Python Built-in Functions: A Hands-On Guide

Python comes with a set of built-in functions that are always available — no imports needed. These functions handle common tasks like getting the length of a list, converting data types, looping over numbers, and working with user input. Mastering them makes your code shorter, cleaner, and more readable.

Table of Contents

• print() — Display Output
• type() — Check Data Type
• len() — Get Length
• input() — Read User Input
• range() — Generate Number Sequences
• sorted() — Return a Sorted List
• zip() — Combine Iterables
• enumerate() — Loop With Index
• map() — Apply a Function to All Items
• filter() — Keep Items That Match a Condition
• Example Program

print() — Display Output

print() writes text or values to the console. It is the most common way to see what your program is doing.

print("Hello, World!")
print(42)
print("The answer is", 42)

You can pass multiple items separated by commas. print() adds a space between them and a newline at the end by default.

name = "Alice"
age = 25
print(name, "is", age, "years old")
# Output: Alice is 25 years old

Change the separator with sep and the ending with end:

print("apple", "banana", "cherry", sep=", ")
# Output: apple, banana, cherry

print("Loading", end="...")
print("Done")
# Output: Loading...Done

type() — Check Data Type

type() returns the type of any value or variable. Use it to inspect what kind of data you are working with.

print(type(42))        # <class 'int'>
print(type(3.14))      # <class 'float'>
print(type("hello"))   # <class 'str'>
print(type([1, 2, 3])) # <class 'list'>
print(type(True))      # <class 'bool'>

This is especially useful when debugging — if your code crashes, type() helps you confirm the actual data type you are dealing with.

value = input("Enter a number: ")
print(type(value))  # <class 'str'> — input() always returns a string

len() — Get Length

len() returns the number of items in a sequence or collection: strings, lists, tuples, dictionaries, sets, etc.

print(len("Python"))       # 6
print(len([10, 20, 30]))   # 3
print(len({"a": 1, "b": 2})) # 2

Common use cases: checking if a list is empty, validating input length, or counting items before looping.

username = input("Enter username: ")
if len(username) < 3:
    print("Username must be at least 3 characters.")

input() — Read User Input

input() pauses the program and waits for the user to type something. It returns whatever the user enters as a string.

name = input("What is your name? ")
print("Hello,", name)

Remember: the return value is always a string. Convert it to a number if you need math.

age = input("How old are you? ")
age = int(age)  # convert string to integer
print("Next year you will be", age + 1)

Shortcut for converting on the spot:

age = int(input("How old are you? "))

range() — Generate Number Sequences

range() produces a sequence of numbers, commonly used in for loops. It is memory-efficient — it does not create a full list in memory.

for i in range(5):
    print(i)
# Output: 0 1 2 3 4

Three forms:

# Count down
for i in range(10, 0, -1):
    print(i)
print("Liftoff!")

range() is often combined with len() to loop over a list by index:

fruits = ["apple", "banana", "cherry"]
for i in range(len(fruits)):
    print(i, fruits[i])

sorted() — Return a Sorted List

sorted() takes any iterable and returns a new sorted list. The original data stays unchanged.

numbers = [4, 2, 9, 1, 5]
sorted_numbers = sorted(numbers)
print(sorted_numbers)  # [1, 2, 4, 5, 9]
print(numbers)         # [4, 2, 9, 1, 5] — unchanged

Works with strings, tuples, and any iterable:

print(sorted("python"))  # ['h', 'n', 'o', 'p', 't', 'y']

Reverse and custom sorting:

print(sorted([4, 2, 9, 1, 5], reverse=True))  # [9, 5, 4, 2, 1]

# Sort by string length
words = ["cat", "elephant", "dog", "butterfly"]
print(sorted(words, key=len))  # ['cat', 'dog', 'elephant', 'butterfly']

The key parameter accepts a function that transforms each item before comparison.

zip() — Combine Iterables

zip() pairs up items from two or more iterables, stopping when the shortest iterable runs out.

names = ["Alice", "Bob", "Charlie"]
scores = [85, 92, 78]

for name, score in zip(names, scores):
    print(name, score)
# Output:
# Alice 85
# Bob 92
# Charlie 78

Common uses: creating dictionaries, iterating over parallel lists, transposing data.

