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List are used to store multiple items in a single variable. Lists are created using square brackets.
Example 1: create a list:
thislist = ["apple", "banana", "cherry"]
print(thislist) # ['apple', 'banana', 'cherry']
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: 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.
Changeable: we can change, add, and remove items in a list after it has been created.
Allow duplicates: since lists are indexed, lists can have items with the same value.
Example 2: lists allow duplicate values:
thislist = ["apple", "banana", "cherry", "apple", "cherry"]
print(thislist) # ['apple', 'banana', 'cherry', 'apple', 'cherry']
List length: to determine how many items a list has, use the len() function.
Example 3: print the number of items in the list:
thislist = ["apple", "banana", "cherry"]
print(len(thislist)) # 3
List items can be of any data type.
Example 4: string, int and boolean data types:
list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]
A list can contain different data types.
Example 5: a list with strings, integers, and boolean values:
list1 = ["abc", 34, True, 40, "male"]
type(): from Python's perspective, lists are defined as objects with the data type 'list':
Example 6: what is the data type of a list?
mylist = ["apple", "banana", "cherry"]
print(type(mylist)) # class 'list'
The list() constructor can also be used when creating a new list.
Example 7: using the list() constructor to make a list:
thislist = list(("apple", "banana", "cherry"))
# note the double round-brackets
print(thislist) # ['apple', 'banana', 'cherry']
Python collections (arrays): There are four collection data types in the Python programming language:
List is a collection which is ordered and changeable. It allows duplicate members.
Tuple is a collection which is ordered and unchangeable. It allows duplicate members.
Set is a collection which is unordered and unindexed. No duplicate members are allowed.
Dictionary is a collection which is ordered* and changeable. No duplicate members are allowed.
!!! 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.
Access items: list items are indexed and you can access them by referring to the index number.
Example 8: print the second item of the list:
thislist = ["apple", "banana", "cherry"]
print(thislist[1]) # banana
Negative indexing means start from the end: -1 refers to the last item, -2 refers to the second last item, etc.
Example 9: print the last item of the list:
thislist = ["apple", "banana", "cherry"]
print(thislist[-1]) # cherry
Range of indexes: you can specify a range of indexes by specifying where to start and where to end the range. When specifying a range, the return value will be a new list with the specified items.
Example 10: return the third, fourth, and fifth item:
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[2:5]) # ['cherry', 'orange', 'kiwi']
By leaving out the start value, the range will start at the first item.
By leaving out the end value, the range will go on to the end of the list.
Range of negative indexes: specify negative indexes if you want to start the search from the end of the list.
Example 11: returns the items from "orange" (-4) to, but NOT including "mango" (-1):
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "melon", "mango"]
print(thislist[-4:-1])
Check if item exists: to determine if a specified item is present in a list use the "in" keyword.
Example 12: check if "apple" is present in the list:
thislist = ["apple", "banana", "cherry"]
if "apple" in thislist:
print("Yes, 'apple' is in the fruits list") # Yes, 'apple' is in the fruits list
Change item value: to change the value of a specific item, refer to the index number.
Example 13: change the second item:
thislist = ["apple", "banana", "cherry"]
thislist[1] = "blackcurrant"
print(thislist) # ['apple', 'blackcurrant', 'cherry']
Change a range of item values: to change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values.
Example 14: change the values "banana" and "cherry" with the values "blackcurrant" and "watermelon":
thislist = ["apple", "banana", "cherry", "orange", "kiwi", "mango"]
thislist[1:3] = ["blackcurrant", "watermelon"]
print(thislist) # ['apple', 'blackcurrant', 'watermelon', 'orange', 'kiwi', 'mango']
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 15: change the second value by replacing it with two new values:
thislist = ["apple", "banana", "cherry"]
thislist[1:2] = ["blackcurrant", "watermelon"]
print(thislist) # ['apple', 'blackcurrant', 'watermelon', 'cherry']
If you insert less items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly.
