On 16/08/2012 15:56, Robert Kern wrote:
On 8/16/12 2:56 PM, Ian Kelly wrote:
On Thu, Aug 16, 2012 at 6:47 AM, Hans Mulder <han...@xs4all.nl> wrote:
On 8/08/12 04:14:01, Steven D'Aprano wrote:
NoneType raises an error if you try to create a second instance. bool
just returns one of the two singletons (doubletons?) again.
py> type(None)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot create 'NoneType' instances
Why is that?
Why doesn't it just return an existing instance of the type,
like bool, int, str and other built-in non-mutable types do?
Because unlike those other types there is no use case for that. It's
simpler to raise an error.
What are the use cases for the empty-argument versions of bool(), int(),
float(), and str()?
They can be used with defaultdict. For example:
counts = defaultdict(int)
for i in items:
counts[i] += 1
Of course, an alternative would be:
counts = defaultdict(lambda: 0)
--
http://mail.python.org/mailman/listinfo/python-list
