Improve namedtuple's _cast() method with a docstring, new name, and error-checking.
diff --git a/Lib/collections.py b/Lib/collections.py
index 487b119..2a58b86 100644
--- a/Lib/collections.py
+++ b/Lib/collections.py
@@ -54,6 +54,7 @@
seen_names.add(name)
# Create and fill-in the class template
+ numfields = len(field_names)
argtxt = repr(field_names).replace("'", "")[1:-1] # tuple repr without parens or quotes
reprtxt = ', '.join('%s=%%r' % name for name in field_names)
dicttxt = ', '.join('%r: t[%d]' % (name, pos) for pos, name in enumerate(field_names))
@@ -63,7 +64,13 @@
_fields = %(field_names)r \n
def __new__(cls, %(argtxt)s):
return tuple.__new__(cls, (%(argtxt)s)) \n
- _cast = classmethod(tuple.__new__) \n
+ @classmethod
+ def _make(cls, iterable):
+ 'Make a new %(typename)s object from a sequence or iterable'
+ result = tuple.__new__(cls, iterable)
+ if len(result) != %(numfields)d:
+ raise TypeError('Expected %(numfields)d arguments, got %%d' %% len(result))
+ return result \n
def __repr__(self):
return '%(typename)s(%(reprtxt)s)' %% self \n
def _asdict(t):
@@ -71,7 +78,7 @@
return {%(dicttxt)s} \n
def _replace(self, **kwds):
'Return a new %(typename)s object replacing specified fields with new values'
- return self.__class__._cast(map(kwds.get, %(field_names)r, self)) \n\n''' % locals()
+ return self.__class__._make(map(kwds.get, %(field_names)r, self)) \n\n''' % locals()
for i, name in enumerate(field_names):
template += ' %s = property(itemgetter(%d))\n' % (name, i)
if verbose: