Partially unpacking a sequence

**Paul McGuire** · Apr 6 '06, 08:05 PM

Re: Partially unpacking a sequence

<tkpmep@hotmail .com> wrote in message
news:1144352710 .190839.165580@ z34g2000cwc.goo glegroups.com.. .[color=blue]
> I have a list y[color=green][color=darkred]
> >>>y[/color][/color]
> ['20001201', 'ARRO', '04276410', '18.500', '19.500', '18.500',
> '19.500', '224']
>
> from which I want to extract only the 2nd and 4th item by partially
> unpacking the list. So I tried[color=green][color=darkred]
> >>>a,b = y[2,4][/color][/color]
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> TypeError: list indices must be integers
>
> Out of curiosity, I tried[color=green][color=darkred]
> >>>a,b = y[2:4]
> >>>a[/color][/color]
> '04276410'[color=green][color=darkred]
> >>> b[/color][/color]
> '18.500'
>
> Why does this work (to a point - it gives me items 2 and 3, not 2 and
> 4 as I require) and not my first attempt? What is the right syntax to
> use when partially upacking a sequence?
>
> Thanks in advance
>
> Thomas Philips
>[/color]

a,b = y[2],y[4]

or

a,b = y[2:5:2]

or

a,b = ( y[i] for i in (2,4) )

-- Paul

**Fredrik Lundh** · Apr 6 '06, 08:15 PM

Re: Partially unpacking a sequence

tkpmep@hotmail. com wrote:
[color=blue]
> I have a list y[color=green][color=darkred]
> >>>y[/color][/color]
> ['20001201', 'ARRO', '04276410', '18.500', '19.500', '18.500',
> '19.500', '224']
>
> from which I want to extract only the 2nd and 4th item by partially
> unpacking the list. So I tried[color=green][color=darkred]
> >>>a,b = y[2,4][/color][/color]
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> TypeError: list indices must be integers
>
> Out of curiosity, I tried[color=green][color=darkred]
> >>>a,b = y[2:4]
> >>>a[/color][/color]
> '04276410'[color=green][color=darkred]
> >>> b[/color][/color]
> '18.500'
>
> Why does this work (to a point - it gives me items 2 and 3, not 2 and
> 4 as I require) and not my first attempt? What is the right syntax to
> use when partially upacking a sequence?[/color]

if you want two items, fetch two items:

a = y[2]
b = y[4]

y[2:4] is a 2-item slice starting at index 2 and ending *before* index 4.
see the documentation for more on slicing.

</F>

**Paul McGuire** · Apr 6 '06, 08:15 PM

Re: Partially unpacking a sequence

"Paul McGuire" <ptmcg@austin.r r._bogus_.com> wrote in message
news:FkeZf.2791 $_26.1642@torna do.texas.rr.com ...[color=blue]
> <tkpmep@hotmail .com> wrote in message
> news:1144352710 .190839.165580@ z34g2000cwc.goo glegroups.com.. .[color=green]
> > I have a list y[color=darkred]
> > >>>y[/color]
> > ['20001201', 'ARRO', '04276410', '18.500', '19.500', '18.500',
> > '19.500', '224']
> >
> > from which I want to extract only the 2nd and 4th item by partially
> > unpacking the list. So I tried[color=darkred]
> > >>>a,b = y[2,4][/color]
> > Traceback (most recent call last):
> > File "<interacti ve input>", line 1, in ?
> > TypeError: list indices must be integers
> >
> > Out of curiosity, I tried[color=darkred]
> > >>>a,b = y[2:4]
> > >>>a[/color]
> > '04276410'[color=darkred]
> > >>> b[/color]
> > '18.500'
> >
> > Why does this work (to a point - it gives me items 2 and 3, not 2 and
> > 4 as I require) and not my first attempt? What is the right syntax to
> > use when partially upacking a sequence?
> >
> > Thanks in advance
> >
> > Thomas Philips
> >[/color]
>
> a,b = y[2],y[4]
>
> or
>
> a,b = y[2:5:2]
>
> or
>
> a,b = ( y[i] for i in (2,4) )
>
>
> -- Paul
>
>[/color]

Forgot one:

_,_,a,_,b,_,_,_ = y

There actually is some merit to this form. If the structure of y changes
sometime in the future (specifically if a field is added or removed), this
statement will fail noisily, calling your attention to this change. But if
a new field is added, say at the front of the list, the previous methods
will all silently succeed, but now giving you the values formerly known as
y[1] and y[3].

-- Paul

**tkpmep@hotmail.com** · Apr 6 '06, 09:15 PM

Re: Partially unpacking a sequence

Thank you, everyone, for resolving my question. At one point, while
trying to solve the problem, I typed
[color=blue][color=green][color=darkred]
>>> y[1,3][/color][/color][/color]
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
TypeError: list indices must be integers

The error message gave me no clue as to what I was doing wrong (in my
mind, I was just writing out the elements of a range), and I thought
perhaps that my inclusion of a comma was the problem. Perhaps a more
explicit error message would have helped.

