why does a 1-element tuple in python require comma?

[IAMTubby]

Hello,
Why is the comma in tup1 = (1,); required for a single element tuple in python?

#tup1 = (1);
tup1 = (1,);
print "tup1[0]: ", tup1[0]

Thanks.

[CptPicard]

Because otherwise it's ambiguous with the arithmetic expression (1), which is 1 with some extra parentheses.

[IAMTubby]

Because otherwise it's ambiguous with the arithmetic expression (1), which is 1 with some extra parentheses.

hmm...
CptPicard, Sorry I'm being a bit slow here. Say in a data structure like array in any programming language, you want to have only one value, you would just assign the value alone right ? Something like

#include <stdio.h>


int main(void)
{
 int a[] = {1};


 printf("%d",a[0]);


 return 0;
}

How is it different in tuples?

[CptPicard]

Go to the Python shell and type (1). What do you get? How could you tell the difference between the expression of a parenthesized 1 and a single-element tuple if the latter also looks like (1)? You couldn't, so you need to define the tuple as something else... for example (1,).

[ofnuts]

Corollary: to unpack your "unuple" you need the same kind of dangling comma:

>>> t = (1,)
>>> e, = t
>>> e
1

Otherwise:

>>> e = t
>>> e
(1,)

[CptPicard]

And an interesting feature of the tuple syntax I had not thought of before: The tuple actually seems to be just the comma-separated list of things, not the parenthesis around it. So the tuple (1,2) can be written as just "1,2".

[ofnuts]

And an interesting feature of the tuple syntax I had not thought of before: The tuple actually seems to be just the comma-separated list of things, not the parenthesis around it. So the tuple (1,2) can be written as just "1,2".

Never wrote a function that does "return x,y" before?

[CptPicard]

Never wrote a function that does "return x,y" before?

Good point. Then again it's been years since I last touched Python

[IAMTubby]

Corollary: to unpack your "unuple" you need the same kind of dangling comma:

Go to the Python shell and type (1). What do you get?

I did that, I got

>>> a = (1)
>>> print a;
1
>>> b = (1,)
>>> print b
(1,)

How could you tell the difference between the expression of a parenthesized 1 and a single-element tuple if the latter also looks like (1)? You couldn't, so you need to define the tuple as something else... for example (1,).

But isn't a "parenthesized 1" same as a single-element tuple. I'm assuming that a tuple is the same as an array in C except that "A tuple is an immutable list. A tuple can not be changed in any way once it is created.(from google)"

As per my understanding, since I think that a "parenthesized 1" is same as a single-element tuple, I still haven't got it completely.

[spjackson]

But isn't a "parenthesized 1" same as a single-element tuple.

No.

>>> print 1 == (1)
True
>>> print 1 == (1,)
False
>>> print (1,) == (1,)
True
>>> print (1) == (1,)
False

I'm assuming that a tuple is the same as an array in C except that "A tuple is an immutable list.

Using the C analogy, 1 is an int and an arrary of one int is not an int, it is an array.