Question about classes

[Ben]

Consider:

class foo:
x = 5

def getx(r):
a = foo()
if r != 0:
a.x = r
return a

a = getx(8)

------------------------------------------------
the above works (unless I typoed it, I'm tired)

this illustrates instatiation of a class with initialization dependent
upon a passed parameter. Seems quite useful in the general sense (in fact,
I have a use for it, which is what led me to this.) But this doesn't work:

class foo(xstart):
x = 5
if xstart != 0:
x = xstart

a = foo(8)

What I am curious about is why not? What am I missing about classes here?
Is the functionality delivered in some other fashion, or must I:

class foo:
x = 5

a = foo()
a.x = 8

to get the brevity and clarity of...

class foo(xstart):
x = 5
if xstart != 0:
x = xstart

a = foo(8)

Thanks for any input on this.

[Steven Bethard]

Re: Question about classes

Ben wrote:[color=blue]
> class foo(xstart):
> x = 5
> if xstart != 0:
> x = xstart
>
> a = foo(8)
>
> What I am curious about is why not? What am I missing about classes here?
> Is the functionality delivered in some other fashion, or must I:
>
> class foo:
> x = 5
>
> a = foo()
> a.x = 8[/color]

The parentheses after a class name do not indicate a parameter list;
they indicate the list of base classes. So generally, they must by
classes/types:
[color=blue][color=green][color=darkred]
>>> def make_class(base ):[/color][/color][/color]
.... class C(base):
.... pass
.... return C
....[color=blue][color=green][color=darkred]
>>> make_class(obje ct)[/color][/color][/color]
<class '__main__.C'>[color=blue][color=green][color=darkred]
>>> make_class(1)[/color][/color][/color]
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
File "<interacti ve input>", line 2, in make_class
TypeError: Error when calling the metaclass bases
int() takes at most 2 arguments (3 given)

If you want to set a class or instance variable for an object at
instantiation time, you probably want to supply the __init__ method:
[color=blue][color=green][color=darkred]
>>> class C(object):[/color][/color][/color]
.... x = 5
.... def __init__(self, x):
.... if x != 0:
.... C.x = x
....[color=blue][color=green][color=darkred]
>>> c = C(0)
>>> c.x[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> c = C(8)
>>> c.x[/color][/color][/color]
8

It seems strange to me that you want to set a *class* variable here, not
an instance variable, though perhaps you have your reasons. Note that
by doing so, every time you make a new instance, you'll change the x
attribute for all objects:
[color=blue][color=green][color=darkred]
>>> C.x[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> c1 = C(0)
>>> c1.x[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> C.x[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> c2 = C(8)
>>> c2.x[/color][/color][/color]
8[color=blue][color=green][color=darkred]
>>> c1.x[/color][/color][/color]
8[color=blue][color=green][color=darkred]
>>> C.x[/color][/color][/color]
8

If instead, you intended to set an instance variable, you might write it
like:
[color=blue][color=green][color=darkred]
>>> class C(object):[/color][/color][/color]
.... def __init__(self, x):
.... if x != 0:
.... self.x = x
.... else:
.... self.x = 5
....[color=blue][color=green][color=darkred]
>>> c = C(0)
>>> c.x[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> c = C(8)
>>> c.x[/color][/color][/color]
8

Steve

[Steve Holden]