# Build a dictionary from two lists
students = dict(zip(names, scores))
print(students)  # {'Alice': 85, 'Bob': 92, 'Charlie': 78}

# Unzip back
names2, scores2 = zip(*students.items())
print(names2)   # ('Alice', 'Bob', 'Charlie')
print(scores2)  # (85, 92, 78)

enumerate() — Loop With Index

enumerate() adds a counter to an iterable, returning pairs of (index, item). It saves you from manually tracking an index variable.

fruits = ["apple", "banana", "cherry"]

for index, fruit in enumerate(fruits):
    print(index, fruit)
# Output:
# 0 apple
# 1 banana
# 2 cherry

The counter starts at 0 by default. Change it with the start parameter:

for index, fruit in enumerate(fruits, start=1):
    print(index, fruit)
# Output:
# 1 apple
# 2 banana
# 3 cherry

This is cleaner and less error-prone than range(len(...)).

map() — Apply a Function to All Items

map() applies a function to every item in an iterable and returns an iterator with the results.

def square(x):
    return x * x

numbers = [1, 2, 3, 4]
squared = map(square, numbers)
print(list(squared))  # [1, 4, 9, 16]

You can use map() with a lambda function for simple operations:

numbers = [1, 2, 3, 4]
doubled = map(lambda x: x * 2, numbers)
print(list(doubled))  # [2, 4, 6, 8]

Working with multiple iterables: map() accepts extra iterables that are passed as arguments to the function.

a = [1, 2, 3]
b = [4, 5, 6]
result = map(lambda x, y: x + y, a, b)
print(list(result))  # [5, 7, 9]

Note: map() returns an iterator, not a list. Wrap it with list() when you need to store or reuse the result.

filter() — Keep Items That Match a Condition

filter() returns only the items for which a function returns True.

def is_even(n):
    return n % 2 == 0

numbers = [1, 2, 3, 4, 5, 6]
evens = filter(is_even, numbers)
print(list(evens))  # [2, 4, 6]

Using a lambda for the condition:

numbers = [1, 2, 3, 4, 5, 6]
evens = filter(lambda n: n % 2 == 0, numbers)
print(list(evens))  # [2, 4, 6]

filter() also returns an iterator, so wrap it with list() when needed.

Filtering out falsy values (None, 0, empty strings, etc.):

data = [0, "hello", "", None, 42, False]
clean = filter(None, data)
print(list(clean))  # ['hello', 42]

Passing None as the function tells filter() to use the identity function — it keeps values that are truthy.

Example Program

A small program that brings several of these functions together:

# Collect student scores
names = []
scores = []

for i in range(3):
    name = input(f"Enter student {i+1} name: ")
    score = int(input(f"Enter score for {name}: "))
    names.append(name)
    scores.append(score)

# Display results sorted by score (highest first)
print("\n--- Results ---")
sorted_pairs = sorted(zip(names, scores), key=lambda x: x[1], reverse=True)
for rank, (name, score) in enumerate(sorted_pairs, start=1):
    print(f"{rank}. {name} — {score}")

# Show who passed (score >= 50)
passing = list(filter(lambda x: x[1] >= 50, sorted_pairs))
print(f"\nPassing students: {len(passing)}")
for name, score in passing:
    print(f"  {name} ({score})")

# Double all scores for a fantasy leaderboard
print("\n--- Fantasy League (doubled scores) ---")
fantasy = list(map(lambda x: (x[0], x[1] * 2), sorted_pairs))
for name, score in fantasy:
    print(f"{name}: {score}")