Python – How to Count List Length

pythonpython-2.7

I want to know how len() works.

Does it count from beginning to end of a list every time I call len(), or, since list is also an class, does len() just return a variable in the list object which record the list's length?

Also, I hope someone can tell me where I can find the source code of those built-in functions like 'len()', 'map()', etc.

Best Answer

Download Python 2.7 source code here: http://www.python.org/getit/releases/2.7.4/

list is implemented in ./Include/listobject.h and ./Objects/listobject.c.

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;

list.__len__() just consults ob_size, which is part of PyObject_VAR_HEAD. This makes len() a constant-time operation for lists.

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