Table of Contents
What is a list in Python and how do you create one
A list is a sequence of values that can be accessed through its index. If you have a list of integers, you can access the 6th item in the sequence with the variable 5.Lists are commonly used in both Python and other programming languages as they are an efficient way to store and organize data. Lists are often used to store data in the form of integers and other basic data types. However, lists can also be used for more complicated functions as well.
A list’s definition is very simple in its theoretical nature, but when you see it in something like Python or Perl, it can get a little confusing. In this tutorial, I will show you how to create and use lists in Python.
In Python, a list is defined by writing the list’s items within square brackets “[]”, also known as brackets, or braces. For example, if we wanted to create a list that contained the integers 1, 2 and 3 then we would write [1,2,3]. The list that contains these three integers is called an “empty list”, as it doesn’t contain anything.
How to add items to a list in Python
To add an item to a list in python, you must use the list data type. You know that lists are sequences of values and sequences are immutable, so if you add a new item to the list, it will not change the original value of any items already in the list. To do this we need to tell Python that we want to create a new value in the list. When we add an item to the list, its value will be the last (or newest) item in the list. We don’t want this, we want it to be the first item. A list is called a non-inclusive set when the value of one element (the first) in the set is not equal to all of the elements in the set.
Lets start with creating a simple example:
import datetime def add_to_list(some_list): some_list = [datetime.datetime(2011,12,27)] print “Some List Example” some_list.append(datetime.datetime(2011,10,3)) print “End of some list example” some_list
This will make a list with one item – 2011-12-27. To make a list with dates from 2011-10-1 to 2011-10-31 we should use:
import datetime def add_to_list(some_list): some_list = [datetime.datetime(2011,12,27)] print “Some List Example” some_list.append(datetime.datetime(2011,10,3)) print “End of some list example” some_list
So now you know how to add an item to the end of a list in python. Lets move onto how to add an item to the beginning of a list:
How to remove items from a list in Python
To remove an item from a list in python, you must use the del statement. The del statement is used to delete one or more variables. When the del statement is applied to a list, it removes the first item in the list that matches what has been specified. When removing an item from a list, you are also deleting the value in that position. The del statement allows us to remove items from lists without modifying their original values.
Code:
>>> myList = [‘one’, ‘two’, ‘three’, 4, 5] >>> del myList [‘four’]
The del statement is different than having an extra variable in a list. When the del statement is applied to a list, it removes the first item that matches what has been specified. However, when extra variables are added to a list, they are simply appended to the end of the list. [1, 2, ‘three’, 4] will have four added to the end of the list instead of removing it from the original list. Deletion occurs only when the values match up.
If you want to delete all items from a list and not just remove an item, you can use the clear() function in Python. The clear() function deletes all elements from a List and leaves the list empty. This function is commonly used in the event of a List getting too long and not all items needing to be listed.
Here’s some code:
Code:
>>> myList = [‘one’, ‘two’, ‘three’, 4, 5] >>> myList.clear() >>> myList [‘one’, ‘three’, 4, 5]
How to search for an item in a list in Python
To search for an item in a list in python, you must use the index of the value that you are looking for. Indexes start at 0 and go up by 1 each time. If you have a list with four items, the indexes would be 0, 1, 2, and 3. If you are looking for the fifth item in the list, you would use the index 5. In some cases, the value itself is not needed to find an item that you want to remove or add. Instead, we can just ask Python to get its index at that point and use that number as our index.
Here is an example of finding the index of a value and printing the index:
index = ‘value’ inLst = [‘value1′,’value2′,’value3′,’value4’] print “The index of value is “,index,”in inLst”
This will give us the following response:
The index of value is 3 in inLst
Here is an example that uses a for loop to change the third list item to add two new items to it. You can see the results in real time.
