How to check if a list is in descending order in Python

Python | Check if list is sorted or not

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Python List sort() - Sorts Ascending or Descending List

The list.sort() method sorts the elements of a list in ascending or descending order using the default < comparisons operator between items.

Use the key parameter to pass the function name to be used for comparison instead of the default < operator. Set the reverse parameter to True, to get the list in descending order.

Syntax:

list.sort(key=None, reverse=False)

Parameters:

1. key: (Optional) A function that extracts a comparison key from each list element while sorting.
2. reverse: (Optional) If true, the sorted list will be reversed. By default, it is False.

Return Value:

No return value. It sorts the list itself.

The following example demonstrates the sort() function on numeric lists.

The sort() method can also be used on character lists.

The following example sorts the string list in alphabetical order.

What is the sort() method in Python?

This method takes a list and sorts it in place. This method does not have a return value.

In this example, we have a list of numbers and we can use the sort() method to sort the list in ascending order.

my_list = [67, 2, 999, 1, 15] # this prints the unordered list print("Unordered list: ", my_list) # sorts the list in place my_list.sort() # this prints the ordered list print("Ordered list: ", my_list)

If the list is already sorted then it will return None in the console.

my_list = [6, 7, 8, 9, 10] # this will return None because the list is already sorted print(my_list.sort())

The sort() method can take in two optional arguments called key and reverse.

key has the value of a function that will be called on each item in the list.

In this example, we can use the len() function as the value for the key argument. key=len will tell the computer to sort the list of names by length from smallest to largest.

names = ["Jessica", "Ben", "Carl", "Jackie", "Wendy"] print("Unsorted: ", names) names.sort(key=len) print("Sorted: ", names)

reverse has a boolean value of True or False.

In this example, reverse=True will tell the computer to sort the list in reverse alphabetical order.

names = ["Jessica", "Ben", "Carl", "Jackie", "Wendy"] print("Unsorted: ", names) names.sort(reverse=True) print("Sorted: ", names)

🔹 Purpose and Use Cases

With the sort() method, you can sort a list in either:

• Ascending Order
• Descending Order

This method is used to sort a list in place, which means that it mutates it or modifies it directly without creating additional copies, so remember:

You will learn more about mutation in this article (I promise!), but for now it's very important that you know that the sort() method modifies the list, so its original version is lost.

Because of this, you should only use this method if:

• You want to modify (sort) the list permanently.
• You don't need to keep the original version of the list.

If this fits your needs, then the .sort() method is exactly what you are looking for.

🔸 Syntax and Arguments

Let's see how you can call .sort() to take advantage of its full power.

This is the most basic call (with no arguments):

If you don't pass any arguments, by default:

• The list will be sorted in ascending order.
• The elements of the list will be compared directly using their values with the < operator.

For example:

>>> b = [6, 3, 8, 2, 7, 3, 9] >>> b.sort() >>> b [2, 3, 3, 6, 7, 8, 9] # Sorted!

Custom Arguments

To customize how the sort() method works, you can pass two optional arguments:

• Key
• Reverse

Let's see how they change the behavior of this method. Here we have a method call with these two arguments:

Before explaining how they work, I would like to explain something that you probably noticed in the diagram above – in the method call, the names of the parameters have to be included before their corresponding values, like this:

• key=
• reverse=

This is because they are keyword-only arguments. If you are passing a custom value for them, their names have to be specified in the method call, followed by an equal sign = and their corresponding values, like this:

Otherwise, if you try to pass the arguments directly as we normally do for positional parameters, you will see this error because the function will not know which argument corresponds to which parameter:

TypeError: sort() takes no positional arguments

Reverse

Now that you know what keyword-only arguments are, let's start with reverse.

The value of reverse can be either True or False:

• False means that the list will be sorted in ascending order.
• True means that the list will be sorted in descending (reverse) order.

💡 Tip: By default, its value is False – if you don't pass any arguments for this parameter, the list is sorted in ascending order.

Here we have a few examples:

By default, reverse is False# List of Integers >>> b = [6, 3, 8, 2, 7, 3, 9] >>> b.sort() >>> b [2, 3, 3, 6, 7, 8, 9] # List of Strings >>> c = ["A", "Z", "D", "T", "U"] >>> c.sort() >>> c ['A', 'D', 'T', 'U', 'Z']

💡 Tip: If the elements of the list are strings, they are sorted alphabetically.

To specify that reverse is True, so the list has to be sorted in descending (reverse) order.# List of Integers >>> b = [6, 3, 8, 2, 7, 3, 9] >>> b.sort(reverse=True) >>> b [9, 8, 7, 6, 3, 3, 2] # List of Strings >>> c = ["A", "Z", "D", "T", "U"] >>> c.sort(reverse=True) >>> c ['Z', 'U', 'T', 'D', 'A']

💡 Tip: Notice how the list is sorted in descending order if reverse is True.