Re: Question about classes

Steven Bethard wrote:
[color=blue]
> Ben wrote:
>[color=green]
>> class foo(xstart):
>> x = 5
>> if xstart != 0:
>> x = xstart
>>
>> a = foo(8)
>>
>> What I am curious about is why not? What am I missing about classes here?
>> Is the functionality delivered in some other fashion, or must I:
>>
>> class foo:
>> x = 5
>>
>> a = foo()
>> a.x = 8[/color]
>
>
> The parentheses after a class name do not indicate a parameter list;
> they indicate the list of base classes. So generally, they must by
> classes/types:
>[color=green][color=darkred]
> >>> def make_class(base ):[/color][/color]
> .... class C(base):
> .... pass
> .... return C
> ....[color=green][color=darkred]
> >>> make_class(obje ct)[/color][/color]
> <class '__main__.C'>[color=green][color=darkred]
> >>> make_class(1)[/color][/color]
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> File "<interacti ve input>", line 2, in make_class
> TypeError: Error when calling the metaclass bases
> int() takes at most 2 arguments (3 given)
>
> If you want to set a class or instance variable for an object at
> instantiation time, you probably want to supply the __init__ method:
>[color=green][color=darkred]
> >>> class C(object):[/color][/color]
> .... x = 5
> .... def __init__(self, x):
> .... if x != 0:
> .... C.x = x
> ....[color=green][color=darkred]
> >>> c = C(0)
> >>> c.x[/color][/color]
> 5[color=green][color=darkred]
> >>> c = C(8)
> >>> c.x[/color][/color]
> 8
>
> It seems strange to me that you want to set a *class* variable here, not
> an instance variable, though perhaps you have your reasons. Note that
> by doing so, every time you make a new instance, you'll change the x
> attribute for all objects:
>[color=green][color=darkred]
> >>> C.x[/color][/color]
> 5[color=green][color=darkred]
> >>> c1 = C(0)
> >>> c1.x[/color][/color]
> 5[color=green][color=darkred]
> >>> C.x[/color][/color]
> 5[color=green][color=darkred]
> >>> c2 = C(8)
> >>> c2.x[/color][/color]
> 8[color=green][color=darkred]
> >>> c1.x[/color][/color]
> 8[color=green][color=darkred]
> >>> C.x[/color][/color]
> 8
>
> If instead, you intended to set an instance variable, you might write it
> like:
>[color=green][color=darkred]
> >>> class C(object):[/color][/color]
> .... def __init__(self, x):
> .... if x != 0:
> .... self.x = x
> .... else:
> .... self.x = 5
> ....[color=green][color=darkred]
> >>> c = C(0)
> >>> c.x[/color][/color]
> 5[color=green][color=darkred]
> >>> c = C(8)
> >>> c.x[/color][/color]
> 8
>
> Steve[/color]

Don't forget, though, that due to the attribute resolution order, if an
instance doesn't have a particular attribute but the class does then a
reference to the attribute in a method will get the class attribute.

This has been used to implement class defaults in the past.

regards
another Steve
--

http://www.holdenweb.com

http://pydish.holdenweb.com

Holden Web LLC +1 800 494 3119

[Ben]

Re: Question about classes

On Mon, 22 Nov 2004 06:39:53 +0000, Steven Bethard wrote:
[color=blue]
> If instead, you intended to set an instance variable, you might write it
> like:
>[color=green][color=darkred]
> >>> class C(object):[/color][/color]
> ... def __init__(self, x):
> ... if x != 0:
> ... self.x = x
> ... else:
> ... self.x = 5
> ...[color=green][color=darkred]
> >>> c = C(0)
> >>> c.x[/color][/color]
> 5[color=green][color=darkred]
> >>> c = C(8)
> >>> c.x[/color][/color]
> 8[/color]

Steve,

That's exactly what I wanted. Thank you very much. I'll keep the class one
in mind too.

--Ben


--Ben

[Steven Bethard]

Re: Question about classes

Ben wrote:[color=blue]
> On Mon, 22 Nov 2004 06:39:53 +0000, Steven Bethard wrote:
>[color=green][color=darkred]
>> >>> class C(object):[/color]
>>... def __init__(self, x):
>>... if x != 0:
>>... self.x = x
>>... else:
>>... self.x = 5
>>...[color=darkred]
>> >>> c = C(0)
>> >>> c.x[/color]
>>5[color=darkred]
>> >>> c = C(8)
>> >>> c.x[/color]
>>8[/color]
>
>
> Steve,
>
> That's exactly what I wanted. Thank you very much. I'll keep the class one
> in mind too.[/color]

You're welcome. Depending on your goal here, you might also find that
default argument values are helpful:
[color=blue][color=green][color=darkred]
>>> class C(object):[/color][/color][/color]
.... def __init__(self, x=5):
.... self.x = x
....[color=blue][color=green][color=darkred]
>>> c = C()
>>> c.x[/color][/color][/color]
5[color=blue][color=green][color=darkred]
>>> c = C(8)
>>> c.x[/color][/color][/color]
8[color=blue][color=green][color=darkred]
>>> c = C(0)
>>> c.x[/color][/color][/color]
0

Steve