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Diffstat (limited to 'Doc/library/collections.rst')
| -rw-r--r-- | Doc/library/collections.rst | 37 |
1 files changed, 25 insertions, 12 deletions
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst index 5b625ee..cb3a029 100644 --- a/Doc/library/collections.rst +++ b/Doc/library/collections.rst @@ -489,7 +489,7 @@ three additional methods and one attribute. >>> Point._make(t) Point(x=11, y=22) -.. method:: somenamedtuple._asdict() +.. method:: namedtuple._asdict() Return a new dict which maps field names to their corresponding values: @@ -498,7 +498,7 @@ three additional methods and one attribute. >>> p._asdict() {'x': 11, 'y': 22} -.. method:: somenamedtuple._replace(kwargs) +.. method:: namedtuple._replace(kwargs) Return a new instance of the named tuple replacing specified fields with new values: @@ -511,7 +511,7 @@ three additional methods and one attribute. >>> for partnum, record in inventory.items(): ... inventory[partnum] = record._replace(price=newprices[partnum], updated=time.now()) -.. attribute:: somenamedtuple._fields +.. attribute:: namedtuple._fields Tuple of strings listing the field names. This is useful for introspection and for creating new named tuple types from existing named tuples. @@ -541,15 +541,28 @@ When casting a dictionary to a named tuple, use the double-star-operator [#]_:: 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: - -:: - - >>> Point = namedtuple('Point', 'x y') - >>> Point.__repr__ = lambda self: 'Point(%.3f, %.3f)' % self - >>> Point(x=11, y=22) - Point(11.000, 22.000) +functionality with a subclass. Here is how to add a calculated field and +a fixed-width print format:: + + >>> 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),'\n', Point(2, 5), '\n', Point(9./7, 6) + Point(x=3.000, y=4.000, hypot=5.000) + Point(x=2.000, y=5.000, hypot=5.385) + Point(x=1.286, y=6.000, hypot=6.136) + +Another use for subclassing is to replace performance critcal methods with +faster versions that bypass error-checking and localize variable access:: + + >>> class Point(namedtuple('Point', 'x y')): + _make = classmethod(tuple.__new__) + def _replace(self, _map=map, **kwds): + return self._make(_map(kwds.pop, ('x', 'y'), self)) Default values can be implemented by starting with a prototype instance and customizing it with :meth:`_replace`: |