Python tutorial for beginners

What Python is and what it is used for

Python is a high-level, dynamically typed language first released by Guido van Rossum in 1991. Code is run by an interpreter (the reference one is CPython), so there is no separate compile step. It is used for data analysis and machine learning (NumPy, pandas, PyTorch), web back ends (Django, Flask, FastAPI), automation scripts, scientific computing and teaching. Python 2 reached end of life in 2020, so always learn Python 3.

Install Python and run Hello, world

Download an installer from python.org (Windows, macOS) or use your package manager on Linux. Check the install with python3 --version (on Windows the command is often py or python). Save this file as hello.py:

print("Hello, world")

python3 hello.py
# Hello, world

Running python3 with no file opens the interactive prompt (REPL), which is useful for trying one line at a time.

Variables and basic types

A variable is created by assigning to it. You do not declare a type; the value carries its type, and type(x) tells you what it is.

name = "Ada"
age = 36
height = 1.65
print(f"{name} is {age} years old")  # Ada is 36 years old
print(7 / 2, 7 // 2, 7 % 2)          # 3.5 3 1
print(int("12") + 1)                 # 13

Control flow: if, for and while

Blocks start with a colon and are defined by indentation (4 spaces by convention). for loops iterate over any sequence; range(n) gives the numbers 0 to n−1.

score = 72
if score >= 90:
    grade = "A"
elif score >= 70:
    grade = "B"
else:
    grade = "C"
print(grade)  # B
for i in range(3):
    print(i)  # 0, then 1, then 2
n = 10
while n > 1:
    n = n // 2
print(n)  # 1

Functions

Define functions with def. Parameters can have default values, and callers can pass arguments by name. A function without return returns None.

def greet(name, greeting="Hello"):
    return f"{greeting}, {name}!"
print(greet("Lin"))                 # Hello, Lin!
print(greet("Lin", greeting="Hi"))  # Hi, Lin!
square = lambda x: x * x
print(square(5))                    # 25

Type hints such as def area(w: float, h: float) -> float: document intent and let tools like mypy check your code, but the interpreter does not enforce them.

Lists, tuples, dicts and sets

nums = [3, 1, 2]
nums.append(5)
nums.sort()
print(nums, nums[0], nums[-1], nums[1:3])  # [1, 2, 3, 5] 1 5 [2, 3]
ages = {"ann": 31, "bob": 25}
ages["cy"] = 40
for person, age in ages.items():
    print(person, age)
print(ages.get("zed", 0))  # 0
squares = [x * x for x in range(5) if x % 2 == 0]
print(squares)  # [0, 4, 16]

The last line is a list comprehension, one of Python's signature features: it builds a list from a loop and an optional filter in one expression. Dict and set comprehensions work the same way.

Signature features: unpacking, with and generators

a, b = 1, 2
a, b = b, a            # swap without a temporary
first, *rest = [1, 2, 3, 4]
print(first, rest)     # 1 [2, 3, 4]
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("line one\n")   # file is closed automatically
def countdown(n):
    while n > 0:
        yield n
        n -= 1
print(list(countdown(3)))  # [3, 2, 1]

The with statement runs setup and cleanup code for you (here, closing the file even if an error happens). A function that uses yield is a generator: it produces values lazily, one at a time, which saves memory on large data.

Error handling with try and except

Errors are raised as exceptions. Catch the specific exception types you expect, and use finally for code that must always run.

def parse_age(text):
    try:
        age = int(text)
    except ValueError:
        print(f"not a number: {text!r}")
        return None
    if age < 0:
        raise ValueError("age cannot be negative")
    return age
print(parse_age("41"))   # 41
print(parse_age("abc"))  # not a number: 'abc', then None

A small complete program: word counter

This script reads a text file given on the command line and prints the five most common words. It uses the standard library only.

import sys
from collections import Counter
def top_words(path, n=5):
    with open(path, encoding="utf-8") as f:
        words = f.read().lower().split()
    cleaned = [w.strip(".,;:!?\"'()") for w in words]
    return Counter(w for w in cleaned if w).most_common(n)
if __name__ == "__main__":
    if len(sys.argv) != 2:
        sys.exit("usage: python3 count.py FILE")
    try:
        for word, count in top_words(sys.argv[1]):
            print(f"{count:5}  {word}")
    except FileNotFoundError:
        sys.exit(f"file not found: {sys.argv[1]}")

Run it with python3 count.py notes.txt. The if __name__ == "__main__": check lets the file be imported as a module without running the script part.

Common beginner mistakes

• Mixing tabs and spaces, or inconsistent indentation, causes IndentationError. Configure your editor to insert 4 spaces.
• = assigns and == compares. Use is only for None (if x is None).
• input() always returns a string; convert with int() or float() before doing math.
• Copying a list with b = a does not copy it; both names point to the same list. Use b = a.copy() or b = list(a).
• Install packages inside a virtual environment (python3 -m venv .venv) instead of into the system Python.

Where to go next

Work through the official Python Tutorial, then browse the Library Reference to learn modules such as pathlib, json, datetime and itertools. Learn pip and virtual environments for third-party packages, and write tests with unittest or pytest. After that, pick a direction: web (Django or FastAPI), data (pandas), or automation.