Feasible in Python ? list of object , with two exeptional objects

[Paul McGuire]

"Osiris" <nono at hotmail.com> wrote in message 
news:6eo5p2di7lrcs5smouhg8ju83901b4hld0 at 4ax.com...
> Is the following intuitively feasible in Python:
> I have an array (I come from C) of identical objects, called sections.
> These sections have some feature, say a length, measured in mm, which
> is calculated by a method A_length of the instantiation of the
> Section's class.
> Only, two elements in the array (or list ?) have a length that must be
> calculated according to a totally different procedure, a different
> function or method.
> After calculation of ALL the lengths, they must be added together and
> output.
> The calculation procedure screams for a FOR or a WHILE loop allong the
> list of sections, only those two sections mentioned make life
> difficult.
>

Any reason you don't just use simple inheritance?


from random import choice
class BasicSection(object):
    def __str__(self):
        return "%s: %s" % (self.__class__.__name__, self.A_length())

class OrdinarySection(BasicSection):
    def __init__(self):
        # boring numbers
        self.mundaneLengthValue = choice(range(10))
    def A_length(self):
        return self.mundaneLengthValue

class ExceptionalSection(BasicSection):
    def __init__(self):
        # exceptional numbers!
        self.extraordinaryLengthValue = 
choice([1.414,3.14159,2.71828,1.6180339,])
    def A_length(self):
        return self.extraordinaryLengthValue

# create list of Sections, randomly choosing Ordinary or Exceptional ones
listOfSections = [ choice( (OrdinarySection,ExceptionalSection) )()
                    for i in range(10) ]

# now iterate over the list and get length for each
totalLength = 0
for sec in listOfSections:
    print "Adding length of " + str(sec)
    totalLength += sec.A_length()
print "total =", totalLength

# or more Pythonically, use a generator expression
totalLength = sum( sec.A_length() for sec in listOfSections )
print "total =", totalLength

One sample run gives this result (although because we randomly choose which 
class to create when adding elements to the list, the results are different 
every time):
Adding length of OrdinarySection: 6
Adding length of OrdinarySection: 2
Adding length of OrdinarySection: 0
Adding length of OrdinarySection: 4
Adding length of OrdinarySection: 1
Adding length of ExceptionalSection: 3.14159
Adding length of OrdinarySection: 4
Adding length of ExceptionalSection: 3.14159
Adding length of ExceptionalSection: 2.71828
Adding length of ExceptionalSection: 1.414
total = 27.41546
total = 27.41546


In truth, it is not even necessary for both classes to subclass from 
BasicSubject (as would be the case in C++ or Java).  Python's duck-typing 
will take any object that implements A_length() - I just created a common 
superclass to put the __str__ method in, and to more classically follow 
common inheritance patterns.

-- Paul