List Comprehensions in Python (Basic) with Examples

List comprehensions provide a concise way to create lists in Python. They are more readable and often faster than using loops.

Basic Syntax:

python

[expression for item in iterable if condition]

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Example 1: Simple List Comprehension

Create a list of squares from 0 to 9.

Using Loop:

python

squares = []
for x in range(10):
    squares.append(x ** 2)
print(squares)  # Output: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Using List Comprehension:

python

squares = [x ** 2 for x in range(10)]
print(squares)  # Output: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

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Example 2: List Comprehension with Condition

Create a list of even numbers from 0 to 10.

Using Loop:

python

evens = []
for x in range(11):
    if x % 2 == 0:
        evens.append(x)
print(evens)  # Output: [0, 2, 4, 6, 8, 10]

Using List Comprehension:

python

evens = [x for x in range(11) if x % 2 == 0]
print(evens)  # Output: [0, 2, 4, 6, 8, 10]

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Example 3: Nested List Comprehension

Convert a 2D list into a 1D list (flattening).

Given:

python

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Using Loop:

python

flattened = []
for row in matrix:
    for num in row:
        flattened.append(num)
print(flattened)  # Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

Using List Comprehension:

python

flattened = [num for row in matrix for num in row]
print(flattened)  # Output: [1, 2, 3, 4, 5, 6, 7, 8, 9]

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Example 4: List Comprehension with if-else

Convert numbers to “even” or “odd” in a list.

Using Loop:

python

numbers = [1, 2, 3, 4, 5]
result = []
for x in numbers:
    if x % 2 == 0:
        result.append("even")
    else:
        result.append("odd")
print(result)  # Output: ['odd', 'even', 'odd', 'even', 'odd']

Using List Comprehension:

python

numbers = [1, 2, 3, 4, 5]
result = ["even" if x % 2 == 0 else "odd" for x in numbers]
print(result)  # Output: ['odd', 'even', 'odd', 'even', 'odd']

1. Filtering with Multiple Conditions

Get numbers between 10 and 50 that are divisible by 3 or 7.

Using Loop:

python

result = []
for num in range(10, 51):
    if num % 3 == 0 or num % 7 == 0:
        result.append(num)

Using List Comprehension:

python

result = [num for num in range(10, 51) if num % 3 == 0 or num % 7 == 0]
# Output: [12, 14, 15, 18, 21, 24, 27, 28, 30, 33, 35, 36, 39, 42, 45, 48, 49]

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2. Applying Functions to Elements

Convert a list of strings to uppercase and exclude short words (length < 4).

Using Loop:

python

words = ["apple", "cat", "dog", "banana", "kiwi"]
filtered = []
for word in words:
    if len(word) >= 4:
        filtered.append(word.upper())

Using List Comprehension:

python

words = ["apple", "cat", "dog", "banana", "kiwi"]
filtered = [word.upper() for word in words if len(word) >= 4]
# Output: ['APPLE', 'BANANA', 'KIWI']

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3. Creating a List of Tuples (Cartesian Product)

Generate all possible (x, y) pairs where x is from [1, 2, 3] and y is from ['a', 'b'].

Using Loop:

python

pairs = []
for x in [1, 2, 3]:
    for y in ['a', 'b']:
        pairs.append((x, y))

Using List Comprehension:

python

pairs = [(x, y) for x in [1, 2, 3] for y in ['a', 'b']]
# Output: [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'b')]

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4. Flattening a 2D List with Condition

Extract even numbers from a nested list.

Given:

python

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Using Loop:

python

evens = []
for row in matrix:
    for num in row:
        if num % 2 == 0:
            evens.append(num)

Using List Comprehension:

python

evens = [num for row in matrix for num in row if num % 2 == 0]
# Output: [2, 4, 6, 8]

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5. Dictionary Comprehension Inside List Comprehension

Convert a list of names into a list of dictionaries with name and length keys.

Using Loop:

python

names = ["Alice", "Bob", "Charlie"]
result = []
for name in names:
    result.append({"name": name, "length": len(name)})

Using List Comprehension:

python

names = ["Alice", "Bob", "Charlie"]
result = [{"name": name, "length": len(name)} for name in names]
# Output: [{'name': 'Alice', 'length': 5}, {'name': 'Bob', 'length': 3}, {'name': 'Charlie', 'length': 7}]

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6. Using enumerate() in List Comprehension

Get only even-indexed elements from a list.

Using Loop:

python

fruits = ["apple", "banana", "cherry", "date", "elderberry"]
filtered = []
for i, fruit in enumerate(fruits):
    if i % 2 == 0:
        filtered.append(fruit)

Using List Comprehension:

python

fruits = ["apple", "banana", "cherry", "date", "elderberry"]
filtered = [fruit for i, fruit in enumerate(fruits) if i % 2 == 0]
# Output: ['apple', 'cherry', 'elderberry']

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7. Simulating zip() with List Comprehension

Combine two lists element-wise.

Given:

python

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]

Using Loop:

python

combined = []
for name, age in zip(names, ages):
    combined.append(f"{name} is {age} years old")

Using List Comprehension:

python

combined = [f"{name} is {age} years old" for name, age in zip(names, ages)]
# Output: ['Alice is 25 years old', 'Bob is 30 years old', 'Charlie is 35 years old']

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8. Handling Exceptions in List Comprehension

Convert strings to integers safely, ignoring invalid entries.

Using Loop:

python

data = ["10", "20", "thirty", "40"]
numbers = []
for item in data:
    try:
        numbers.append(int(item))
    except ValueError:
        pass

Using List Comprehension (with a helper function):

python

def safe_int(x):
    try:
        return int(x)
    except ValueError:
        return None

data = ["10", "20", "thirty", "40"]
numbers = [safe_int(x) for x in data if safe_int(x) is not None]
# Output: [10, 20, 40]