Question

How did you figure out that (1,2) + (3,) gives you (1,2,3)?

Answer 1

Very interesting question . You deserve several medals for this one. The syntactical representation of lists and tuples in python are for the most part equivalent. But sometimes they are subtly different. Consider:

x = [1,2]
y = [3,4]

x is a list, y is a list
if i add (concatenate) x to y I also get a list

x + y (equals)

[1,2,3,4]

The above pattern is exactly the same for tuples

x = (1,2)
y = (3,4)

x is a tuple, y is a tuple
if I add (concatenate) x to y I also get a tuple

x+ y (equals)

(1,2,3,4)

Now consider:

x =[1,2]
y = [3]
x is a list and so is y

if I add (concatenate) x to y I also get a list

x + y (equals)

[1,2,3]

But now it gets interesting. If I declare:

x =(1,2)
y=(3)

and add x + y, I would expect it to give me the tuple:

(1,2,3).

Try it and you will get a "type error". Why?

The statement y=(3) looks like you are declaring a tuple with one member, 3. But y it turns out is not a tuple at all. It's just 3. If you want a tuple with one member then you have to declare it as

y = (3,)

Then you can go ahead an add x to y and you will get:

(1,2,3).

Excellent Question!

Answer 2

wow, the line of thinking makes terrific sense. y=(3) is not a tuple, and now I see that because parenthesis are used as a arithmetical operator, unless you include a comma. brackets on the other hand are not used with arithmetic operations.

Apparently, this can be illustrated by the two expressions:
>>> (1+2)*3
9
>>> [1+2]*3
[3, 3, 3]

Thanks!

Answer 3

To reinforce the point, if you type in the interpreter

x =[1]
y=1

x==y

you will get
False

It's what we would expect since x is a list with one member, namely 1 and y is the number 1.
x and y are not the same

but if you type

x=(1)
y=1
x==y

you will get True

because x is 1 and y is also 1
one is deceived into thinking that we are creating a tuple with the statement

x=(1)

in the statement above x=(1) is equivalent to x=1

Answer 4

no need for further reinforcement, though what you say all seems to be true. i believe your first reply made a light switch come on in my head.

thanks again!

Answer 5

http://docs.python.org/tutorial/datastructures.html#tuples-and-sequences

Answer 6

I read this section:
"A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective."

and immediately favorited the link you sent as a resource for python.

Thanks bwCA!

Answer 7

that documentation should be installed on your computer. If using windows and idle, F1 from within idle.