[Zope] ZClass

[Thomas B. Passin]

From: Stephan Richter <srichter@cbu.edu>
To: Tom Deprez <tom.deprez@uz.kuleuven.ac.be>; zope@zope.org <zope@zope.org>


>> Why does Zope asks for a value to enter for a new property when you're
>> making a class? I understand that you've to enter a value when you're
>> making a Class instance? But while defining a class??? Is this meant as a
>> default value??
>Okay, the problem is that Python does not know about data types. You cannot
>declare an"empty" (variable without value) variable in Python, since a
>statement like 'int test' does not exist.I have heard from DC (someone correct
[snip]

Sure you can declare an empty variable in Python, and you can test for it.  Of
course you can't test for its data type, but so what - it can become any data
type you need.

Run the intrepreter and  try this:

>>> aa
Traceback (innermost last):
  File "<stdin>", line 1, in ?
NameError: aa
>>>

# aa was not declared so it does not exist.

>>> ob=None
>>> ob
# ob was declared as an empty object (that's what None is: a standard empty
object) and gives no error when referred to.

>>> ob==None
1
>>>
# The statement ob==None is true

>>> aa==None
Traceback (innermost last):
  File "<stdin>", line 1, in ?
NameError: aa
# The statement aa==None is an error since aa has not been created.

Finally, create this script as ob.py and run it:

try:
 aa==None
except:
 print "aa hasn't been defined"

Run it:
C:>python ob
aa hasn't been defined

So in Python you CAN define an empty variable and check for its existence, but
whether or not that is a useful or even possible thing to do in Zope is another
matter.
Tom Passin