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 and unindexed. No duplicate members.
- Dictionary is a collection which is unordered, changeable and indexed. No duplicate members.
When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.
A list is a collection which is ordered and changeable. In Python lists are written with square brackets.
Create a List:
You access the list items by referring to the index number:
Print the second item of the list:
Change Item ValueTo change the value of a specific item, refer to the index number:
Change the second item:
thislist = "blackcurrant"
Loop Through a List
You can loop through the list items by using a
Print all items in the list, one by one:
for x in thislist:
You will learn more about
for loops in out Python For Loops Chapter.
Check if Item Exists
To determine if a specified item is present in a list use the
Check if "apple" is present in the list:
if "apple" in thislist:
print("Yes, 'apple' is in the fruits list")
To determine how many items a list has, use the
Print the number of items in the list:
To add an item to the end of the list, use the append() method:
Using the append() method to append an item:
To add an item at the specified index, use the insert() method:
Insert an item as the second position:
There are several methods to remove items from a list:
remove() method removes the specified item:
pop() method removes the specified
index, (or the last item if index is not specified):
del keyword removes the specified
del keyword can also delete the list
clear() method empties the list:
Copy a List
You cannot copy a list simply by typing
list2 will only be a
list1, and changes made in
list1 will automatically also be made in
There are ways to make a copy, one way is to use the built-in List
Make a copy of a list with the
mylist = thislist.copy()
Another way to make a copy is to use the built-in method
Make a copy of a list with the
mylist = list(thislist)
The list() Constructor
It is also possible to use the list() constructor to make a new list.
Using the list() constructor to make a List:
Python has a set of built-in methods that you can use on lists.
|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|