[Tutor] A class that instantiates conditionally ?

[Izz ad-Din Ruhulessin]

Of course that would be:

> foo = lambda condition: MyClass() if condition else None


2011/3/3 Izz ad-Din Ruhulessin <izzaddin.ruhulessin at gmail.com>

> Maybe:
>
> 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
>>  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.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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>
>
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