THE ULTIMATE PYTHON TUTORIAL

3 Ways To Create a Dictionary in Python

Make the appropriate choice for different situations

Introduction

The dictionary (or dict in short) is a core data structure in Python. It stores key-value pairs and handles data efficiently. Creating dictionaries is the first step to take advantage of the powerful data structure.

Creating a dictionary looks like a simple task. However, the real data could be very complicated. One method may not be appropriate for all situations.

Fortunately, Python gives us enough flexibility to create dictionaries in different ways under different scenarios. Being familiar with all the approaches is significant for writing elegant and Pythonic code.

This article will dive into the three methods for creating dictionaries and discuss their pros and cons. After reading, it will be easy for you to choose the best method for a specific usage scenario.

1. Create a Dictionary by Curly Brackets

This is an intuitive and basic way to create a dictionary. We just need to put all key-value pairs into a curly bracket and separate them by commas.

Since the dict is a mutable data structure, we don’t need to fill all data in the creation time. If the data of a dict hasn’t been gotten yet, we can create an empty dict and update it later.

This method is simple, but if we get a long list of data, the typing work is too much for this method.

2. Create a Dictionary by the Dict Function

If a list of key-value data is already prepared, the dict function can help us convert it to a dictionary, no more typing is needed.

If the cities list just contains a few names of cities rather than key-value tuples, we can use the dict.fromkeys() method as following:

If there are two lists, one includes names of cities and the other contains names of countries, do we need to type them into a dict one by one?

Not at all. With the help of the zip function, it’s very easy to build a dict based on two lists:

3. Create a Dictionary by Dict Comprehension

Sometimes, we just need some specific items from the original data set to build a dict. In this scenario, of course we can pick and type the items into a dictionary one by one, but it’s not a senior Python developer’s method.

A senior developer will use the dict comprehension, which is a Pythonic technique, to create a dict from other lists.

As the above example shown, with the help of dict comprehension, we can create a dict which only contains the cities in the UK by just one line of code.

The advantage of the dict comprehension is that it gives us more flexibility to filter our data, since it can contain the if statement. The template of the dict comprehension is as following:

D = {key: value for key,value in iterable (if condition)}

There are three components of the above template:

• A for-loop to iterate an iterable
• A key:value style expression to deal with the items
• An optional if condition

For the previous example, if the capitalization of the words is incorrect, we just need to change the dict comprehension a little and get the right result:

Conclusion

There are three major ways to create a dictionary in Python, we should choose the appropriate way for different situations.

My suggestion is:

• Create a dict with a few key-value pairs: use curly brackets
• Convert existed data into a dict: use the dict function
• Extract data from other data sets into a dict: use the dict comprehension

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