extend Python namedtuple with many @properties?

问题:

How can namedtuples be extended or subclassed with many additional @properties ?
For a few, one can just write the text below; but there are many, so I'm looking for a generator or property factory. One way would be to generate text from _fields and exec it; another would be an add_fields with the same effect at runtime.
(My @props are to get rows and fields in a database scattered across several tables, so that rec.pname is persontable[rec.personid].pname; but namedtuples-with-smart-fields would have other uses too.)

""" extend namedtuple with many @properties ? """
from collections import namedtuple

Person = namedtuple( "Person", "pname paddr" )  # ...
persontable = [
    Person( "Smith", "NY" ),
    Person( "Jones", "IL" )
    ]

class Top( namedtuple( "Top_", "topid amount personid" )):
    """ @property 
        .person -> persontable[personid]
        .pname -> person.pname ...
    """
    __slots__ = ()
    @property
    def person(self):
        return persontable[self.personid]

    # def add_fields( self, Top.person, Person._fields ) with the same effect as these ?
    @property
    def pname(self):
        return self.person.pname
    @property
    def paddr(self):
        return self.person.paddr
    # ... many more

rec = Top( 0, 42, 1 )
print rec.person, rec.pname, rec.paddr

回答1:

The answer to your question

How can namedtuples be extended or subclassed with additional @properties ?

is: exactly the way you're doing it! What error are you getting? To see a simpler case,

>>> class x(collections.namedtuple('y', 'a b c')):
...   @property
...   def d(self): return 23
... 
>>> a=x(1, 2, 3)
>>> a.d
23
>>> 

回答2:

How about this?

class Top( namedtuple( "Top_", "topid amount personid" )): 
    """ @property  
        .person -> persontable[personid] 
        .pname -> person.pname ... 
    """ 
    __slots__ = () 
    @property 
    def person(self): 
        return persontable[self.personid] 

    def __getattr__(self,attr):
        if attr in Person._fields:
            return getattr(self.person, attr)
        raise AttributeError("no such attribute '%s'" % attr)

回答3:

Here's one approach, a little language: turn this into Python text like the above, and exec it.
(Expanding text-to-text is easy to do, and easy to test — you can look at the intermediate text.)
I'm sure there are similar if not-so-little such, links please ?

# example of a little language for describing multi-table databases 3feb
# why ?
# less clutter, toprec.pname -> persontable[toprec.personid].pname
# describe in one place: easier to understand, easier to change

Top:
    topid amount personid
    person: persontable[self.personid] + Person
        # toprec.person = persontable[self.personid]
        # pname = person.pname
        # locid = person.locid
        # todo: chaining, toprec.city -> toprec.person.loc.city

Person:
    personid pname locid
    loc: loctable[self.locid] + Loc

Loc:
    locid zipcode province city