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]

Ben wrote:

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

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:

def make_class(base): .... class C(base):
.... pass
.... return C
.... make_class(object) <class '__main__.C'> make_class(1) Traceback (most recent call last):
File "<interactive input>", line 1, in ?
File "<interactive 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:
class C(object): .... x = 5
.... def __init__(self, x):
.... if x != 0:
.... C.x = x
.... c = C(0)
c.x 5 c = C(8)
c.x 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:
C.x 5 c1 = C(0)
c1.x 5 C.x 5 c2 = C(8)
c2.x 8 c1.x 8 C.x 8

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

8

Steve

[Steve Holden]

Steven Bethard wrote:

Ben wrote:

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

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:

>>> def make_class(base): .... class C(base):
.... pass
.... return C
.... >>> make_class(object) <class '__main__.C'> >>> make_class(1) Traceback (most recent call last):
File "<interactive input>", line 1, in ?
File "<interactive 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:
>>> class C(object): .... x = 5
.... def __init__(self, x):
.... if x != 0:
.... C.x = x
.... >>> c = C(0)
>>> c.x 5 >>> c = C(8)
>>> c.x 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:
>>> C.x 5 >>> c1 = C(0)
>>> c1.x 5 >>> C.x 5 >>> c2 = C(8)
>>> c2.x 8 >>> c1.x 8 >>> C.x 8

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

8

Steve

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 [phone]

[Ben]

On Mon, 22 Nov 2004 06:39:53 +0000, Steven Bethard wrote:

If instead, you intended to set an instance variable, you might write it
like:

>>> class C(object): ... def __init__(self, x):
... if x != 0:
... self.x = x
... else:
... self.x = 5
... >>> c = C(0)
>>> c.x 5 >>> c = C(8)
>>> c.x

8

Steve,

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

--Ben
--Ben

[Steven Bethard]

Ben wrote:

On Mon, 22 Nov 2004 06:39:53 +0000, Steven Bethard wrote:

>>> class C(object):

... def __init__(self, x):
... if x != 0:
... self.x = x
... else:
... self.x = 5
...

>>> c = C(0)
>>> c.x

5

>>> c = C(8)
>>> c.x

8

Steve,

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

You're welcome. Depending on your goal here, you might also find that
default argument values are helpful:

class C(object): .... def __init__(self, x=5):
.... self.x = x
.... c = C()
c.x 5 c = C(8)
c.x 8 c = C(0)
c.x

0

Steve