Python Dictionary Comprehensions: How and When to Use Them :

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November 13, 2024
Python Dictionary Comprehensions: How and When to Use Them
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Dictionary comprehensions are a concise and quick way to create, transform, and filter dictionaries in Python. They can significantly enhance your code’s conciseness and readability compared to using regular for loops to process your dictionaries.

Understanding dictionary comprehensions is crucial for you as a Python developer because they’re a Pythonic tool for dictionary manipulation and can be a valuable addition to your programming toolkit.

In this tutorial, you’ll learn how to:

  • Create dictionaries using dictionary comprehensions
  • Transform existing dictionaries with comprehensions
  • Filter key-value pairs from dictionaries using conditionals
  • Decide when to use dictionary comprehensions

To get the most out of this tutorial, you should be familiar with basic Python concepts, such as for loops, iterables, and dictionaries, as well as list comprehensions.

Creating and Transforming Dictionaries in Python

In Python programming, you’ll often need to create, populate, and transform dictionaries. To do this, you can use dictionary literals, the dict() constructor, and for loops. In the following sections, you’ll take a quick look at how to use these tools. You’ll also learn about dictionary comprehensions, which are a powerful way to manipulate dictionaries in Python.

Creating Dictionaries With Literals and dict()

To create new dictionaries, you can use literals. A dictionary literal is a series of key-value pairs enclosed in curly braces. The syntax of a dictionary literal is shown below:

Python Syntax 
{key_1: value_1, key_2: value_2,..., key_N: value_N}

The keys must be hashable objects and are commonly strings. The values can be any Python object, including other dictionaries. Here’s a quick example of a dictionary:

Python 
>>> likes = {"color": "blue", "fruit": "apple", "pet": "dog"}
>>> likes
{'color': 'blue', 'fruit': 'apple', 'pet': 'dog'}

>>> likes["hobby"] = "guitar"
>>> likes
{'color': 'blue', 'fruit': 'apple', 'pet': 'dog', 'hobby': 'guitar'}

In this example, you create dictionary key-value pairs that describe things people often like. The keys and values of your dictionary are string objects. You can add new pairs to the dictionary using the dict[key] = value syntax.

You can also create new dictionaries using the dict() constructor:

Python 
>>> dict(apple=0.40, orange=0.35, banana=0.25)
{'apple': 0.4, 'orange': 0.35, 'banana': 0.25}

In this example, you create a new dictionary using dict() with keyword arguments. In this case, the keys are strings and the values are floating-point numbers. It’s important to note that the dict() constructor is only suitable for those cases where the dictionary keys can be strings that are valid Python identifiers.

Using for Loops to Populate Dictionaries

Sometimes, you need to start with an empty dictionary and populate it with key-value pairs dynamically. To do this, you can use a for loop. For example, say that you want to create a dictionary in which keys are integer numbers and values are powers of 2.

Here’s how you can do this with a for loop:

Python 
>>> powers_of_two = {}

>>> for integer in range(1, 10):
...     powers_of_two[integer] = 2**integer
...

>>> powers_of_two
{1: 2, 2: 4, 3: 8, 4: 16, 5: 32, 6: 64, 7: 128, 8: 256, 9: 512}

In this example, you create an empty dictionary using an empty pair of curly braces. Then, you run a loop over a range of integer numbers from 1 to 9. Inside the loop, you populate the dictionary with the integer numbers as keys and powers of two as values.

The loop in this example is readable and clear. However, you can also use dictionary comprehension to create and populate a dictionary like the one shown above.

Introducing Dictionary Comprehensions

Read the full article at https://realpython.com/python-dictionary-comprehension/ »

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November 13, 2024 at 07:30PM
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