Example 16: change the second and third value by replacing it with one value:
thislist = ["apple", "banana", "cherry"]
thislist[1:3] = ["watermelon"]
print(thislist) # ['apple', 'watermelon']
Insert items: to insert a new list item, without replacing any of the existing values, we can use the insert() method, which inserts an item at the specified index.
Example 17: insert "watermelon" as the third item.
thislist = ["apple", "banana", "cherry"]
thislist.insert(2, "watermelon")
print(thislist) # ['apple', 'banana', 'watermelon', 'cherry']
Append items: to add an item to the end of the list, use the append() method.
Example 18: using the append() method to append an item:
thislist = ["apple", "banana", "cherry"]
thislist.append("orange")
print(thislist) # ['apple', 'banana', 'cherry', 'orange']
Insert items: to insert a list item at a specified index, use the insert() method, which inserts an item at the specified index.
Example 19: insert an item as the second position:
thislist = ["apple", "banana", "cherry"]
thislist.insert(1, "orange")
print(thislist) # ['apple', 'orange', 'banana', 'cherry']
Extend list: to append elements from another list to the current list, use the extend() method. The elements will be added to the end of the list.
Example 20: add the elements of "tropical" to "thislist":
thislist = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
thislist.extend(tropical)
print(thislist) # ['apple', 'banana', 'cherry', 'mango', 'pineapple', 'papaya']
Add any iterable: the extend() method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries, etc.).
Example 21: add elements of a tuple to a list:
thislist = ["apple", "banana", "cherry"]
thistuple = ("kiwi", "orange")
thislist.extend(thistuple)
print(thislist) # ['apple', 'banana', 'cherry', 'kiwi', 'orange']
Remove specified item using the remove() method.
Example 22: remove "banana":
thislist = ["apple", "banana", "cherry"]
thislist.remove("banana")
print(thislist) # ['apple', 'cherry']
Remove specified index using the pop() method. If you do not specify the index, the pop() method removes the last item.
Example 23: remove the second item:
thislist = ["apple", "banana", "cherry"]
thislist.pop(1)
print(thislist) # ['apple', 'cherry']
The "del" keyword also removes the specified index. The "del" keyword can also delete the list completely.
Example 24: remove the first item:
thislist = ["apple", "banana", "cherry"]
del thislist[0]
print(thislist) # ['banana', 'cherry']
Example 25: delete the entire list:
thislist = ["apple", "banana", "cherry"]
del thislist
Clear the list: the clear() method empties the list. The list still remains, but it has no content.
Example 26: clear the list content:
thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist) # []
Loop through the list items by using a for loop.
Example 27: print all items in the list, one by one:
thislist = ["apple", "banana", "cherry"]
for x in thislist:
print(x) # apple banana cherry
Loop through the index numbers: you can also loop through the list items by referring to their index number. Use the range() and len() functions to create a suitable iterable.
Example 28: print all items by referring to their index number:
thislist = ["apple", "banana", "cherry"]
for i in range(len(thislist)):
print(thislist[i]) # apple banana cherry
Using a while loop: you can loop through the list items by using a while loop. Use the len() function to determine the length of the list, then start at 0 and loop through the list items by refering to their indexes. Remember to increase the index by 1 after each iteration.
Example 29: print all items, using a while loop to go through all the index numbers
thislist = ["apple", "banana", "cherry"]
i = 0
while i < len(thislist):
print(thislist[i])
i = i + 1
Looping using list comprehension: list comprehension offers the shortest syntax for looping through lists.
Example 30: 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 offers a shorter syntax when you want to create a new list based on the values of an existing list. The return value is a new list, leaving the old list unchanged.
