two newbie questions "cyclic class dependencies" and "date from string"

[Daan Hoogland]

H everyone,

I'm new as the title sugests. And I'd like two classes to be able to
instantiate each other:

<file name='both.py'>
class A:
  def __init__(self):
    self.b = B();


class B:
  def delegate(self, a):
    a = A();
    return a;

x = A();
y = None;
y = x.b.delegate(y);
print x;
print y;
</file>

this works. Now I want to have these lines in three files. so adding some
import statements seemed the obvious. It failed for equally obvious reasons:

ImportError: cannot import name A                                               

<file name="A.py">
from B import B;
 
class A:
  def __init__(self):
    self.b = B();
</file>

<file name="B.py">
from A import A;
 
class B:
  def delegate(self, a):
    a = A();
</file>

<file name="C.py">
from A import A;
 
x = A();
y = None;
y = x.b.delegate(y);
print x;
print y;
</file>

<stderr>
bash-2.04$ python C.py
Traceback (most recent call last):
  File "C.py", line 1, in ?
    from A import A;
  File "/data/home/daan/tmp/A.py", line 1, in ?
    from B import B;
  File "/data/home/daan/tmp/B.py", line 1, in ?
    from A import A;
ImportError: cannot import name A                                               
</stderr>

Is there a trick around this? Any advice on reading on object orientation in
python? My oo is good, i think.


I can't find a function to convert a date string back to a datetime object. The
date i have is returned by postgresql in the (well known) format "YYYY-MM-DD
hh:mm:ss:mmmmmm". Am i missing something in the help? Or is there an extended
Date package that does this?

thanks,