Add subclassing example to docs for named tuples.
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 0205cf1..1f9e82d 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -510,15 +510,22 @@
Point(x=11, y=22)
Since a named tuple is a regular Python class, it is easy to add or change
-functionality. For example, the display format can be changed by overriding
-the :meth:`__repr__` method:
+functionality with a subclass. Here is how to add a calculated field and
+a custom fixed-width print format:
::
- >>> Point = namedtuple('Point', 'x y')
- >>> Point.__repr__ = lambda self: 'Point(%.3f, %.3f)' % self
- >>> Point(x=11, y=22)
- Point(11.000, 22.000)
+ >>> class Point(namedtuple('Point', 'x y')):
+ @property
+ def hypot(self):
+ return (self.x ** 2 + self.y ** 2) ** 0.5
+ def __repr__(self):
+ return 'Point(x=%.3f, y=%.3f, hypot=%.3f)' % (self.x, self.y, self.hypot)
+
+ >>> print Point(3, 4)
+ Point(x=3.000, y=4.000, hypot=5.000)
+ >>> Point(2, 5)
+ Point(x=2.000, y=5.000, hypot=5.385)
Default values can be implemented by starting with a prototype instance
and customizing it with :meth:`_replace`:
diff --git a/Lib/collections.py b/Lib/collections.py
index 0b86898..1701952 100644
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -116,8 +116,15 @@
assert p == loads(dumps(p))
# test and demonstrate ability to override methods
- Point.__repr__ = lambda self: 'Point(%.3f, %.3f)' % self
- print p
+ class Point(namedtuple('Point', 'x y')):
+ @property
+ def hypot(self):
+ return (self.x ** 2 + self.y ** 2) ** 0.5
+ def __repr__(self):
+ return 'Point(x=%.3f, y=%.3f, hypot=%.3f)' % (self.x, self.y, self.hypot)
+
+ print Point(3, 4)
+ print Point(2, 5)
import doctest
TestResults = namedtuple('TestResults', 'failed attempted')