Dict Comprehension

# Basic dictionary comprehension
{key_expr: value_expr for variable in iterable}

# Dictionary comprehension with a condition
{key_expr: value_expr for variable in iterable if condition}

# Generate from an existing dictionary
{key: transform(value) for key, value in dictionary.items()}

sample_dict_comprehension.py

# Create a dictionary that stores the square of each number.
squares = {x: x**2 for x in range(1, 6)}
print(squares)  # Outputs: {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# Create a dictionary of only even numbers with a condition.
even_squares = {x: x**2 for x in range(1, 11) if x % 2 == 0}
print(even_squares)  # Outputs: {2: 4, 4: 16, 6: 36, 8: 64, 10: 100}

# Transform the values of an existing dictionary (multiply power levels by 50).
power_levels = {'Son Goku': 9000, 'Vegeta': 8000, 'Krillin': 1500}
super_saiyan = {k: v * 50 for k, v in power_levels.items()}
print(super_saiyan)  # Outputs: {'Son Goku': 450000, 'Vegeta': 400000, 'Krillin': 75000}

# Swap keys and values.
original = {'a': 1, 'b': 2, 'c': 3}
flipped = {v: k for k, v in original.items()}
print(flipped)  # Outputs: {1: 'a', 2: 'b', 3: 'c'}

# Combine two lists with zip() to create a dictionary.
names = ['Son Goku', 'Vegeta', 'Bulma']
scores = [85, 92, 78]
result = {name: score for name, score in zip(names, scores)}
print(result)  # Outputs: {'Son Goku': 85, 'Vegeta': 92, 'Bulma': 78}

# Combine BGM lists with zip() to create a dictionary.
songs = ['Cha-La Head-Cha-La', 'Vegeta no Theme', 'Bulma no Theme']
song_map = {name: song for name, song in zip(names, songs)}
print(song_map)  # Outputs: {'Son Goku': 'Cha-La Head-Cha-La', 'Vegeta': 'Vegeta no Theme', 'Bulma': 'Bulma no Theme'}

# Use a list of strings as keys and their lengths as values.
words = ['python', 'java', 'javascript']
word_lengths = {w: len(w) for w in words}
print(word_lengths)  # Outputs: {'python': 6, 'java': 4, 'javascript': 10}

# Extract a dictionary containing only specific keys.
profile = {'name': 'Son Goku', 'age': 23, 'org': 'Capsule Corporation', 'job': 'Fighter'}
allowed_keys = {'name', 'org'}
filtered = {k: v for k, v in profile.items() if k in allowed_keys}
print(filtered)  # Outputs: {'name': 'Son Goku', 'org': 'Capsule Corporation'}

python3 dict_comprehension.py
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
{2: 4, 4: 16, 6: 36, 8: 64, 10: 100}
{'Son Goku': 450000, 'Vegeta': 400000, 'Krillin': 75000}
{1: 'a', 2: 'b', 3: 'c'}
{'Son Goku': 85, 'Vegeta': 92, 'Bulma': 78}
{'Son Goku': 'Cha-La Head-Cha-La', 'Vegeta': 'Vegeta no Theme', 'Bulma': 'Bulma no Theme'}
{'python': 6, 'java': 4, 'javascript': 10}
{'name': 'Son Goku', 'org': 'Capsule Corporation'}

Dictionary comprehensions are one of Python's powerful features, allowing you to create a dictionary in a single line by combining a loop with an optional condition. Compared to building a dictionary with a regular for loop, this syntax is concise, readable, and considered Pythonic.

Because any expression can be used for both keys and values, dictionary comprehensions are useful in many situations — transforming existing dictionaries, filtering entries, and creating reverse-lookup dictionaries. If duplicate keys may occur, the last one processed takes precedence. Keys must also be hashable objects (such as strings, numbers, or tuples); lists cannot be used as keys.

For copying and merging dictionaries, see 'dict.copy() / dict() / Merging Dictionaries'.