[Tutor] A class that instantiates conditionally ?
Izz ad-Din Ruhulessin
izzaddin.ruhulessin at gmail.com
Thu Mar 3 07:59:43 CET 2011
foo = lambda x: MyClass() if condition else None
2011/3/3 David <bouncingcats at gmail.com>
> 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
> 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
> and then use "if" tests in the rest of the code that processes the objects
> 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
> > because it is bound to the class object itself (MyClass_2 in this
> > 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
> 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
> "self" when getting their example code to work for me. In Section 3.4.1
> 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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