Improve namedtuple's _cast() method with a docstring, new name, and error-checking.
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 628dbf5..34c3118 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -393,7 +393,13 @@
def __new__(cls, x, y):
return tuple.__new__(cls, (x, y))
- _cast = classmethod(tuple.__new__)
+ @classmethod
+ def _make(cls, iterable):
+ 'Make a new Point object from a sequence or iterable'
+ result = tuple.__new__(cls, iterable)
+ if len(result) != 2:
+ raise TypeError('Expected 2 arguments, got %d' % len(result))
+ return result
def __repr__(self):
return 'Point(x=%r, y=%r)' % self
@@ -404,7 +410,7 @@
def _replace(self, **kwds):
'Return a new Point object replacing specified fields with new values'
- return self.__class__._cast(map(kwds.get, ('x', 'y'), self))
+ return self.__class__._make(map(kwds.get, ('x', 'y'), self))
x = property(itemgetter(0))
y = property(itemgetter(1))
@@ -426,34 +432,28 @@
EmployeeRecord = namedtuple('EmployeeRecord', 'name, age, title, department, paygrade')
import csv
- for emp in map(EmployeeRecord._cast, csv.reader(open("employees.csv", "rb"))):
+ for emp in map(EmployeeRecord._make, 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 map(EmployeeRecord._cast, cursor.fetchall()):
+ for emp in map(EmployeeRecord._make, cursor.fetchall()):
print emp.name, emp.title
In addition to the methods inherited from tuples, named tuples support
-three additonal methods and one attribute.
+three additional methods and one attribute.
-.. method:: namedtuple._cast(iterable)
+.. method:: namedtuple._make(iterable)
- Class method returning a new instance taking the positional arguments from the
- *iterable*. Useful for casting existing sequences and iterables to named tuples.
-
- This fast constructor does not check the length of the inputs. To achieve the
- same effect with length checking, use the star-operator instead.
+ Class method that makes a new instance from an existing sequence or iterable.
::
- >>> t = [11, 22]
- >>> Point._cast(t) # fast conversion
- Point(x=11, y=22)
- >>> Point(*t) # slow conversion with length checking
- Point(x=11, y=22)
+ >>> t = [11, 22]
+ >>> Point._make(t)
+ Point(x=11, y=22)
.. method:: somenamedtuple._asdict()