Key

Now that you know how to work with the reverse parameter, let's see the key parameter.

This parameter is a little bit more detailed because it determines how the elements of the list are be compared during the sorting process.

Basic Syntax

The value of key is either:

• None, which means that the elements of the list will be compared directly. For example, in a list of integers, the integers themselves can be used for the comparison.
• A function of one argument that generates an intermediate value for each element. This intermediate value is calculated only once and it's used to make the comparisons during the entire sorting process. We use this when we don't want to compare the elements directly, for example, when we want to compare strings based on their length (the intermediate value).

💡 Tip: By default, the value of key is None, so the elements are compared directly.

For example:

Let's say that we want to sort a list of strings based on their length, from the shortest string to the longest string. We can pass the function len as the value of key, like this:

>>> d = ["aaa", "bb", "c"] >>> d.sort(key=len) >>> d ['c', 'bb', 'aaa']

💡 Tip: Notice that we are only passing the name of the function (len) without parenthesis because we are not calling the function. This is very important.

Notice the difference between comparing the elements directly and comparing their length (see below). Using the default value of key (None) would have sorted the strings alphabetically (left), but now we are sorting them based on their length (right):

What happens behind the scenes? Each element is passed as an argument to the len() function, and the value returned by this function call is used to perform the comparisons during the sorting process:

This results in a list with a different sorting criteria: length.

Here we have another example:

Another interesting example is sorting a list of strings as if they were all written in lowercase letters (for example, making "Aa" equivalent to "aa").

According to lexicographical order, capital letters come before lowercase letters:

>>> "E" < "e" True

So the string "Emma" would come before "emily" in a sorted list, even if their lowercase versions would be in the opposite order:

>>> "Emma" < "emily" True >>> "emma" < "emily" False

To avoid distinguishing between capital and lowercase letters, we can pass the function str.lower as key. This will generate a lowercase version of the strings that will be used for the comparisons:

>>> e = ["Emma", "emily", "Amy", "Jason"] >>> e.sort(key=str.lower) >>> e ['Amy', 'emily', 'Emma', 'Jason']

Notice that now, "emily" comes before "Emma" in the sorted list, which is exactly what we wanted.

💡 Tip: if we had used the default sorting process, all the strings that started with an uppercase letter would have come before all the strings that started with a lowercase letter:

>>> e = ["Emma", "emily", "Amy", "Jason"] >>> e.sort() >>> e ['Amy', 'Emma', 'Jason', 'emily']

Here is an example using Object-Oriented Programming (OOP):

If we have this very simple Python class:

>>> class Client: def __init__(self, age): self.age = age

And we create four instances:

>>> client1 = Client(67) >>> client2 = Client(23) >>> client3 = Client(13) >>> client4 = Client(35)

We can make a list that references them:

>>> clients = [client1, client2, client3, client4]

Then, if we define a function to get the age of these instances:

>>> def get_age(client): return client.age

We can sort the list based on their age by passing the get_age function an an argument:

>>> clients.sort(key=get_age)

This is the final, sorted version of the list. We use a for loop to print the age of the instances in the order that they appear in the list:

>>> for client in clients: print(client.age) 13 23 35 67

Exactly what we wanted – now the list is sorted in ascending order based on the age of the instances.

💡 Tip: Instead of defining a get_age function, we could have used a lambda function to get the age of each instance, like this:

>>> clients.sort(key=lambda x: x.age)

Lambda functions are small and simple anonymous functions, which means that they don't have a name. They are very helpful for these scenarios when we only want to use them in particular places for a very short period of time.

This is the basic structure of the lambda function that we are using to sort the list:

Basic Structure of a Lambda Function

Passing Both Arguments

Awesome! Now you know to customize the functionality of the sort() method. But you can take your skills to a whole new level by combining the effect of key and reverse in the same method call:

>>> f = ["A", "a", "B", "b", "C", "c"] >>> f.sort(key=str.lower, reverse=True) >>> f ['C', 'c', 'B', 'b', 'A', 'a']Sort the list in reverse order as if the strings were in all lowercase.

These are the different combinations of the arguments and their effect:

The Order of Keyword-Only Arguments Doesn't Matter

Since we are specifying the names of the arguments, we already know which value corresponds to which parameter, so we can include either key or reverse first in the list and the effect will be exactly the same.

So this method call:

Is equivalent to:

This is an example:

>>> a = ["Zz", "c", "y", "o", "F"] >>> a.sort(key=str.lower, reverse=True) >>> a ['Zz', 'y', 'o', 'F', 'c']

If we change the order of the arguments, we get the exact same result:

>>> a = ["Zz", "c", "y", "o", "F"] >>> a.sort(reverse=True, key=str.lower) >>> a ['Zz', 'y', 'o', 'F', 'c']