Raymond Hettinger <raymond.hettinger <at> verizon.net> writes:
> * Add a pure python named_tuple class to the collections module. I've been
> using the class for about a year and found that it greatly improves the
> usability of tuples as records.
> http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/500261
The implementation of this recipe is really clean and I like it a lot
(I even think of including it in our codebase), but there are a few issues
that I would like to point out.
1. I find the camelcase confusing and I think that NamedTuple should be
spelled namedtuple, since is a function, not a class. The fact that it
returns classes does not count ;)
2. I agree with Giovanni Bajo, the constructor signature should be consistent
with regular tuples. For instance I want to be able to map a named tuple
over a record set as returned by fetchall.
3. I would like to pass the list of the fields as a sequence, not as
a string. It would be more consistent and it would make easier
the programmatic creation of NamedTuple classes at runtime.
4. I want help(MyNamedTuple) to work well; in particular it should
display the right module name. That means
that in the m dictionary you should add a __module__ attribute:
__module__ = sys._getframe(1).f_globals['__name__']
5. The major issue is that pickle does work with named tuples since the
__module__ attribute is wrong. The suggestion in #4 would solve even
this issue for free.
6. The ability to pass a show function to the __repr__ feems over-engineering
to me.
In short, here is how I would change the recipe:
import sys
from operator import itemgetter
def namedtuple(f):
"""Returns a new subclass of tuple with named fields.
>>> Point = namedtuple('Point x y'.split())
>>> Point.__doc__ # docstring for the new class
'Point(x, y)'
>>> p = Point((11,), y=22) # instantiate with positional args or keywords
>>> p[0] + p[1] # works just like the tuple (11, 22)
33
>>> x, y = p # unpacks just like a tuple
>>> x, y
(11, 22)
>>> p.x + p.y # fields also accessable by name
33
>>> p # readable __repr__ with name=value style
Point(x=11, y=22)
"""
typename, field_names = f[0], f[1:]
nargs = len(field_names)
def __new__(cls, args=(), **kwds):
if kwds:
try:
args += tuple(kwds[name] for name in field_names[len(args):])
except KeyError, name:
raise TypeError(
'%s missing required argument: %s' % (typename, name))
if len(args) != nargs:
raise TypeError(
'%s takes exactly %d arguments (%d given)' %
(typename, nargs, len(args)))
return tuple.__new__(cls, args)
template = '%s(%s)' % (
typename, ', '.join('%s=%%r' % name for name in field_names))
def __repr__(self):
return template % self
m = dict(vars(tuple)) # pre-lookup superclass methods (for faster lookup)
m.update(__doc__= '%s(%s)' % (typename, ', '.join(field_names)),
__slots__ = (), # no per-instance dict
__new__ = __new__,
__repr__ = __repr__,
__module__ = sys._getframe(1).f_globals['__name__'],
)
m.update((name, property(itemgetter(index)))
for index, name in enumerate(field_names))
return type(typename, (tuple,), m)
if __name__ == '__main__':
import doctest
TestResults = namedtuple(['TestResults', 'failed', 'attempted'])
print TestResults(doctest.testmod())
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