Another classic case of trying something not the best way, due to inexperience. 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.ga...@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.ga...@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 <st...@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 <knack...@googlemail.com> wrote: > > I think that's too clever ;-). I agree now .. but it was a useful experiment. Thanks for the tute. _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
