diff options
Diffstat (limited to 'Doc/library/collections.rst')
| -rw-r--r-- | Doc/library/collections.rst | 49 |
1 files changed, 28 insertions, 21 deletions
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst index 1982187..e01c52e 100644 --- a/Doc/library/collections.rst +++ b/Doc/library/collections.rst @@ -421,27 +421,31 @@ Example:: __slots__ = () - _fields = ('x', 'y') - def __new__(cls, x, y): return tuple.__new__(cls, (x, y)) + _cast = classmethod(tuple.__new__) + def __repr__(self): return 'Point(x=%r, y=%r)' % self - def _asdict(self): + def _asdict(t): 'Return a new dict which maps field names to their values' - return dict(zip(('x', 'y'), self)) + return {'x': t[0], 'y': t[1]} def _replace(self, **kwds): 'Return a new Point object replacing specified fields with new values' - return Point(*map(kwds.get, ('x', 'y'), self)) + return Point._cast(map(kwds.get, ('x', 'y'), self)) + + @property + def _fields(self): + return ('x', 'y') x = property(itemgetter(0)) y = property(itemgetter(1)) >>> p = Point(11, y=22) # instantiate with positional or keyword arguments - >>> p[0] + p[1] # indexable like the regular tuple (11, 22) + >>> p[0] + p[1] # indexable like the plain tuple (11, 22) 33 >>> x, y = p # unpack like a regular tuple >>> x, y @@ -456,33 +460,30 @@ by the :mod:`csv` or :mod:`sqlite3` modules:: EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade') - from itertools import starmap import csv - for record in starmap(EmployeeRecord, csv.reader(open("employees.csv", "rb"))): + for emp in map(EmployeeRecord._cast, csv.reader(open("employees.csv", "rb"))): print(emp.name, emp.title) import sqlite3 conn = sqlite3.connect('/companydata') cursor = conn.cursor() cursor.execute('SELECT name, age, title, department, paygrade FROM employees') - for emp in starmap(EmployeeRecord, cursor.fetchall()): + for emp in map(EmployeeRecord._cast, cursor.fetchall()): print emp.name, emp.title -When casting a single record to a named tuple, use the star-operator [#]_ to unpack -the values:: +In addition to the methods inherited from tuples, named tuples support +three additonal methods and a read-only attribute. - >>> t = [11, 22] - >>> Point(*t) # the star-operator unpacks any iterable object - Point(x=11, y=22) +.. method:: namedtuple._cast(iterable) -When casting a dictionary to a named tuple, use the double-star-operator:: + Class method returning a new instance taking the positional arguments from the *iterable*. + Useful for casting existing sequences and iterables to named tuples: - >>> d = {'x': 11, 'y': 22} - >>> Point(**d) - Point(x=11, y=22) +:: -In addition to the methods inherited from tuples, named tuples support -two additonal methods and a read-only attribute. + >>> t = [11, 22] + >>> Point._cast(t) + Point(x=11, y=22) .. method:: somenamedtuple._asdict() @@ -529,6 +530,12 @@ function: >>> getattr(p, 'x') 11 +When casting a dictionary to a named tuple, use the double-star-operator [#]_:: + + >>> d = {'x': 11, 'y': 22} + >>> Point(**d) + 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: @@ -551,5 +558,5 @@ and customizing it with :meth:`_replace`: .. rubric:: Footnotes -.. [#] For information on the star-operator see +.. [#] For information on the double-star-operator see :ref:`tut-unpacking-arguments` and :ref:`calls`. |