Lorenzo Di Gregorio wrote:
On Jun 20, 8:43 pm, Dave Angel <da...@ieee.org> wrote:
Lorenzo Di Gregorio wrote:
Hi,
I'm wondering what would be the preferred way to solve the following
forward reference problem:
---------------------------------------
class BaseA(object):
def __init__(self):
return
class DebugA(BaseA):
def __init__(self):
return
# here I would have a prototype of class A which is the same as class
BaseA
class B(object):
def __init__(self):
self.obj =()
return
if __name__ ="__main__":
# class A(BaseA): # Uncomment this for using BaseA objects
# pass
class A(DebugA): # Uncomment this for using DebugA objects
pass
---------------------------------------
I can figure out some ways to fix this but none seems satisfying.
Either they are too specific or too cumbersome.
A runtime redefinition of class A does not seem to work either.
What would be the most "pythonesque" solution other than sorting out
the class order?
Best Regards,
Lorenzo
You haven't shown us any problem. class B works fine with a forward
reference to A. Now if you were trying to subclass A before defining
it, that'd be a problem. Or if you were trying to make an instance of B
before defining A.
Better put some code together with enough meat to actually show a
symptom. And tell us what sys.version says. I'm testing with 2.6.2
(r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit (Intel)], running
on Win XP.- Hide quoted text -
- Show quoted text -
Thank you for your help: I'm working on a rather large source, but I
think I have isolated the problem now.
This listing generates an error:
-----------------------------------------------
class BaseA(object):
def __init__(self):
return
class DebugA(BaseA):
def __init__(self):
return
class B(object):
def __init__(self,test=A()):
self.obj =()
return
if __name__ ="__main__":
# class A(BaseA): # Uncomment this for using BaseA objects
# pass
class A(DebugA): # Uncomment this for using DebugA objects
pass
-----------------------------------------------
The error happens because Python apparently evaluates the named
arguments before running the script.
I think I have read something about this some (long) time ago but I
can't find it anymore.
Suggestions?
BTW, my Python version is 2.6.1 (with latest PyDev).
Thx!
Lorenzo
This error is caused because a default argument uses class A. Default
arguments of class methods are evaluated during the definition of the
class, and not later when the class is instantiated. Thus the problem.
To work around that specific problem, you may want to use the following:
class B(object):
def __init__(self,test=None):
if test==None:
test = A()
self.obj =()
return
This is actually different than what you had, since what you had would
have used the same A() object for all instances of B that didn't supply
their own test() parameter. Maybe that's what you wanted, and maybe
not, but default arguments set to mutable values are frequently a bug.
But I'm wondering if you're just looking for problems. Why not put the
commented switch early in the file, and test for it wherever you need to
use it?
import x, y, z
_debug = False
#_debug = True
then as soon as BaseA and DebugA are defined, do the following:
if _debug:
class A(DebugA):
pass
else:
class A(BaseA)
pass
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