Why tuple with one item is no tuple

[Gregor Horvath]

Hi,

type(['1']) <type 'list'>
type(('1') )

<type 'str'>

I wonder why ('1') is no tuple????

Because I have to treat this "special" case differently in my code.

--
Greg

[Fuzzyman]

('1',) is a tuple... you need the comma to make it a tuple.

regards,

Fuzzy
http://www.voidspace.org.uk/python/index.shtml

[Bill Mill]

On Tue, 15 Mar 2005 16:16:34 GMT, Gregor Horvath <g.*******@mx.a t> wrote:

Hi,

>>>type(['1']) <type 'list'>
>>>type(('1') ) <type 'str'>

I wonder why ('1') is no tuple????
because, syntactically, those parens are for grouping, and do not
unambiguously define a tuple. It's a python gotcha. To define a
one-tuple, put a comma after the '1':

type(('1', ))

<type 'tuple'>

Because I have to treat this "special" case differently in my code.

you shouldn't have to; post your code if you still think you do.

Peace
Bill Mill
bill.mill at gmail.com

[deelan]

Gregor Horvath wrote:

Hi,

>>>type(['1']) <type 'list'>
>>>type(('1') ) <type 'str'>

I wonder why ('1') is no tuple????

Because I have to treat this "special" case differently in my code.

you need to tell python that ('1') isn't a string inside
a couple parens but a tuple, look:

t = ('1', )
type(t) <type 'tuple'>

if there's no ambiguity you can omit the parens:
t = '1',
type(t)

<type 'tuple'>

HTH,
deelan

--
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
<#me> a foaf:Person ; foaf:nick "deelan" ;
foaf:weblog <http://blog.deelan.com/> .

[Roy Smith]

Gregor Horvath <g.*******@mx.a t> wrote:

Hi,

type(['1'])<type 'list'>
type(('1') )

<type 'str'>

I wonder why ('1') is no tuple????

You need to say ('1',). In just plain ('1'), the parens are
interpreted as grouping, not as tuple creation. Depending on your
point of view, this is either a "special case", or an "ugly wart" in
the syntax.

a = () # tuple of zero elements
a = (1,) # tuple of one element
a = 1, # tuple of one element
a = (1) # scalar
a = (1, 2) # tuple of two elements
a = 1, 2 # tuple of two elements
a = , # syntax error

The big question is, is it the parens that make it a tuple, or is it
the comma? If you go along with the parens school of thought, then
(1,) is the special case. If you believe in commas, then the () is
the special case. In either case, it's a bit ugly, but we learn to
overlook the occasional cosmetic blemishes of those we love :-)

[Gregor Horvath]

thanks are given to all....

"problem" solved...
--
Greg

[Paddy]

Hmm,
going 'the other way', you are allowed an extra , but you can't have
(,) as the empty tuple.:

(1,2,) (1, 2) (1,) (1,) (,)

....
Traceback ( File "<interacti ve input>", line 1
(,)
^
SyntaxError: invalid syntax
-- Pad.

[James Stroud]

On Tuesday 15 March 2005 08:25 am, Roy Smith wrote:

a = () * * * # tuple of zero elements
a = (1,) * * # tuple of one element
a = 1, * * * # tuple of one element
a = (1) * * *# scalar
a = (1, 2) * # tuple of two elements
a = 1, 2 * * # tuple of two elements
a = , * * * *# syntax error

The big question is, is it the parens that make it a tuple, or is it
the comma? *If you go along with the parens school of thought, then
(1,) is the special case. *If you believe in commas, then the () is
the special case. *In either case, it's a bit ugly, but we learn to
overlook the occasional cosmetic blemishes of those we love :-)

The answer is obvious, the naked comma should be an empty tuple.
--
James Stroud, Ph.D.
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

[Bill Mill]

On Tue, 15 Mar 2005 10:47:28 -0800, James Stroud <js*****@mbi.uc la.edu> wrote:

On Tuesday 15 March 2005 08:25 am, Roy Smith wrote:

a = () # tuple of zero elements
a = (1,) # tuple of one element
a = 1, # tuple of one element
a = (1) # scalar
a = (1, 2) # tuple of two elements
a = 1, 2 # tuple of two elements
a = , # syntax error

The big question is, is it the parens that make it a tuple, or is it
the comma? If you go along with the parens school of thought, then
(1,) is the special case. If you believe in commas, then the () is
the special case. In either case, it's a bit ugly, but we learn to
overlook the occasional cosmetic blemishes of those we love :-)

The answer is obvious, the naked comma should be an empty tuple.

The other answer, that parens should be required to surround all
tuples, is obvious too.

Neither is particularly appealing; a lone comma creating a data
structure seems counter-intuitive, but it's nice to do a, b = b, a
instead of (a, b) = (b, a) . In this case, since the need to create
empty tuples is vanishingly rare, I'm okay with a little
inconsistency.

Peace
Bill Mill
bill.mill at gmail.com

[Reinhold Birkenfeld]

Bill Mill wrote:

On Tue, 15 Mar 2005 10:47:28 -0800, James Stroud <js*****@mbi.uc la.edu> wrote:

On Tuesday 15 March 2005 08:25 am, Roy Smith wrote:

> a = () # tuple of zero elements
> a = (1,) # tuple of one element
> a = 1, # tuple of one element
> a = (1) # scalar
> a = (1, 2) # tuple of two elements
> a = 1, 2 # tuple of two elements
> a = , # syntax error
>
> The big question is, is it the parens that make it a tuple, or is it
> the comma? If you go along with the parens school of thought, then
> (1,) is the special case. If you believe in commas, then the () is
> the special case. In either case, it's a bit ugly, but we learn to
> overlook the occasional cosmetic blemishes of those we love :-)

The answer is obvious, the naked comma should be an empty tuple.

The other answer, that parens should be required to surround all
tuples, is obvious too.

Neither is particularly appealing; a lone comma creating a data
structure seems counter-intuitive, but it's nice to do a, b = b, a
instead of (a, b) = (b, a) . In this case, since the need to create
empty tuples is vanishingly rare, I'm okay with a little
inconsistency.

And if you don't like it at all, you can still use tuple() to "create"
an empty tuple.

Reinhold