Lists

Python Lists

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.

List

A list is a collection which is ordered and changeable. In Python lists are written with square brackets.

Example

Create a List:

thislist = ["apple", "banana", "cherry"]
print(thislist)

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Access Items

You access the list items by referring to the index number:

Example

Print the second item of the list:

thislist = ["apple", "banana", "cherry"]
print(thislist[1])

Change Item Value

To change the value of a specific item, refer to the index number:

Example

Change the second item:

thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist)

Loop Through a List

You can loop through the list items by using a for loop:

Example

Print all items in the list, one by one:

thislist = ["apple", "banana", "cherry"]
for x in thislist:
  print(x)

List Length

To determine how many items a list have, use the len() method:

Example

Print the number of items in the list:

thislist = ["apple", "banana", "cherry"]
print(len(thislist))

Add Items

To add an item to the end of the list, use the append() method:

Example

Using the append() method to append an item:

thislist = ["apple", "banana", "cherry"]
thislist.append("orange")
print(thislist)

To add an item at the specified index, use the insert() method:

Example

Insert an item as the second position:

thislist = ["apple", "banana", "cherry"]
thislist.insert(1, "orange")
print(thislist)

Remove Item

There are several methods to remove items from a list:

Example

The remove() method removes the specified item:

thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
print(thislist)

Example

The pop() method removes the specified index, (or the last item if index is not specified):

thislist = ["apple", "banana", "cherry"]
thislist.pop()
print(thislist)

Example

The del keyword removes the specified index:

thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)

Example

The del keyword can also delete the list completely:

thislist = ["apple", "banana", "cherry"]
del thislist
print(thislist) #this will cause an error because "thislist" no longer exists.

Example

The clear() method empties the list:

thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)

The list() Constructor

It is also possible to use the list() constructor to make a list.

Example

Using the list() constructor to make a List:

thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)

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List Methods

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