Another solution, I suppose, is to use a list comprehension. Store the
indices in a list x. For example, let x = [1,5,4]. Then

a,b,c = [y[i] for i in x]

Thanks

Thomas Philips

**Fredrik Lundh** · Apr 6 '06, 09:55 PM

Re: Partially unpacking a sequence

Paul McGuire wrote:
[color=blue]
> Forgot one:
>
> _,_,a,_,b,_,_,_ = y
>
> There actually is some merit to this form. If the structure of y changes
> sometime in the future (specifically if a field is added or removed), this
> statement will fail noisily, calling your attention to this change. But if
> a new field is added, say at the front of the list, the previous methods
> will all silently succeed, but now giving you the values formerly known as
> y[1] and y[3].[/color]

if this is likely to happen, an straightforward assert is a lot easier to parse than
a one-two-a-three-oops-one-two-a-four-five-etc number of underscores:

assert len(y) == 8
a = y[2]
b = y[4]

</F>

**Sybren Stuvel** · Apr 6 '06, 09:55 PM

Re: Partially unpacking a sequence

tkpmep@hotmail. com enlightened us with:[color=blue][color=green][color=darkred]
>>>> y[1,3][/color][/color]
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> TypeError: list indices must be integers
>
> The error message gave me no clue as to what I was doing wrong (in
> my mind, I was just writing out the elements of a range), and I
> thought perhaps that my inclusion of a comma was the problem.[/color]

You thought correct. List indices must be integers, not tuples.
[color=blue]
> Perhaps a more explicit error message would have helped.[/color]

Why? You thought the right thing.

Sybren
--
The problem with the world is stupidity. Not saying there should be a
capital punishment for stupidity, but why don't we just take the
safety labels off of everything and let the problem solve itself?
Frank Zappa

**jUrner@arcor.de** · Apr 6 '06, 10:05 PM

Re: Partially unpacking a sequence

>> Thank you, everyone, for resolving my question. At one point, while[color=blue][color=green]
>> trying to solve the problem, I typed
>>[color=darkred]
>>>>> y[1,3][/color]
>>
>> Traceback (most recent call last):
>> File "<interacti ve input>", line 1, in ?
>> TypeError: list indices must be integers
>>
>> The error message gave me no clue as to what I was doing wrong (in my
>>mind, I was just writing out the elements of a range), and I thought
>> perhaps that my inclusion of a comma was the problem. Perhaps a more
>>explicit error message would have helped.[/color][/color]

The error message is correct because in y[1, 3] "1, 3" is recognized by
the interpreter
as tuple. Python goodie or snakebite that is...

**Fredrik Lundh** · Apr 7 '06, 06:55 AM

Re: Partially unpacking a sequence

tkpmep@hotmail. com wrote:
[color=blue]
> Thank you, everyone, for resolving my question. At one point, while
> trying to solve the problem, I typed
>[color=green][color=darkred]
> >>> y[1,3][/color][/color]
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> TypeError: list indices must be integers
>
> The error message gave me no clue as to what I was doing wrong (in my
> mind, I was just writing out the elements of a range), and I thought
> perhaps that my inclusion of a comma was the problem. Perhaps a more
> explicit error message would have helped.[/color]

the problem is that the *compiler* doesn't know what "y" is, and y[1,3]
is a perfectly valid way to access e.g. a dictionary or a multidimensiona l
array. so it's the list implementation that has to do the complaining, and
all it knows is that it wants an integer index, and got something else.

maybe something like

TypeError: list indices must be integers (got tuple)

would have been less confusing; feel free to add a suggestion to the bug
tracker:

http://sourceforge.net/tracker/?grou...70&atid=105470

</F>

**bruno at modulix** · Apr 7 '06, 08:45 AM

Re: Partially unpacking a sequence

tkpmep@hotmail. com wrote:[color=blue]
> I have a list y
>[color=green][color=darkred]
>>>>y[/color][/color]
>
> ['20001201', 'ARRO', '04276410', '18.500', '19.500', '18.500',
> '19.500', '224']
>
> from which I want to extract only the 2nd and 4th item
>
> by partially
> unpacking the list. So I tried
>[color=green][color=darkred]
>>>>a,b = y[2,4][/color][/color][/color]

Mmm, so lovely and meaningful names !-)

FWIW, and since nobody seemed to mention it, list indexes are
zero-based, so the second element of a list is at index 1 and the fourth
at index 3.

Also, a GoodPractice(tm ) is to use named constants instead of magic
numbers. Here we don't have a clue about why these 2 elements are so
specials. Looking at the example list (which - semantically - should be
a tuple, not a list) I could wild-guess that the 2nd item is a reference
and the fourth a price, so:

REF_INDEX = 1 # lists are zero-based
PRICE_INDEX = 3

ref, price = y[REF_INDEX], y[PRICE_INDEX]

And finally, since this list is clearly structured data, wrapping it
into an object to hide away implementation details *may* help -
depending on the context, of course !-)

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

Partially unpacking a sequence

Partially unpacking a sequence

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