Maybe: foo = lambda x: MyClass() if condition else None
2011/3/3 David <bouncingc...@gmail.com> > 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 >
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