class
                  > class A(object):
                  > pass
                  >
                  > # the new-style __dict__ is a dictproxy object
                  > A.__dict__[meth_name] = lambda self: type(self)[color=green][color=darkred]
                  > >>Traceback (most recent call last):
                  > >> File "<interacti ve input>", line 1, in ?
                  > >>TypeError: object does not support item assignment[/color][/color]
                  >
                  > Is there other way to add/change methods to new-style classes
                  > dynamically?[/color]
                  [color=blue][color=green][color=darkred]
                  >>> import new
                  >>> dir(new)[/color][/color][/color]
                  ['__builtins__', '__doc__', '__file__', '__name__', 'classobj', 'code', 'function', 'instance', 'instancemethod ', 'module'][color=blue][color=green][color=darkred]
                  >>> def foo(self):[/color][/color][/color]
                  .... print "FOO!"[color=blue][color=green][color=darkred]
                  >>> A.foo = new.instancemet hod(foo, None, A) # func, object, class
                  >>> A.foo[/color][/color][/color]
                  <unbound method A.foo>[color=blue][color=green][color=darkred]
                  >>> a = A()
                  >>> a.foo[/color][/color][/color]
                  <bound method A.foo of <__main__.A object at 0xb7e0b4cc>>[color=blue][color=green][color=darkred]
                  >>> a.foo()[/color][/color][/color]
                  FOO!

                  -jackdied


                    Comment

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