What's the difference between lists enclosed by square brackets and parentheses in Python?

>>> x=[1,2]
>>> x[1]
2
>>> x=(1,2)
>>> x[1]
2

Are they both valid? Is one preferred for some reason?

Square brackets are lists while parentheses are tuples.

A list is mutable, meaning you can change its contents:

>>> x = [1,2]
>>> x.append(3)
>>> x
[1, 2, 3]

while tuples are not:

>>> x = (1,2)
>>> x
(1, 2)
>>> x.append(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'tuple' object has no attribute 'append'

The other main difference is that a tuple is hashable, meaning that you can use it as a key to a dictionary, among other things. For example:

>>> x = (1,2)
>>> y = [1,2]
>>> z = {}
>>> z[x] = 3
>>> z
{(1, 2): 3}
>>> z[y] = 4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Note that, as many people have pointed out, you can add tuples together. For example:

>>> x = (1,2)
>>> x += (3,)
>>> x
(1, 2, 3)

However, this does not mean tuples are mutable. In the example above, a new tuple is constructed by adding together the two tuples as arguments. The original tuple is not modified. To demonstrate this, consider the following:

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

Whereas, if you were to construct this same example with a list, y would also be updated:

>>> x = [1, 2]
>>> y = x
>>> x += [3]
>>> x
[1, 2, 3]
>>> y
[1, 2, 3]

One interesting difference :

lst=[1]
print lst          // prints [1]
print type(lst)    // prints <type 'list'>

notATuple=(1)
print notATuple        // prints 1
print type(notATuple)  // prints <type 'int'>
                                         ^^ instead of tuple(expected)

A comma must be included in a tuple even if it contains only a single value. e.g. (1,) instead of (1).

They are not lists, they are a list and a tuple. You can read about tuples in the Python tutorial. While you can mutate lists, this is not possible with tuples.

In [1]: x = (1, 2)

In [2]: x[0] = 3
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/home/user/<ipython console> in <module>()

TypeError: 'tuple' object does not support item assignment

Another way brackets and parentheses differ is that square brackets can describe a list comprehension, e.g. [x for x in y]

Whereas the corresponding parenthetic syntax specifies a tuple generator: (x for x in y)

You can get a tuple comprehension using: tuple(x for x in y)

See: Why is there no tuple comprehension in Python?

The first is a list, the second is a tuple. Lists are mutable, tuples are not.

Take a look at the Data Structures section of the tutorial, and the Sequence Types section of the documentation.