[Tutor] Class Tips

[David]

Alan Gauld wrote:
> 
> "David" <david at abbottdavid.com> wrote
> 
>> I took this program that determines a fertilizer application rate;
>> ...
>> And converted it to class/object to learn how they work. Just looking 
>> for some pointers, if I did it correctly etc.
> 
> For such a small program its hard to see what else you could have done. 
> Technically itys not too bad, I've made a couple of suggestions below. 
> However, leaving the fact that its an excercise I'd say that really this 
> is just a function rather than a class.
> 
>> class FertRate:
>>     def __init__(self, rate, nitrogen, acre, bag):
>>         self.area = 43.560
>>         self.app = rate / (nitrogen / 100.00)
>>         self.acre = acre
>>         self.bag = bag
>>
>>     def result(self):
>>         result = self.app * self.area * self.acre / self.bag
>>         print 'You should apply %0.2f bags.' % result
> 
> Its never a good idea to include printing in the same function as 
> calculation. IT would be better to have two functions one to calculate 
> and one to generate the printable output. (The actual printing is best 
> done outside the class altogether, it improves the reusability. For 
> example prints don't work in a GUI but a formatted string can be used.
> 
> But even if you keep it as one function at lest return the result as a 
> string rather than print it
> 
> 
>> def main():
>>     rate, nitrogen, acre, bag = get_inputs()
>>     frate = FertRate(rate, nitrogen, acre, bag)
> 
> This could have just been:
> 
> frate = FertRate( get_inputs() )
> 
> which saves the need for the variables.
> 
>>     frate.result()
> 
> So if you took my advice about returning a string this becomes
> 
> print frate.result()
> 
> But as I say you could just have made FertRate a function:
> 
> print FertRate( get_inputs() )
> 
> and it would have been just as effective, classes really only start to 
> be useful on slightly larger programs than this.
> 
> 
> But as an example of using a class its nearly OK, just the tweak of the 
> return value to think about - and that applies to the function version too!
> 
> 
Thanks Alan,

this works;
def main():
     rate, nitrogen, acre, bag = get_inputs()
     frate = FertRate(rate, nitrogen, acre, bag)
     result = frate.result()
     print 'You should apply %0.2f bags.' % result

but I get an error here;

def main():
     frate = FertRate(get_inputs())
     result = frate.result()
     print 'You should apply %0.2f bags.' % result


Traceback (most recent call last):
   File "./fert_class.py", line 26, in <module>
     main()
   File "./fert_class.py", line 15, in main
     frate = FertRate(get_inputs())
TypeError: __init__() takes exactly 5 arguments (2 given)



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