[Tutor] A class that instantiates conditionally ?

David bouncingcats at gmail.com
Thu Mar 3 01:47:33 CET 2011

Another classic case of trying something not the best way, due to
But it has been a good process: I learned something new from
setting myself the initial puzzle and then finding a solution,and then
learned more from the great tutoring here. Thanks very much for all
the replies.

On 2 March 2011 03:31, Alan Gauld <alan.gauld at btinternet.com> wrote:
>> class MyClass_2(object):
>>    def __new__(self, condition):
>>         if condition:
>>               return object.__new__(self)
>>         else:
>>               return None
> Thats pretty much how I'd do it.

Thanks for reviewing my code.

On 2 March 2011 03:35, Alan Gauld <alan.gauld at btinternet.com> wrote:
> Oops, sent too soon.
> I meant to add that you should realize that the implication of your
> design is that the user of the class now has to check each object
> to see if it is a valid reference or None. You could raise an exception
> instead of returning None which allows a try/except style...
> This extra overhead is one reason these kinds of "clever" tricks
> are usually avoided. A valid object with null content is often
> preferrable, or a singleton style pattern. But occasionally your
> style is needed, just be aware of the extra overhead you
> introduce by using it.

Spot on. It would require two "if" tests, one inside __new__() and
another in the code.

I found your mention of try/except there especially helpful, because
it was a pertinent reminder that I was not thinking in "ask forgiveness
not permission" mode. This (newbie mistake) occurred because I
wanted my application to continue, not abort with an exception, but
after your prompt I recalled that "except" doesn't have to raise exceptions
it can do other things.

So I went in the direction you suggested and I am happy with the results.
Basically my application is interpreting binary file data by instantiating a
structured object for each file in a fixed list of binary files, and I
was looking
for a neat way to ignore any errors on files that might not be present
(failed to open). So, after feedback here my solution now is to use
try/except in the class __init__() to create a valid object with null content,
and then use "if" tests in the rest of the code that processes the objects to
just ignore them if they are null, which is a nice clear way to do it.

On 2 March 2011 20:44, Steven D'Aprano <steve at pearwood.info> wrote:
> By convention, the name of the first argument to __new__ is cls, not self,
> because it is bound to the class object itself (MyClass_2 in this example)
> rather than the instance. The instance doesn't yet exist, so that's not
> surprising!

Thanks for pointing that out. In 2.6 Language Reference 3.4.3 they used
"mcs" (metaclass?) which I didn't comprehend at all at the time (the mindset
of just wanting to get some code working to fix a problem is not the most
helpful mindset for decoding a large body of new information), so I just used
"self" when getting their example code to work for me. In Section 3.4.1 they
use "cls" which I now see clearly and understand thanks.

On 3 March 2011 03:03, Knacktus <knacktus at googlemail.com> wrote:
> I think that's too clever ;-).

I agree now .. but it was a useful experiment. Thanks for the tute.

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