python list

By: Ashley J Printer Friendly Format

A Python list is a mutable ordered sequence of items. The items of a list are arbitrary objects and may be of different types. To denote a list, use a series of expressions (the items of the list) separated by commas (,), within square brackets ([]); if every item is a literal, the whole assembly is a list literal. You may optionally place a redundant comma after the last item. To denote an empty list, use an empty pair of brackets. Here are some example list literals:

[42, 3.14,
'hello']       
# List with three items

[100]                     
# List with one item

[]                        
# Empty list

You can also call the built-in type list to create a list. For example:


list('wow')

This builds a list equal to that denoted by the list literal:

['w', 'o', 'w']
list() without arguments creates and returns an empty list, like []. When x is iterable, list(x) creates and returns a new list whose items are the same as those x.

MODIFYING A LIST

You can modify a single item in a list by assigning to an indexing. For instance:

x = [1, 2, 3, 4]

x[1] =
42               
# x is now [1, 42, 3, 4]

You can also modify a range of elements in a list as below. For example:


x = [1, 2, 3, 4]

x[1:3] = [22, 33, 44]     # x is
now [1, 22, 33, 44, 4]

x[1:4] = [8,
9]          
# x is now [1, 8, 9, 4]

SORTING A LIST

A list’s method sort() causes the list to be sorted in-place (reordering items to place them in increasing order).

mylist = ['alpha', 'Beta', 'GAMMA']

mylist.sort()                 
#  ['Beta', 'GAMMA', 'alpha']

mylist.sort(key=str.lower)    
#  ['alpha', 'Beta', 'GAMMA']

Python also provides the built-in function sorted() to produce a sorted list from any input iterable. sorted, after the first argument (which is the iterable supplying the items), accepts the same arguments as a list’s method sort.

Other List Operations

Method	Description
Nonmutating
`L` `.count(` `x` `)`	Returns the number of items of `L` that are equal to `x`.
`L` `.index(` `x` `)`	Returns the index of the first occurrence of an item in `L` that is equal to `x`, or raises an exception if `L` has no such item.
Mutating
`L` `.append(` `x` `)`	Appends item `x` to the end of `L` ; like `L``[len(``L``):]=[``x``]`.
`L` `.extend(` `s` `)`	Appends all the items of iterable `s` to the end of `L` ; like `L``[len(``L``):]=``s` or `L += s`.
`L``.insert(``i``,` `x``)`	Inserts item `x` `in` `L` before the item at index `i`, moving following items of `L` (if any) “rightward” to make space (increases `len(``L``)` by one, does not replace any item, does not raise exceptions; acts just like `L``[``i``:``i``]=[``x``]`).
`L` `.remove(` `x` `)`	Removes from `L` the first occurrence of an item in `L` that is equal to `x``,` or raises an exception if `L` has no such item.
`L.pop(i=-1)`	Returns the value of the item at index `i` and removes it from `L`; when you omit `i`, removes and returns the last item; raises an exception if `L` is empty or `i` is an invalid index in `L`.
`L` `.reverse()`	Reverses, in place, the items of `L`.
`L``.sort(cmp=cmp, key=None, reverse=False)`	Sorts, in-place, the items of `L`, comparing items pairwise via—v2, only —the function passed as `cmp` (by default, the built-in function `cmp`). When argument `key` is not `None`, what gets compared for each item `x` is `key``(``x``)`, not `x` itself.