Python newbie advice needed!

I'm tring to write what I would have expressed in Perl as

my ($a, $b, $c) = @array;

This is very similar to the python statement

a, b, c = array

BUT, the Python will throw an exception if array isn't exactly 3
elements long, wheras the Perl will work for any length of @array,
either throwing away excess elements or setting the variables to
undef, ie like this

if len(array) >= 1:
a = array[0]
else:
a = None
if len(array) >= 2:
b = array[1]
else:
b = None
if len(array) >= 3:
c = array[2]
else:
c = None

This works if array has >= 3 elements

a, b, c = array[:3]

And this works however many elements array has

a, b, c = (array + 3*[None])[:3]

but it doesn't seem very Pythonic - is there a better way?

--
Nick Craig-Wood
ncw@axis.demon.co.uk

In the specific case where you're trying to map a variable-length
argument list to local variables in a function, you can do the more
Pythonic:

def myfunc (a = None, b = None, c = None)

I'd say the answer is similar for other tasks. There's probably a
Pythonic way to do the overall task as opposed to a
statement-by-statement translation.
[color=blue]
> I'm tring to write what I would have expressed in Perl as
>
> my ($a, $b, $c) = @array;
>
> This is very similar to the python statement
>
> a, b, c = array
>
> BUT, the Python will throw an exception if array isn't exactly 3
> elements long, wheras the Perl will work for any length of @array,
> either throwing away excess elements or setting the variables to
> undef, ie like this
>
> if len(array) >= 1:
> a = array[0]
> else:
> a = None
> if len(array) >= 2:
> b = array[1]
> else:
> b = None
> if len(array) >= 3:
> c = array[2]
> else:
> c = None
>
> This works if array has >= 3 elements
>
> a, b, c = array[:3]
>
> And this works however many elements array has
>
> a, b, c = (array + 3*[None])[:3]
>
> but it doesn't seem very Pythonic - is there a better way?
>
> --
> Nick Craig-Wood
> ncw@axis.demon.co.uk
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

"Nick Craig-Wood" <ncw@axis.demon.co.uk> wrote in message
news:slrnc2enr8.bmp.ncw@irishsea.home.craig-wood.com...[color=blue]
> Python newbie advice needed!
>
> I'm tring to write what I would have expressed in Perl as
>
> my ($a, $b, $c) = @array;
>
> This is very similar to the python statement
>
> a, b, c = array
>
> BUT, the Python will throw an exception if array isn't exactly 3 elements[/color]
long,

This is an intentional feature. Mismatches are often bugs.
[color=blue]
> wheras the Perl will work for any length of @array,
> either throwing away excess elements or setting the variables to
> undef, ie like this[/color]

Guido's philosophy is that the interpreter should resist guessing like this
when code is at least half likely to be buggy.
[color=blue]
> And this works however many elements array has
>
> a, b, c = (array + 3*[None])[:3]
>
> but it doesn't seem very Pythonic - is there a better way?[/color]

Being explicitly generic is Pythonic to me. Or do something like

a=b=c=None
try: c=array[2]
try: b=array[1]
try: a=array[1]
except: pass
except: pass
except: pass

but I prefer the one liner here. It is easier to extend to more names.

Terry J. Reedy

Wayne Folta <wfolta@netmail.to> wrote:[color=blue]
> In the specific case where you're trying to map a variable-length
> argument list to local variables in a function, you can do the more
> Pythonic:
>
> def myfunc (a = None, b = None, c = None)[/color]

Interesting... I was thinking in particular of sys.argv with this
example, but the above gave me the idea below which would work quite
well (imagine sys.argv in place of L below)
[color=blue][color=green][color=darkred]
>>> def f(a=None,b=None,c=None,*extra): print a,b,c[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> L=[]; apply(f,L)[/color][/color][/color]
None None None[color=blue][color=green][color=darkred]
>>> L=[1]; apply(f,L)[/color][/color][/color]
1 None None[color=blue][color=green][color=darkred]
>>> L=[1,2]; apply(f,L)[/color][/color][/color]
1 2 None[color=blue][color=green][color=darkred]
>>> L=[1,2,3]; apply(f,L)[/color][/color][/color]
1 2 3[color=blue][color=green][color=darkred]
>>> L=[1,2,3,4]; apply(f,L)[/color][/color][/color]
1 2 3[color=blue][color=green][color=darkred]
>>> L=[1,2,3,4,5]; apply(f,L)[/color][/color][/color]
1 2 3

--
Nick Craig-Wood
ncw@axis.demon.co.uk

"Nick Craig-Wood" <ncw@axis.demon.co.uk> wrote in message
news:slrnc2h9id.um.ncw@irishsea.home.craig-wood.com...[color=blue][color=green]
> > def myfunc (a = None, b = None, c = None)[/color][/color]
[color=blue][color=green][color=darkred]
> >>> def f(a=None,b=None,c=None,*extra): print a,b,c[/color][/color][/color]
...[color=blue][color=green][color=darkred]
> >>> L=[]; apply(f,L)[/color][/color][/color]

You can now write 'apply(f,L)' as f(*L) and be future-proof in case apply
disappears

tjr