List Items
List items are ordered, changeable, and allow duplicate values.
List items are indexed, the first item has index [0], the second item has index [1] etc.
Ordered
When we say that lists are ordered, it means that the items have a defined order, and that order will not change.
If you add new items to a list, the new items will be placed at the end of the list.
Python Collections (Arrays)
There are four collection data types in the Python programming language:
- List is a collection which is ordered and changeable. Allows duplicate members.
- Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
- Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
- Dictionary is a collection which is ordered** and changeable. No duplicate members.
Python has a set of built-in methods that you can use on lists.
| Method | Description |
|---|---|
| append() | Adds an element at the end of the list |
| clear() | Removes all the elements from the list |
| copy() | Returns a copy of the list |
| count() | Returns the number of elements with the specified value |
| extend() | Add the elements of a list (or any iterable), to the end of the current list |
| index() | Returns the index of the first element with the specified value |
| insert() | Adds an element at the specified position |
| pop() | Removes the element at the specified position |
| remove() | Removes the item with the specified value |
| reverse() | Reverses the order of the list |
| sort() | Sorts the list |
*Set items are unchangeable, but you can remove and/or add items whenever you like.
**As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
The list() Constructor
It is also possible to use the list() constructor when creating a new list.
Example
Using the list() constructor to make a List:
thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:
Example
Change the second value by replacing it with two new values:
thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print(thislist)Result: thislist = ["apple", "blackcurrant", "watermelon", "cherry"]
Del
The del keyword also removes the specified index:
Example
Remove the first item:
thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)The del keyword can also delete the list completely.
Example
Delete the entire list:
thislist = ["apple", "banana", "cherry"]
del thislist Looping Using List Comprehension
List Comprehension offers the shortest syntax for looping through lists:
Example
A short hand for loop that will print all items in a list:
thislist = ["apple", "banana", "cherry"]
[print(x) for x in thislist] List Comprehension
List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.
Example:
Based on a list of fruits, you want a new list, containing only the fruits with the letter "a" in the name.
Without list comprehension you will have to write a for statement with a conditional test inside:
Example
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = []
for x in fruits:
if "a" in x:
newlist.append(x)
print(newlist) With list comprehension you can do all that with only one line of code:
Example
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
print(newlist) The Syntax
newlist = [expression for item in iterable if condition == True]The return value is a new list, leaving the old list unchanged.
Condition
The condition is like a filter that only accepts the items that valuate to True.
Example
Only accept items that are not "apple":
newlist = [x for x in fruits if x != "apple"] The condition if x != "apple" will return True for all elements other than "apple", making the new list contain all fruits except "apple".
The condition is optional and can be omitted:
Example
With no if statement:
newlist = [x for x in fruits] 