Python Basics: A Beginner's Guide to Writing Your First Programs

Key Python terms: interpreter, script, REPL and dynamic typing

• Interpreter: the program, usually called python or python3, that reads your code and runs it.
• Script: a file ending in .py that you run from a terminal with python3 hello.py.
• REPL (Read-Eval-Print Loop): the interactive prompt you get by running python3 with no file. It evaluates each line straight away, which makes it ideal for quick experiments.
• Dynamic typing: you never declare a type. A variable is just a name that refers to a value, and the value carries its own type.

$ python3
>>> 2 + 3
5
>>> name = "Asha"
>>> f"Hello, {name}"
'Hello, Asha'
>>> exit()

The >>> prompt shows that the REPL is waiting for input. The same ideas appear in every language: our guide to PHP and the JavaScript beginner guide cover variables and functions with a different syntax.

How Python stores data: variables, core types and collections

You create a variable by assigning to it with =. Python has a small set of built-in types you will use constantly, and the type() function tells you which one a value has.

count = 10            # int: whole number
price = 249.50        # float: decimal number
city = "Pune"         # str: text
is_member = True      # bool: True or False
discount = None       # None: "no value yet"

print(type(price))    # <class 'float'>
count = "ten"         # allowed: the name now refers to a str

Two collections cover most beginner needs. A list is an ordered sequence you can change, and a dict maps keys to values. Both are explained in depth in our data structures guide.

marks = [72, 88, 95]
marks.append(64)
print(marks[0], len(marks))      # 72 4

student = {"name": "Ravi", "marks": 88}
student["city"] = "Indore"
print(f"{student['name']} from {student['city']} scored {student['marks']}")
print(f"Average: {sum(marks) / len(marks):.2f}")   # Average: 79.75

An f-string is a string prefixed with f; anything inside braces is evaluated and inserted. The :.2f part formats a number to two decimal places. Lines starting with # are comments, which Python ignores.

Making decisions and repeating work: if, for and while

Python marks a block with a colon followed by indented lines, conventionally four spaces. There are no braces: the indentation is the structure, so inconsistent indentation causes an IndentationError.

score = 78

if score >= 90:
    grade = "A"
elif score >= 75:
    grade = "B"
elif score >= 50:
    grade = "C"
else:
    grade = "F"
print(grade)                 # B

for mark in [72, 88, 95]:
    print(mark)

for i in range(1, 6):        # 1, 2, 3, 4, 5 (the end is excluded)
    print(i * i)

attempts = 0
while attempts < 3:
    attempts += 1
    print("Attempt", attempts)

Use for when you are looping over a known collection or range, and while when you keep going until a condition changes. range(start, stop) stops one before stop, which is a frequent source of off-by-one errors.

Functions, user input and modules

A function packages steps under a name. You define it with def, give it parameters, and send a result back with return. A function without a return statement returns None.

import math

def average(numbers):
    """Return the mean of a non-empty list of numbers."""
    return sum(numbers) / len(numbers)

def greet(name, greeting="Namaste"):
    return f"{greeting}, {name}!"

print(average([72, 88, 95]))   # 85.0
print(greet("Meera"))          # Namaste, Meera!
print(math.sqrt(16))           # 4.0

age = int(input("Your age: "))
print(f"Next year you will be {age + 1}")

input() always returns a string, so convert it with int() or float() before doing arithmetic. A module is any .py file; import math loads one from the standard library. Third-party packages are installed with pip, as shown below.

Setting up Python step by step: install, venv and pip

1. Download a current Python 3 release from python.org. On Windows the py launcher is installed with it; on macOS and Linux you can also use your package manager.
2. Open a terminal and check the version with python3 --version (or py --version on Windows).
3. Create a project folder and, inside it, a virtual environment with python3 -m venv .venv. This keeps the project's packages separate from every other project.
4. Activate it: source .venv/bin/activate on macOS and Linux, or .venv\Scripts\activate on Windows. Your prompt shows (.venv) when it is active.
5. Install packages with pip install requests, then record them with pip freeze > requirements.txt so anyone can recreate the setup with pip install -r requirements.txt.
6. Write your code in a file such as main.py using an editor with Python support, and run it with python main.py while the environment is active.

Habits that keep Python code readable

When a program misbehaves, read the traceback from the bottom up: the last line names the error and the lines above show where it happened. Our guide to debugging code walks through this in detail.

Practical exercise: build a tip calculator

Save this as tip.py, run it with python tip.py, and then extend it.

def calculate_tip(bill, percent, people):
    tip = bill * percent / 100
    total = bill + tip
    return tip, total, total / people


def main():
    bill = float(input("Bill amount (Rs): "))
    percent = float(input("Tip percentage: "))
    people = int(input("Number of people: "))
    tip, total, share = calculate_tip(bill, percent, people)
    print(f"Tip: Rs {tip:.2f}")
    print(f"Total: Rs {total:.2f}")
    print(f"Each person pays: Rs {share:.2f}")


if __name__ == "__main__":
    main()

• Done looks like this: entering 1200, 10 and 4 prints a tip of Rs 120.00, a total of Rs 1320.00 and Rs 330.00 per person.
• Extension one: wrap the input in a while loop with try and except ValueError so typing letters asks again instead of crashing.
• Extension two: reject zero people, which would otherwise raise ZeroDivisionError.
• Extension three: turn it into a grade calculator that reads five marks into a list and prints the average and an if/elif grade.

Once this feels comfortable, move on to classes and objects in Python. SkillNewton has no Python course, but the PHP Foundation course teaches the same programming fundamentals in PHP.

Common mistakes

• Mixing tabs and spaces for indentation, which leads to confusing IndentationError or TabError messages.
• Doing arithmetic on the result of input() without converting it, so "5" + 1 raises a TypeError.
• Using = (assignment) where == (comparison) was meant inside an if condition.
• Installing packages globally instead of in a virtual environment, so projects end up needing conflicting versions.
• Naming your own file after a module you import, such as random.py, which hides the standard library module.

Frequently asked questions