Fri Dec 20, 2013
Tags: python development
We’ve all been there - working on a one-off script or personal project where you deal with lots of tuples: maybe points on a coordinate grid or time-series data, or anything imported from a
csv. You do the lazy thing and keep track of everything in a list of tuples, shrugging off that object-oriented nagging in the back of your head.
Python, as always, has a solution for your troubles, hidden in a rarely-accessed
Behold, ye mighty, and despair – I present to you
namedtuple, a perfect middleground for developers like myself who are too precocious to live with poorly-documented tuple monstrosities and too lazy to do anything about it.
>>> from collections import namedtuple >>> Point = namedtuple("Point", ("x", "y")) >>> foo = Point(0, 4) >>> foo Point(x=0, y=4) >>> foo.x 0 >>> foo 4
Behind the scenes,
namedtuple actually creates an entire subclass of
tuple, which you can verify by adding
class Point(tuple): 'Point(x, y)' __slots__ = () _fields = ('x', 'y') def __new__(_cls, x, y): 'Create new instance of Point(x, y)' return _tuple.__new__(_cls, (x, y)) @classmethod def _make(cls, iterable, new=tuple.__new__, len=len): 'Make a new Point object from a sequence or iterable' result = new(cls, iterable) if len(result) != 2: raise TypeError('Expected 2 arguments, got %d' % len(result)) return result def __repr__(self): 'Return a nicely formatted representation string' return 'Point(x=%r, y=%r)' % self def _asdict(self): 'Return a new OrderedDict which maps field names to their values' return OrderedDict(zip(self._fields, self)) def _replace(_self, **kwds): 'Return a new Point object replacing specified fields with new values' result = _self._make(map(kwds.pop, ('x', 'y'), _self)) if kwds: raise ValueError('Got unexpected field names: %r' % kwds.keys()) return result def __getnewargs__(self): 'Return self as a plain tuple. Used by copy and pickle.' return tuple(self) x = _property(_itemgetter(0), doc='Alias for field number 0') y = _property(_itemgetter(1), doc='Alias for field number 1')
And, since they’re subclasses of tuples, you get all of the wonderful interactions baked right in:
>>> [foo, Point(-2, 2)] [Point(x=0, y=4), Point(x=-2, y=2)] >>> map(abs, Point(-2, 2)) [2, 2]
Simple, elegant, and way more self-documenting than referencing
line and hoping you remember that 8 is the invoice amount and not the revenue figure (especially thirty days after you write the damn thing).