Redesign factory to throw error on load time instead of on execution time

Question

I am using a factory pattern to get objects that shouldn't be instantiated other than the factory class. These objects are of type ViolationType, which represent violations on a set of rule. Here is the summary of the factory and the available violation types:

class ViolationFactory:
    def __init__(self):
        raise ValueError('%s cannot be instantiated' % self.__class__.__name__)

    class ViolationType:
        def __init__(self, name):
            self.name = name

    _violations = (
        ViolationType('RULE_X'),
        ViolationType('RULE_Y'),
        # the rest of the types...
    )
    _violations_dict = {v.name: v for v in _violations}

    @staticmethod
    def get(violation_name):
        if violation_name not in ViolationFactory._violations_dict:
            raise ValueError('Invalid ViolationType')
        return ViolationFactory._violations_dict[violation_name]

A ViolationType is primarily used to instantiate a Violation:

violation = Violation(ViolationFactory.get('RULE_X'), **kwargs)

The problem with this is that if I misspell the string passed to the get() factory method I won't know there is an error until that particular piece of code is ran. So if I try doing ViolationFactory.get('RULE_Z') and such type doesn't exist, it should throw an error when running the program

I think the current design simply doesn't allow such behavior, so: how could I redesign it so that I get a load time error rather than a run time one?

docs.python.org/3/library/enum.html enumeration ? I don't really know however if your IDE will show it or a syntax error will be thrown on loading thescript before running it. — Walfrat
– Walfrat, Commented Jun 22, 2017 at 11:48

Vladimir Stokic · Accepted Answer · 2017-06-23 07:07:08Z

You could use enums as rule identifiers. That way, you would know if you made a mistake at compile time. Something along these lines:

enum Rules
{
   Rule_X = 0,
   Rule_Y = 1,
   Rule_Z = 2
}

Then, the call would look like this:

violation = Violation(ViolationFactory.get(Rules.Rule_X), **kwargs)

Alternatively, you could implement every rule as a plugin, in their own assembly, and load the assemblies dynamically. Then, if the plugin is not loaded correctly, throw an exception and you will know that there was a problem at load time. The downside is that if you have many rules, you will have many assemblies.

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