How to understand expression lists in Python

Question

When I read the Python document today, I found Expression lists on Python Documents, the description on the site like this:

expression_list ::= expression ( "," expression )* [","]

An expression list containing at least one comma yields a tuple. The length of the tuple is the number of expressions in the list. The expressions are evaluated from left to right.

The trailing comma is required only to create a single tuple (a.k.a. a singleton); it is optional in all other cases. A single expression without a trailing comma doesn’t create a tuple, but rather yields the value of that expression. (To create an empty tuple, use an empty pair of parentheses: ().)

Because the examples are not given on the site, So I just wondering anyone can give a brief description about this, and give a example about its usage. Thank a lots.

Answer 1

Here are some samples to help you understand what is going on:

An expression list containing at least one comma yields a tuple.

This means, that if you have 1,2, this will create a tuple. The length is how many items you have.

The trailing comma is required only to create a single tuple (a.k.a. a singleton); it is optional in all other cases.

This means that if you want to create a tuple with one item, you need to have a comma at the end, like this 1,, otherwise:

A single expression without a trailing comma doesn’t create a tuple, but rather yields the value of that expression.

So 1 is not creating a tuple, what will happen is the express will be evaluated. This sounds like an obvious point but it makes sense if you write (1) and then expect it to be evaluated as a tupe (in the case of (1), it will be evaluated to the integer value 1).

Finally

(To create an empty tuple, use an empty pair of parentheses: ().)

If for some reason you want to create an empty tuple as an expression, use the following special syntax ()

It is a common practice to surround expressions (especially in the case of tuples) with (), but this is not required - although sometimes it helps with readability. 1,2 and (1,2) are equal:

>>> a = 1,2
>>> type(a)
<type 'tuple'>

>>> b = (1,2)
>>> type(b)
<type 'tuple'>

>>> a == b
True

Answer 2

This talks about how you write tuples.

For instance,

>>> 1, 2
(1, 2)

is a two element tuple, as is

>>> 7*8, 5-6
(56, -1)

Tuples are usually written with parentheses around them for clarity, but they are unnecessary; except in case of the 0-element tuple, ().

One-element tuples are another exception, as there it is mandatory to have the comma:

>>> 1,
(1, )

Without the comma it wouldn't be possible to distinguish this from the normal number 1. You can add an extra comma after multi-element tuples too, but it doesn't do anything in that case:

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