19.8: Named tuples
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Many simple objects are basically collections of related values. For example, the Point object defined in Chapter 15 contains two numbers,
y. When you define a class like this, you usually start with an init method and a str method:
class Point: def __init__(self, x=0, y=0): self.x = x self.y = y def __str__(self): return '(%g, %g)' % (self.x, self.y)
This is a lot of code to convey a small amount of information. Python provides a more concise way to say the same thing:
from collections import namedtuple Point = namedtuple('Point', ['x', 'y'])
The first argument is the name of the class you want to create. The second is a list of the attributes Point objects should have, as strings. The return value from
namedtuple is a class object:
>>> Point <class '__main__.Point'>
Point automatically provides methods like
__str__ so you don’t have to write them.
To create a Point object, you use the Point class as a function:
>>> p = Point(1, 2) >>> p Point(x=1, y=2)
The init method assigns the arguments to attributes using the names you provided. The str method prints a representation of the Point object and its attributes.
You can access the elements of the named tuple by name:
>>> p.x, p.y (1, 2)
But you can also treat a named tuple as a tuple:
>>> p, p (1, 2) >>> x, y = p >>> x, y (1, 2)
Named tuples provide a quick way to define simple classes. The drawback is that simple classes don’t always stay simple. You might decide later that you want to add methods to a named tuple. In that case, you could define a new class that inherits from the named tuple:
class Pointier(Point): # add more methods here
Or you could switch to a conventional class definition.