Why date do not construct from date?

[Lie Ryan]

Gabriel Genellina wrote:
> En Tue, 02 Jun 2009 03:14:22 -0300, Chris Rebert <clp2 at rebertia.com>
> escribió:
> 
>> On Mon, Jun 1, 2009 at 10:25 PM, Alexandr N Zamaraev
>> <tonal at promsoft.ru> wrote:
> 
>>>>>> import datetime as dt
>>>>>> 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)
> 
>>> 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.
> 
> That doesn't convince me. It's not very consistent along the various
> types: int("3ab0",16) is rather different than int(3.2) but they're the
> same function...

Strictly speaking int("3ab0",16) does not create an int from an int,
instead it creates an int from a string.

Maybe you want to say int(3) -> 3 ?

>> 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.
> 
> Isn't the same for all other examples (int, float, str, Decimal...)?
> They're all immutable types, and some have several and rather different
> constructor signatures:

int(ob), float(ob), and str(ob) are type casting (strictly speaking it
is not a type casting, but you get the idea); while date() is a
constructor for the date object. Strictly speaking int(ob), float(ob),
and str(ob) merely calls the special ob.__int__, ob.__float__, and
ob.__str__. These special functions are there to convert the current
object into int, float, or str wherever defined. It just happens that
calling int.__int__, float.__float__, and str.__str__ just returns
themselves.

For Decimal, (I think) it is as a symmetry to float since Decimal is
intended to be used whenever IEEE 764 behavior does not suit you.