Example 31: only one line of code:
fruits = ["apple", "banana", "cherry", "kiwi", "mango"]
newlist = [x for x in fruits if "a" in x]
print(newlist) # ['apple', 'banana', 'mango']
The syntax: newlist = [expression "for" item "in" iterable "if" condition == "True"]
Condition: the condition is like a filter that only accepts the items that valuate to True. The condition is optional and can be omitted:
Example 32: only accept items that are not "apple":
newlist = [x "for" x "in" fruits "if" x != "apple"]
Iterable：the iterable can be any iterable object, like a list, tuple, set, etc.
Example 33: you can use the range() function to create an iterable:
newlist = [x for x in range(10)]
Example 34: same example, but with a condition: accept only numbers lower than 5:
newlist = [x for x in range(10) if x < 5]
Expression: the expression is the current item in the iteration, but it is also the outcome, which you can manipulate before it ends up like a list item in the new list.
Example 35: set the values in the new list to upper case:
newlist = [x.upper() for x in fruits]
The expression can also contain conditions, not like a filter, but as a way to manipulate the outcome.
Example 36: return "orange" instead of "banana":
newlist = [x if x != "banana" else "orange" for x in fruits]
Sort list alphanumerically: list objects have a sort() method that will sort the list alphanumerically, "ascending" by default.
Example 37: sort the list alphabetically:
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort()
print(thislist) # ['banana', 'kiwi', 'mango', 'orange', 'pineapple']
Example 38: sort the list numerically:
thislist = [100, 50, 65, 82, 23]
thislist.sort()
print(thislist) # [23, 50, 65, 82, 100]
Sort descending: to sort descending, use the keyword argument "reverse = True".
Example 39: sort the list descending:
thislist = ["orange", "mango", "kiwi", "pineapple", "banana"]
thislist.sort(reverse = True)
print(thislist) # ['pineapple', 'orange', 'mango', 'kiwi', 'banana']
Example 40: sort the list descending:
thislist = [100, 50, 65, 82, 23]
thislist.sort(reverse = True)
print(thislist) # [100, 82, 65, 50, 23]
Customize sort function: you can also customize your own function by using the keyword argument "key = function". The function will return a number that will be used to sort the list (the lowest number first).
Example 41: sort the list based on how close the number is to 50:
def myfunc(n):
return abs(n - 50)
thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc)
print(thislist) # [50, 65, 23, 82, 100]
Case insensitive sort: by default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters.
Example 42: case sensitive sorting can give an unexpected result:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort()
print(thislist) # ['Kiwi', 'Orange', 'banana', 'cherry']
We can use built-in functions as key functions when sorting a list. So if you want a case-insensitive sort function, use str.lower as a key function:
Example 43: perform a case-insensitive sort of the list:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.sort(key = str.lower)
print(thislist) # ['banana', 'cherry', 'Kiwi', 'Orange']
Reverse order: what if you want to reverse the order of a list, regardless of the alphabet? The reverse() method reverses the current sorting order of the elements.
Example 44: reverse the order of the list items:
thislist = ["banana", "Orange", "Kiwi", "cherry"]
thislist.reverse()
print(thislist) # ['cherry', 'Kiwi', 'Orange', 'banana']
Copy a list: there are ways to make a copy: one way is to use the built-in List method copy(), another way to make a copy is to use the built-in method list().
Example 45: make a copy of a list with the copy() method:
thislist = ["apple", "banana", "cherry"]
mylist = thislist.copy()
print(mylist)
Example 46: make a copy of a list with the list() method:
thislist = ["apple", "banana", "cherry"]
mylist = list(thislist)
print(mylist)
Join two lists: three ways: 1/use the + operator, 2/ append all the items from list2 into list1, one by one, 3/ use the extend() method, which purpose is to add elements from one list to another list.
Example 47: join two list:
list1 = ["a", "b", "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3) # ['a', 'b', 'c', 1, 2, 3]
Example 48: append list2 into list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1) # ['a', 'b', 'c', 1, 2, 3]
Example 49: use the extend() method to add list2 at the end of list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1) # ['a', 'b', 'c', 1, 2, 3]
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