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From: Thomas Troeger on 4 Jul 2008 07:48
Hello,

I have a class that looks like this:

class A(object):
def __init__(self, a=0, b=1):
self.a, self.b=a, b

def __str__(self):
return "%s(%d,%d)" % (type(a).__name__, self.a, self.b)

I want to have a list of such classes instantiated automatically on
startup of my program. My current (most probably clumsy) implementation
looks like this:

bla=[A(x[0], x[1]) for x in ((1, 2), (3, 4))]

giving the following:

>>> map(str, bla)
['A(1,2)', 'A(3,4)']

Is there a better way to construct a list of such classes? Basically
what I want is something similar to the following C example:

struct {
int a;
int b;
} bla[]={ {1, 2}, {3, 4} };

Regards,
T.
From: Bruno Desthuilliers on 4 Jul 2008 08:26
Thomas Troeger a �crit :
> Hello,
>
> I have a class that looks like this:
>
> class A(object):
> def __init__(self, a=0, b=1):
> self.a, self.b=a, b
>
> def __str__(self):
> return "%s(%d,%d)" % (type(a).__name__, self.a, self.b)

Given the output example you give, I assume there's a typo here and you
meant:
return "%s(%d,%d)" % (type(self).__name__, self.a, self.b)


> I want to have a list of such classes instantiated automatically on
> startup of my program. My current (most probably clumsy) implementation
> looks like this:
>
> bla=[A(x[0], x[1]) for x in ((1, 2), (3, 4))]

Not clumsy at all, and almost perfectly pythonic. The only improvment I
can think of is:

bla = [A(*args) for args in ((1,2), (3,4))]


> giving the following:
>
> >>> map(str, bla)
> ['A(1,2)', 'A(3,4)']
>
> Is there a better way to construct a list of such classes?

Note that it's not a list of classes, but a list of instances of A. But
given your specs, nope, your approach is the right one.

> Basically
> what I want is something similar to the following C example:
>
> struct {
> int a;
> int b;
> } bla[]={ {1, 2}, {3, 4} };

Basically (no pun intended[1]), Python is not C. Trying to write C in
Python will only buy you pain and frustration (and this can be
generalized for any combination of two languages for any known
programming language).

[1] well... in fact, yes... !-)
From: Thomas Troeger on 4 Jul 2008 08:59
Bruno Desthuilliers wrote:
> return "%s(%d,%d)" % (type(self).__name__, self.a, self.b)

Er, yes exactly! I noticed it a few seconds after I had sent the message ;-(

>> I want to have a list of such classes instantiated automatically on

Of course I meant class instances ... sorry :) It's always good to have
an example to compensate for English errors *g*.

> bla = [A(*args) for args in ((1,2), (3,4))]

....

> Note that it's not a list of classes, but a list of instances of A. But
> given your specs, nope, your approach is the right one.

Ah I knew there was something and I couldn't find it in the docs
anymore! Now my potential follow-up question is answered as well, namely
how I can instantiate with variable argument lists, like this:

>>> bla = [A(*args) for args in ((), (1,), (1, 2))]
>>> map(str, bla)
['A(0,1)', 'A(1,1)', 'A(1,2)']

> Basically (no pun intended[1]), Python is not C. Trying to write C in
> Python will only buy you pain and frustration (and this can be
> generalized for any combination of two languages for any known
> programming language).

Hehe. I am trying to develop a program prototype in python because of
it's repaid prototyping properties, and once it's working I will port it
to C, because of speed issues and the fact that it's running on an
embedded machine without space for a python interpreter. I have like 4
Megs left, but until now noone has answered my question how I can cut
down a standard python installation so that it fit's into 4 megs.

Thanks for your quick answer :)
T.
From: Marc 'BlackJack' Rintsch on 4 Jul 2008 09:13
On Fri, 04 Jul 2008 14:59:05 +0200, Thomas Troeger wrote:

> Bruno Desthuilliers wrote:
>>> I want to have a list of such classes instantiated automatically on
>
> Of course I meant class instances ... sorry :) It's always good to have
> an example to compensate for English errors *g*.

Well, "class instances" is still a little bit ambiguous in a language
where classes are objects too. ;-)

> Ah I knew there was something and I couldn't find it in the docs
> anymore! Now my potential follow-up question is answered as well, namely
> how I can instantiate with variable argument lists, like this:
>
> >>> bla = [A(*args) for args in ((), (1,), (1, 2))]
> >>> map(str, bla)
> ['A(0,1)', 'A(1,1)', 'A(1,2)']

Looks like you want default values for the arguments of `A.__init__()`.

Ciao,
Marc 'BlackJack' Rintsch
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