(beginners question) howto set self.field4.subfield8='asdf'?

[Peter Otten]

openopt at ukr.net wrote:

> I have
> class A:
> def __init__(self, objFun, x0):
>         #(I want to have self.primal.f = objFun)
>         #both
>         self.primal.f = objFun
>         #and
>         self.primal = None
>         self.primal.f = objFun

None is a singleton, so if Python were to allow the above f would always be
the objFun of the last created A instance.

> yields error
> what should I do?

Make a dedicated Primal class:

>>> class Primal:
...     pass
...
>>> class A:
...     def __init__(self, fun, x0):
...             self.primal = Primal()
...             self.primal.f = fun
...
>>> def square(x): return x*x
...
>>> a = A(square, 42)
>>> a.primal.f
<function square at 0x401d609c>

Even better, because it makes no assumptions about the internal layout of
Primal:

class Primal:
   def __init__(self, f):
      self.f = f

...
self.primal = Primal(fun)
...

Peter