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.
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])
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Negative Indexing
Negative indexing means beginning from the end, -1 refers to the last item,
-2 refers to the second last item etc.
Example
Print the last item of the list:
thislist = ["apple", "banana", "cherry"]
print(thislist[-1])
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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
Return the third, fourth, and fifth item:
thislist = ["apple", "banana", "cherry", "orange",
"kiwi", "melon", "mango"]
print(thislist[2:5])
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Note: The search will start at index 2 (included) and end at index 5 (not included).
Remember that the first item has index 0.
By leaving out the start value, the range will start at the first item:
Example
This example returns the items from the beginning to "orange":
thislist = ["apple", "banana", "cherry", "orange",
"kiwi", "melon", "mango"]
print(thislist[:4])
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By leaving out the end value, the range will go on to the end of the list:
Example
This example returns the items from "cherry" and to the end:
thislist = ["apple", "banana", "cherry", "orange",
"kiwi", "melon", "mango"]
print(thislist[2:])
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Range of Negative Indexes
Specify negative indexes if you want to start the search from the end of the list:
Example
This example returns the items from index -4 (included) to index -1 (excluded)
thislist = ["apple", "banana", "cherry", "orange",
"kiwi", "melon", "mango"]
print(thislist[-4:-1])
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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)
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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)
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You will learn more about for loops in our 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")
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List Length
To determine how many items a list has, use the
len() function:
Example
Print the number of items in the list:
thislist = ["apple", "banana", "cherry"]
print(len(thislist))
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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)
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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)
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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)
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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)
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Example
The del keyword removes the specified
index:
thislist = ["apple", "banana", "cherry"]
del
thislist[0]
print(thislist)
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Example
The del keyword can also delete the list
completely:
thislist = ["apple", "banana", "cherry"]
del
thislist
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Example
The clear() method empties the list:
thislist = ["apple", "banana", "cherry"]
thislist.clear()
print(thislist)
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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)
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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)
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Join Two Lists
There are several ways to join, or concatenate, two or more lists in Python.
One of the easiest ways are by using the +
operator.
Example
Join two list:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
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Another way to join two lists are by appending all the items from list2 into list1, one by one:
Example
Append list2 into list1:
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1)
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Or you can use the extend()
method,
which purpose is to add elements from one list to another
list:
Example
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)
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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-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 |
