10 · Comprehensions

Most commonly used

Normal loop

products = ["Copper", "Aluminum", "Nickel"]
upper_products = []

for product in products:
    upper_products.append(product.upper())

List comprehension

upper_products = [product.upper() for product in products]

Both create:

["COPPER", "ALUMINUM", "NICKEL"]

List Comprehension

A list comprehension creates a new list from another iterable.

Basic syntax:

[new_value for item in collection]

Example:

numbers = [1, 2, 3, 4]

doubled = [number * 2 for number in numbers]

Result:

[2, 4, 6, 8]

Transform Values

products = ["Copper", "Aluminum", "Nickel"]

upper_products = [
    product.upper()
    for product in products
]

Result:

["COPPER", "ALUMINUM", "NICKEL"]

Each product is converted to uppercase.

Filter with if

A comprehension can keep only items that match a condition.

Syntax:

[item for item in collection if condition]

Example:

quantities = [5, 10, 20, 3, 50]

large_orders = [
    quantity
    for quantity in quantities
    if quantity >= 10
]

Result:

[10, 20, 50]

Transform + Filter

products = [
    "Copper",
    "Aluminum",
    "Nickel",
    "Copper"
]

result = [
    product.upper()
    for product in products
    if product != "Nickel"
]

Result:

["COPPER", "ALUMINUM", "COPPER"]

if / else in Comprehension

A conditional expression can choose different output values.

Syntax:

[value_if_true if condition else value_if_false
 for item in collection]

Example:

quantities = [5, 20, 8, 50]

sizes = [
    "Large" if quantity >= 10 else "Small"
    for quantity in quantities
]

Result:

["Small", "Large", "Small", "Large"]

Dictionary Comprehension

Creates a dictionary using key-value pairs.

Syntax:

{key: value for item in collection}

Example:

products = ["Copper", "Aluminum", "Nickel"]

product_lengths = {
    product: len(product)
    for product in products
}

Result:

{
    "Copper": 6,
    "Aluminum": 8,
    "Nickel": 6
}

Dictionary Comprehension from Dictionary

prices = {
    "Copper": 8500,
    "Aluminum": 2300,
    "Nickel": 16000
}

high_prices = {
    product: price
    for product, price in prices.items()
    if price > 8000
}

Result:

{
    "Copper": 8500,
    "Nickel": 16000
}

Set Comprehension

Creates a set, automatically keeping unique values.

products = [
    "Copper",
    "Aluminum",
    "Copper",
    "Nickel"
]

unique_products = {
    product
    for product in products
}

Result:

{"Copper", "Aluminum", "Nickel"}

Nested Comprehension

matrix = [
    [1, 2],
    [3, 4]
]

numbers = [
    number
    for row in matrix
    for number in row
]

Result:

[1, 2, 3, 4]

The comprehension reads values from nested lists.

When to Use Comprehensions

Use comprehensions when:

Prefer a normal loop when:

Example:

prices = [price * 1.1 for price in prices]

Less readable:

Do not create long, complex comprehensions with many nested conditions.