OO approach to decision sequence?

[Paul McGuire]

Lee C -

Here is a technique for avoiding the if-elseif-elseif...-else method
for building objects.  It is a modified form of ChainOfResponsibility
pattern, in which you have a collection of factory methods that all
have a common signature, or a collection of Factory classes that all
implement a "makeObject" method.  These common methods probably take a
string, and return a generic object.  Fortunately, such type
flexibility is *very* easy in Python. :)

Example:
I want to convert individual strings to native data types.  I want to
detect integers, reals, complex numbers, and booleans (indicated by
'True' or 'False').  This is kind of similar to a parsing problem, but
it could also be used for deserializing or unpickling data of an
unknown original type.

Note the special treatment I have to go through with boolean values - I
needed to write a special makeBool routine, since Python will take any
non-empty string to be True, when what I want is 'True' to yield true
and 'False' to yield false.

Hope this gives you some alternative ideas to your cascading if's.

-- Paul


def makeBool(s):
    if s in ('True','False'):
        return s == 'True'
    raise ValueError

converters = [ int, float, complex, makeBool, str ]

def makeObject(stringVar):
    for conv in converters:
        try:
            val = conv(stringVar)
        except Exception:
            continue
        else:
            break;
    return val

def test(s):
    val = makeObject(s)
    print s, val, type(val)

test('1')
test('1.0')
test('1+2j')
test('1+0j')
test('True')
test('False')
test('A')

prints:
1 1 <type 'int'>
1.0 1.0 <type 'float'>
1+2j (1+2j) <type 'complex'>
1+0j (1+0j) <type 'complex'>
True True <type 'bool'>
False False <type 'bool'>
A A <type 'str'>