Why doesn't this work?

[Ron Garret]

Python 2.3.5 (#1, Jan 30 2006, 13:30:29)
[GCC 3.3 20030304 (Apple Computer, Inc. build 1819)] on darwin
Type "help", "copyright" , "credits" or "license" for more information.

>>from datetime import datetime
>>class ts(datetime):

.... def __init__(self): pass
....

>>ts()

Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: function takes at least 3 arguments (0 given)

>>>

[Larry Bates]

Re: Why doesn't this work?

Because datetime is a new-style class:

The Constructor __new__

If you are like me, then you probably always thought of the __init__ method as
the Python equivalent of what is called a constructor in C++. This isn't the
whole story.

When an instance of a class is created, Python first calls the __new__ method of
the class. __new__ is a static method that is called with the class as its first
argument. __new__ returns a new instance of the class.

The __init__ method is called afterwards to initialize the instance. In some
situations (think "unplickling"!) , no initialization is performed. Also,
immutable types like int and str are completely constructed by the __new__
method; their __init__ method does nothing. This way, it is impossible to
circumvent immutability by explicitly calling the __init__ method after
construction.


I think what you wanted was:

>>class ts(datetime):

.... def __new__(self): pass
....

>>a=ts()

-Larry

[Ron Garret]

Re: Why doesn't this work?

In article <453A507E.40706 02@websafe.com> ,
Larry Bates <larry.bates@we bsafe.comwrote:

Because datetime is a new-style class:

Ah.

The Constructor __new__
>
If you are like me, then you probably always thought of the __init__ method
as
the Python equivalent of what is called a constructor in C++. This isn't the
whole story.
>
When an instance of a class is created, Python first calls the __new__ method
of
the class. __new__ is a static method that is called with the class as its
first
argument. __new__ returns a new instance of the class.
>
The __init__ method is called afterwards to initialize the instance. In some
situations (think "unplickling"!) , no initialization is performed. Also,
immutable types like int and str are completely constructed by the __new__
method; their __init__ method does nothing. This way, it is impossible to
circumvent immutability by explicitly calling the __init__ method after
construction.
>
>
I think what you wanted was:
>

>class ts(datetime):

... def __new__(self): pass
...

>a=ts()

>
-Larry

Thanks!

rg