Re: Why date do not construct from date?
- From: Lie Ryan <lie.1296@xxxxxxxxx>
- Date: Tue, 02 Jun 2009 11:23:27 GMT
Lie Ryan wrote:
Gabriel Genellina wrote:
En Tue, 02 Jun 2009 03:14:22 -0300, Chris Rebert <clp2@xxxxxxxxxxxx>
escribió:
On Mon, Jun 1, 2009 at 10:25 PM, Alexandr N ZamaraevThat doesn't convince me. It's not very consistent along the various
<tonal@xxxxxxxxxxx> wrote:
Probably because the function signatures would be so different. str(),Traceback (most recent call last):import datetime as dt
d = dt.date(2009, 10, 15)
dt.date(d)
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?
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.
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 toIsn't the same for all other examples (int, float, str, Decimal...)?
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.
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.
In fact, the doc of int and float says "Convert a string or number to an
integer, if possible" and "Convert a string or number to a floating
point number, if possible" respectively. There is no mention that they
are constructors at all...
While the doc for str says "Return a nice string representation of the
object." the argument still holds since the "nice string representation"
for a string is the string itself...
Decimal is the rotten apple here since it just mimics float(). But that
is why Decimal is in separate module and there is no decimal() built-in.
Classes in modules are free to do anything they want to do... including
mimicking float() or deciding not to accept its own self as a valid
initializer.
.
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- Re: Why date do not construct from date?
- From: Gabriel Genellina
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- From: Lie Ryan
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