On Mon, Jun 1, 2009 at 10:25 PM, Alexandr N Zamaraev <to...@promsoft.ru> wrote: > Simple example: > [code] > Python 2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit (Intel)] > on win32 >>>> import datetime as dt >>>> dt.date(2009, 10, 15) > datetime.date(2009, 10, 15) >>>> d = dt.date(2009, 10, 15) >>>> dt.date(d) > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: function takes exactly 3 arguments (1 given) >>>> > [/code] > Why int form int, str from str, Decumal from Decumal can construct bat date > from date not?
Probably because the function signatures would be so different. str(), int(), etc *always* take *exactly one* argument -- the object to convert. In contrast, date() takes several integers corresponding to the year, month, and day. Adding a second signature to it that took exactly one argument (of type `date`) and copied it would be significantly different from its other signature; in idiomatic Python, one would typically make a separate, new function for this drastically different signature. However, the `date` type is immutable, so there's no reason at all to try and copy a new instance from an existing one anyway, thus a single-argument copy-constructor is completely unnecessary, hence why there isn't one. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
