What is the underlying data structure for Python lists?

List objects are implemented as arrays. They are optimized for fast fixed-length operations and incur O(n) memory movement costs for pop(0) and insert(0, v) operations which change both the size and position of the underlying data representation.

See also: http://docs.python.org/library/collections.html#collections.deque

Btw, I find it interesting that the Python tutorial on data structures recommends using pop(0) to simulate a queue but does not mention O(n) or the deque option.

http://docs.python.org/tutorial/datastructures.html#using-lists-as-queues

CPython:

typedef struct {
    PyObject_VAR_HEAD
    /* Vector of pointers to list elements.  list[0] is ob_item[0], etc. */
    PyObject **ob_item;

    /* ob_item contains space for 'allocated' elements.  The number
     * currently in use is ob_size.
     * Invariants:
     *     0 <= ob_size <= allocated
     *     len(list) == ob_size
     *     ob_item == NULL implies ob_size == allocated == 0
     * list.sort() temporarily sets allocated to -1 to detect mutations.
     *
     * Items must normally not be NULL, except during construction when
     * the list is not yet visible outside the function that builds it.
     */
    Py_ssize_t allocated;
} PyListObject;

As can be seen on the following line, the list is declared as an array of pointers to PyObjects.

PyObject **ob_item;

In the Jython implementation, it's an ArrayList<PyObject>.

Although it may be obvious, worth stating that Python lists are Dynamic arrays (as opposed to Static arrays). This is an important distinction that comes up in job interview questions/academia.

Because the array is dynamic, Python reserves an amount of memory upon declaration, eg:

somelist = []

Because extra memory already is set aside, performing somelist.append() simply writes to the next reserved memory slot(s) and hence is O(1) most of the time. For a static array, typically the array is full (ie. if have 4 bytes, then array size is 4) and appends would always be O(n) because they require reserving an entirely new set of memory (maybe 5 bytes now) and copying contents over.

The list is an inbuilt data structure in python. But can be used to create user-defined data structures. Two main user-defined data structures created by lists are stacks and queues.