What are some (concrete) use-cases for metaclasses?

Question

I have a friend who likes to use metaclasses, and regularly offers them as a solution.

I am of the mind that you almost never need to use metaclasses. Why? because I

Answer 1

I recently had to use a metaclass to help declaratively define an SQLAlchemy model around a database table populated with U.S. Census data from http://census.ire.org/data/bulkdata.html

IRE provides database shells for the census data tables, which create integer columns following a naming convention from the Census Bureau of p012015, p012016, p012017, etc.

I wanted to a) be able to access these columns using a model_instance.p012017 syntax, b) be fairly explicit about what I was doing and c) not have to explicitly define dozens of fields on the model, so I subclassed SQLAlchemy's DeclarativeMeta to iterate through a range of the columns and automatically create model fields corresponding to the columns:

from sqlalchemy.ext.declarative.api import DeclarativeMeta

class CensusTableMeta(DeclarativeMeta):
    def __init__(cls, classname, bases, dict_):
        table = 'p012'
        for i in range(1, 49):
            fname = "%s%03d" % (table, i)
            dict_[fname] = Column(Integer)
            setattr(cls, fname, dict_[fname])

        super(CensusTableMeta, cls).__init__(classname, bases, dict_)

I could then use this metaclass for my model definition and access the automatically enumerated fields on the model:

CensusTableBase = declarative_base(metaclass=CensusTableMeta)

class P12Tract(CensusTableBase):
    __tablename__ = 'ire_p12'

    geoid = Column(String(12), primary_key=True)

    @property
    def male_under_5(self):
        return self.p012003

    ...