What are dictionary comprehensions in Python?

Dictionary comprehension — is a compact syntax for creating dictionaries based on an iterable object. It allows generating "key: value" pairs using expressions and optional filtering.

Syntax:

# basic syntax
dict = {key_expression: value_expression for item in iterable}

# with condition
dict = {key_expression: value_expression for item in iterable if condition}

Examples:

Creating a dictionary of squares of numbers from 0 to 4:

# dictionary where key is the number, value is its square
squares = {x: x**2 for x in range(5)}
# squares = {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Creating a dictionary only for even numbers:

# dictionary of squares of only even numbers
even_squares = {x: x**2 for x in range(10) if x % 2 == 0}
# even_squares = {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

Swapping keys and values:

original_dict = {'a': 1, 'b': 2, 'c': 3}
# dictionary with keys and values reversed
inverted_dict = {value: key for key, value in original_dict.items()}
# inverted_dict = {1: 'a', 2: 'b', 3: 'c'}

Advantages:

• Conciseness: More compact than equivalent for loops.
• Readability: Often clearer code for simple transformations.
• Efficiency: Can be slightly faster than for loops in some cases.

Disadvantages:

• Readability for complex cases: Can become less readable for very complex logic.
• No side effects: Not intended for actions unrelated to dictionary creation (e.g., printing something).