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)
Show Python »C:\Users\My Name>python demo_list.py
['apple', 'banana', 'cherry']
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])
Show Python »C:\Users\My Name>python demo_list_access.py
banana
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)
Show Python »C:\Users\My Name>python demo_list_change.py
['apple', 'blackcurrant', 'cherry']
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)
Show Python »C:\Users\My Name>python demo_list_loop.py
apple
banana
cherry
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")
Show Python »C:\Users\My Name>python demo_list_in.py
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))
Show Python »C:\Users\My Name>python demo_list_len.py
3
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)
Show Python »C:\Users\My Name>python demo_list_append.py
['apple', 'banana', 'cherry', 'orange']
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)
Show Python »C:\Users\My Name>python demo_list_insert.py
['apple', 'orange', 'banana', 'cherry']
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)
Show Python »C:\Users\My Name>python demo_list_remove.py
['apple', 'cherry']
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)
Show Python »C:\Users\My Name>python demo_list_pop.py
['apple', 'banana']
Example
The
del keyword removes the specified index:
thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist)
Show Python »C:\Users\My Name>python demo_list_del.py
['banana', 'cherry']
Example
The
del keyword can also delete the list completely:
thislist = ["apple", "banana", "cherry"]
del thislist
Show Python »C:\Users\My Name>python demo_list_del2.py
Traceback (most recent call last):
File "demo_list_del2.py", line 3, in <module>
print(thislist) #this will cause an error because you have succsesfully
deleted "thislist".
NameError: name 'thislist' is not defined
Example
The
clear() method empties the list:
thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)
Show Python »C:\Users\My Name>python demo_list_clear.py
[]
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)
Show Python »C:\Users\My Name>python demo_list_copy.py
['apple', 'banana', 'cherry']
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)
Show Python »C:\Users\My Name>python demo_list_copy2.py
['apple', 'banana', 'cherry']
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)
Show Python »C:\Users\My Name>python demo_list_constructor.py
['apple', 'banana', 'cherry']
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 |
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