map() — Transform Every Element!

Returns a map object — wrap it in list() to get a list.

What is map()?

map() applies a function to every element of a list and returns a new list of results.

numbers = [1, 2, 3, 4, 5]

# Old way
doubled = []
for num in numbers:
    doubled.append(num * 2)

# With map()
doubled = list(map(lambda x: x * 2, numbers))
print(doubled)  # [2, 4, 6, 8, 10]

Syntax

map(function, iterable)

Returns a map object — wrap it in list() to get a list.

Basic Examples

Multiplication and Exponentiation

numbers = [1, 2, 3, 4, 5]

doubled = list(map(lambda x: x * 2, numbers))
print(doubled)  # [2, 4, 6, 8, 10]

squares = list(map(lambda x: x ** 2, numbers))
print(squares)  # [1, 4, 9, 16, 25]

Type Conversion

# str → int
strings = ["1", "2", "3", "4", "5"]
numbers = list(map(int, strings))
print(numbers)  # [1, 2, 3, 4, 5]

# int → str
numbers = [10, 20, 30]
strings = list(map(str, numbers))
print(strings)  # ['10', '20', '30']

# float → int (drops the decimal part)
floats = [1.5, 2.7, 3.9]
integers = list(map(int, floats))
print(integers)  # [1, 2, 3]

Strings

words = ["python", "javascript", "go"]

# Uppercase (no lambda — pass the function directly)
upper_words = list(map(str.upper, words))
print(upper_words)  # ['PYTHON', 'JAVASCRIPT', 'GO']

# Word lengths
lengths = list(map(len, words))
print(lengths)  # [6, 10, 2]

# Capitalization
names = ["alice", "bob", "carl"]
capitalized = list(map(str.capitalize, names))
print(capitalized)  # ['Alice', 'Bob', 'Carl']

map() with Multiple Lists

map() can take multiple iterables — the function receives one element from each:

numbers1 = [1, 2, 3]
numbers2 = [10, 20, 30]

sums = list(map(lambda x, y: x + y, numbers1, numbers2))
print(sums)  # [11, 22, 33]

products = list(map(lambda x, y: x * y, numbers1, numbers2))
print(products)  # [10, 40, 90]

If the lists differ in length, map() stops at the shortest:

short = [1, 2]
long  = [10, 20, 30, 40]

result = list(map(lambda x, y: x + y, short, long))
print(result)  # [11, 22]  ← only 2 elements

Practical Examples

Processing Prices

prices = [100, 200, 150, 300]

# 20% discount
discounted = list(map(lambda p: round(p * 0.8, 2), prices))
print(discounted)  # [80.0, 160.0, 120.0, 240.0]

# 10% tax
with_tax = list(map(lambda p: round(p * 1.1, 2), prices))
print(with_tax)  # [110.0, 220.0, 165.0, 330.0]

Working with Dicts

students = [
    {"name": "Alice", "grade": 95},
    {"name": "Bob",   "grade": 87},
    {"name": "Carl",  "grade": 92}
]

# Extract names
names = list(map(lambda s: s["name"], students))
print(names)  # ['Alice', 'Bob', 'Carl']

# Add status
with_status = list(map(
    lambda s: {**s, "status": "Excellent" if s["grade"] >= 90 else "Good"},
    students
))
# [{'name': 'Alice', 'grade': 95, 'status': 'Excellent'}, ...]

map() + filter()

numbers = [1, 2, 3, 4, 5, 6]

# Double the even numbers
result = list(map(lambda x: x * 2, filter(lambda x: x % 2 == 0, numbers)))
print(result)  # [4, 8, 12]

# map + reduce: double → sum
from functools import reduce

doubled = map(lambda x: x * 2, numbers)
total = reduce(lambda acc, x: acc + x, doubled, 0)
print(total)  # 42

map() vs List Comprehension

numbers = [1, 2, 3, 4, 5]

doubled_map  = list(map(lambda x: x * 2, numbers))
doubled_comp = [x * 2 for x in numbers]

print(doubled_map == doubled_comp)  # True

map() is convenient when a ready-made function already exists:

numbers = ["1", "2", "3"]
integers = list(map(int, numbers))  # Cleaner than [int(x) for x in numbers]

Comprehension is better when you need a condition or complex logic:

even_doubled = [x * 2 for x in numbers if x % 2 == 0]

Common Mistakes

Mistake 1: Forgot list()

numbers = [1, 2, 3]

result = map(lambda x: x * 2, numbers)
print(result)  # <map object at 0x...>  — not a list!

# ✅ CORRECT
result = list(map(lambda x: x * 2, numbers))
print(result)  # [2, 4, 6]

Mistake 2: Function Doesn’t Return a Value

def double_bad(x):
    x * 2  # Forgot return!

result = list(map(double_bad, [1, 2, 3]))
print(result)  # [None, None, None]

# ✅ CORRECT
def double_good(x):
    return x * 2

result = list(map(double_good, [1, 2, 3]))
print(result)  # [2, 4, 6]