[python] 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?

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.

Comma-separated items enclosed by ( and ) are tuples, those enclosed by [ and ] are lists.

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?