Am 01.03.2011 07:49, schrieb David:
I have an idea that might clean up my code slightly, if I can make one
of my classes
clever enough to refuse to instantiate itself if a necessary condition
is not met.
I think that's too clever ;-). Instead, you could create a function
which has the only and explicit purpose to decide wether or not to
create the class, e.g.
def class_for_condition(condition):
if condition:
return MyClass()
return None
and use this throughout your code, e.g.
my_object = class_for_condition(condition)
But to me, it sounds a bit like trouble in general. As Alan said, you
have to deal with two options for my_object in the remaining code. I
can't really judge without seeing what you want to do later on, but
maybe you have the chance to branch on a higher abstraction level?
if condition:
do_all_this_stuff() # do the things with my_object
else:
do_the_other_stuff() # do the things you don't need my_object
Cheers,
Jan
Below I propose some example code that seems to achieve this, and I am
asking here
for feedback on it, because I have not written much python. So I might be doing
something unwise due to fiddling with things I don't totally understand.
My aim is that instead of writing this:
class MyClass:
pass
condition = 0
if condition:
my_object = MyClass()
else:
my_object = None
I could instead write this:
class MyClass_2:
# put the if-test here inside the class
my_object_2 = MyClass_2(condition)
to achieve the goal that (my_object_2 == None) when (condition == False).
I read the (ver 2.6) Python Language Reference
Section 3.4.1. Basic customization
Section 3.4.3. Customizing class creation
Most of the content there is way beyond my current understanding, but I came up
with the following code, which seems to work:
class MyClass_2(object):
def __new__(self, condition):
if condition:
return object.__new__(self)
else:
return None
condition = 0
my_object_2 = MyClass_2(condition)
print my_object_2
condition = 1
my_object_2 = MyClass_2(condition)
print my_object_2
Can anyone see any technical or style issues with that? Or
alternatively reassure me that it is completely ok?
Thanks.
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