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)
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)
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
in keyword:Example
Check if "apple" is present in the list:
thislist = ["apple", "banana", "cherry"]
if "apple" in thislist:
print("Yes, 'apple' is in the fruits list")
List Length
To determine how many items a list has, 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
Example
The
clear() method empties the list:
thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)
Copy a List
You cannot copy a list simply by typing
list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.
There are ways to make a copy, one way is to use the built-in List method
copy().Example
Make a copy of a list with the
copy() method:
thislist = ["apple", "banana", "cherry"]
mylist = thislist.copy()
print(mylist)
Another way to make a copy is to use the built-in method
list().Example
Make a copy of a list with the
list() method:
thislist = ["apple", "banana", "cherry"]
mylist = list(thislist)
print(mylist)
The list() Constructor
It is also possible to use the list() constructor to make a new list.
Example
Using the list() constructor to make a List:
thislist = list(("apple", "banana", "cherry")) # note the double round-bracketsprint(thislist)