What are named tuples?
A named tuple is a tuple.
It does everything a tuple can.
But it's more than just a tuple.
It's a specific subclass of a tuple that is programmatically created to your specification, with named fields and a fixed length.
This, for example, creates a subclass of tuple, and aside from being of fixed length (in this case, three), it can be used everywhere a tuple is used without breaking. This is known as Liskov substitutability.
New in Python 3.6, we can use a class definition with typing.NamedTuple to create a namedtuple:
from typing import NamedTuple
class ANamedTuple(NamedTuple):
"""a docstring"""
foo: int
bar: str
baz: list
The above is the same as the below, except the above additionally has type annotations and a docstring. The below is available in Python 2+:
>>> from collections import namedtuple
>>> class_name = 'ANamedTuple'
>>> fields = 'foo bar baz'
>>> ANamedTuple = namedtuple(class_name, fields)
This instantiates it:
>>> ant = ANamedTuple(1, 'bar', [])
We can inspect it and use its attributes:
>>> ant
ANamedTuple(foo=1, bar='bar', baz=[])
>>> ant.foo
1
>>> ant.bar
'bar'
>>> ant.baz.append('anything')
>>> ant.baz
['anything']
Deeper explanation
To understand named tuples, you first need to know what a tuple is. A tuple is essentially an immutable (can't be changed in-place in memory) list.
Here's how you might use a regular tuple:
>>> student_tuple = 'Lisa', 'Simpson', 'A'
>>> student_tuple
('Lisa', 'Simpson', 'A')
>>> student_tuple[0]
'Lisa'
>>> student_tuple[1]
'Simpson'
>>> student_tuple[2]
'A'
You can expand a tuple with iterable unpacking:
>>> first, last, grade = student_tuple
>>> first
'Lisa'
>>> last
'Simpson'
>>> grade
'A'
Named tuples are tuples that allow their elements to be accessed by name instead of just index!
You make a namedtuple like this:
>>> from collections import namedtuple
>>> Student = namedtuple('Student', ['first', 'last', 'grade'])
You can also use a single string with the names separated by spaces, a slightly more readable use of the API:
>>> Student = namedtuple('Student', 'first last grade')
How to use them?
You can do everything tuples can do (see above) as well as do the following:
>>> named_student_tuple = Student('Lisa', 'Simpson', 'A')
>>> named_student_tuple.first
'Lisa'
>>> named_student_tuple.last
'Simpson'
>>> named_student_tuple.grade
'A'
>>> named_student_tuple._asdict()
OrderedDict([('first', 'Lisa'), ('last', 'Simpson'), ('grade', 'A')])
>>> vars(named_student_tuple)
OrderedDict([('first', 'Lisa'), ('last', 'Simpson'), ('grade', 'A')])
>>> new_named_student_tuple = named_student_tuple._replace(first='Bart', grade='C')
>>> new_named_student_tuple
Student(first='Bart', last='Simpson', grade='C')
A commenter asked:
In a large script or programme, where does one usually define a named tuple?
The types you create with namedtuple are basically classes you can create with easy shorthand. Treat them like classes. Define them on the module level, so that pickle and other users can find them.
The working example, on the global module level:
>>> from collections import namedtuple
>>> NT = namedtuple('NT', 'foo bar')
>>> nt = NT('foo', 'bar')
>>> import pickle
>>> pickle.loads(pickle.dumps(nt))
NT(foo='foo', bar='bar')
And this demonstrates the failure to lookup the definition:
>>> def foo():
... LocalNT = namedtuple('LocalNT', 'foo bar')
... return LocalNT('foo', 'bar')
...
>>> pickle.loads(pickle.dumps(foo()))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
_pickle.PicklingError: Can't pickle <class '__main__.LocalNT'>: attribute lookup LocalNT on __main__ failed
Why/when should I use named tuples instead of normal tuples?
Use them when it improves your code to have the semantics of tuple elements expressed in your code.
You can use them instead of an object if you would otherwise use an object with unchanging data attributes and no functionality.
You can also subclass them to add functionality, for example:
class Point(namedtuple('Point', 'x y')):
"""adding functionality to a named tuple"""
__slots__ = ()
@property
def hypot(self):
return (self.x ** 2 + self.y ** 2) ** 0.5
def __str__(self):
return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y, self.hypot)
Why/when should I use normal tuples instead of named tuples?
It would probably be a regression to switch from using named tuples to tuples. The upfront design decision centers around whether the cost from the extra code involved is worth the improved readability when the tuple is used.
There is no extra memory used by named tuples versus tuples.
Is there any kind of "named list" (a mutable version of the named tuple)?
You're looking for either a slotted object that implements all of the functionality of a statically sized list or a subclassed list that works like a named tuple (and that somehow blocks the list from changing in size.)
A now expanded, and perhaps even Liskov substitutable, example of the first:
from collections import Sequence
class MutableTuple(Sequence):
"""Abstract Base Class for objects that work like mutable
namedtuples. Subclass and define your named fields with
__slots__ and away you go.
"""
__slots__ = ()
def __init__(self, *args):
for slot, arg in zip(self.__slots__, args):
setattr(self, slot, arg)
def __repr__(self):
return type(self).__name__ + repr(tuple(self))
# more direct __iter__ than Sequence's
def __iter__(self):
for name in self.__slots__:
yield getattr(self, name)
# Sequence requires __getitem__ & __len__:
def __getitem__(self, index):
return getattr(self, self.__slots__[index])
def __len__(self):
return len(self.__slots__)
And to use, just subclass and define __slots__:
class Student(MutableTuple):
__slots__ = 'first', 'last', 'grade' # customize
>>> student = Student('Lisa', 'Simpson', 'A')
>>> student
Student('Lisa', 'Simpson', 'A')
>>> first, last, grade = student
>>> first
'Lisa'
>>> last
'Simpson'
>>> grade
'A'
>>> student[0]
'Lisa'
>>> student[2]
'A'
>>> len(student)
3
>>> 'Lisa' in student
True
>>> 'Bart' in student
False
>>> student.first = 'Bart'
>>> for i in student: print(i)
...
Bart
Simpson
A
