How can you sort a list of dictionaries by a key contained in each dictionary using Python? Can you do it in 1 line to show off your friends? Of course!

To sort a list of dictionaries use the sorted() function with the key parameter set to a lambda function that uses the common key found in each dictionary that you want to sort the list by.

Previously I posted a task where I needed to sort a list after the second element onwards, in this article I’ll use the same techniques but apply this to a specific key in each dictionary.

Let’s start with a data set first:

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>>> data = [{ 'Surname': 'Jones', 'Address': '1 Smith Way', '2020-Census': 5, '2016-Census': 3, '2012-Census': 2}, {'Surname': 'Smith', 'Address': '2 Smith Way', '2020-Census': 2, '2016-Census': 2, '2012-Census': 2}, {'Surname': 'Doe', 'Address': '3 Smith Way', '2020-Census': 5, '2016-Census': 4, '2012-Census': 4}]

How would you go about sorting this data if you needed the data list to be sorted by the Dict value represented by the key named Surname?

Thankfully the sorted() function comes in handy here. The parameters for the function are as follows:

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sorted(iterable, key, reverse)

The first parameter iterable takes an iterable object such as a list, or even a dictionary (more on this later). The second parameter key takes a function or key value that instructs how to sort. And finally, the reverse parameter takes a boolean value on whether the sort is in descending order, by default this value is False.

The great thing about the sorted function is that it doesn’t mutate the list passed in at the first parameter. Therefore, we could achieve the desired result of sorting by the key name Surname by simply writing the following:

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>>> sorted_data = sorted(data, key=lambda x:x['Surname'])
>>> print(sorted_data)

[{'Surname': 'Doe', 'Address': '3 Smith Way', '2020-Census': 5, '2016-Census': 4, '2012-Census': 4},
{'Surname': 'Jones', 'Address': '1 Smith Way', '2020-Census': 5, '2016-Census': 3, '2012-Census': 2},
{'Surname': 'Smith', 'Address': '2 Smith Way', '2020-Census': 2, '2016-Census': 2, '2012-Census': 2}]

The function used is a simple one-liner that performs the necessary task and uses a lambda function to get the job done.

What is a lambda function?

A lambda function is a function that simply performs operations without needing to be defined by a name.

In our working example, the lambda function accepts one parameter which we’ve labelled as x and the value being passed in is each Dict item from the List.

As the value is each Dict passed in we can then set the key that will be used to be the ordering condition, which we set as x['Surname'].

Summary

Sorting our list of dictionaries in Python can easily be achieved using the sorted function. By passing in our list as the first parameter, and using a lambda function as the key parameter we can achieve the desired result of getting a sorted list of dictionaries in Python in one line.

Explore my other article on how you can sort a simple dictionary by key or value using Python.