Lesson 1 · Python Foundations

Introduction to Python

What Python is, how the interpreter runs your code, and your first program.

Beginner20 min

What you will be able to do

Say what Python is and what it is commonly used for
Explain what the interpreter does with your file
Tell the REPL and a .py file apart, and know when to use each
Write and run your first program
Use print(), write comments, and indent a block correctly
Recognise that keywords are reserved and cannot be used as names

The idea, in plain English

Python is a programming language you write as plain text. Nothing you write is understood by your computer directly - something has to translate it, and in Python that something is called the interpreter. The usual one is CPython, which is what you get from python.org, and it reads your file top to bottom and carries out each line as it goes.

That "as it goes" is the difference people mean when they call Python interpreted rather than compiled. There is no separate build step producing an executable. You save a file, you run it, it does something. The cost is that a mistake on line 40 is not discovered until line 40 actually runs.

Python is used for backend APIs, data analysis, machine learning, automation, and scripting - which is a broad enough list that "what is Python for" has no short answer. What those uses share is that Python optimises for the reader rather than the machine, which is why it is a reasonable first language and still a reasonable twentieth one.

One thing surprises almost everyone coming from another language: in Python, indentation is not formatting. It is syntax. The spaces at the start of a line are how the interpreter knows which lines belong to an if or a loop, so getting them wrong is an error rather than an untidy diff.

Worked example: A three-line program that greets you by name.

The REPL and a .py file

Typing python with no arguments opens the REPL - read, evaluate, print, loop. You type one line, it answers immediately, and nothing is saved. It is the right tool for checking what a function does or what type something is.

A .py file is a program you keep. You write the whole thing, save it, then run it with python myfile.py. Everything past this lesson is a file; the REPL stays useful for the ten seconds when you just want to know whether len("hello") is 5.

Tip: In the REPL, an expression prints its value without print(). In a file it does not. That difference confuses a lot of beginners who test in one place and run in the other.

Syntax and examples

Check what you have

python3 --version    # should print 3.10 or later
python3              # opens the REPL; exit() to leave

Your first program - hello.py

# This is a comment. Python ignores it; it is for people.
print("Hello, Python!")

Run it

python3 hello.py

Three lines that do something

name = "Chandu"
print("Hello,", name)
print("Your name is", len(name), "characters long.")

Indentation is syntax

age = 20

if age >= 18:
    print("Adult")      # indented: belongs to the if
    print("Can vote")   # same block

print("Done")           # not indented: always runs

Finding your way around, from inside Python

x = 42

print(type(x))        # <class 'int'>  - what kind of thing is it
print(id(x))          # a number identifying this exact object
help(print)           # the documentation for print, in your terminal

Watch out: On many systems python means Python 2 or nothing at all, while python3 is the one you want. If python3 --version works and python does not, use python3 everywhere in this course.

Built-in functions used in this lesson

Built-ins are always available - you never import them. These four are the ones worth knowing on day one.

print()

Writes its arguments to the screen, separated by spaces, then a newline.

print("Hello,", name)

type()

Tells you what kind of object something is.

type(42)  # <class 'int'>

id()

The identity of an object - effectively its address. Useful later for understanding when two names point at one object.

id(x)

help()

Prints the documentation for anything you pass it.

help(len)

Keywords you cannot use as names

These 35 words are reserved by the language. Trying to use one as a variable name is a syntax error - which is a good error to have seen once.

Control flow

if, elif, else, for, while, break, continue, pass, match, case

Functions and classes

def, return, lambda, class, yield

Values

True, False, None

Logic

and, or, not, in, is

Errors

try, except, finally, raise, assert

Scope and modules

import, from, as, global, nonlocal, del, with

Async

async, await

Try it yourself

The code does not change. Swap the content string and the program does something else entirely.

Change the output

“print("Hello,", "world", "!")”

Do some arithmetic

“print(2 + 3 * 4)”

Ask what something is

“print(type(3.14))”

Break it on purpose

“print("unclosed string)”

What usually goes wrong

Mixing tabs and spaces

Python treats them as different, so a block that looks aligned can still be an IndentationError. Set your editor to insert 4 spaces for Tab and never think about it again.

Forgetting the colon

Every line that opens a block - if, for, while, def, class - ends in a colon. Leaving it off gives a SyntaxError pointing at the next line, which is why it takes beginners so long to spot.

✗ if age >= 18
    print("Adult")

✓ if age >= 18:
    print("Adult")

Using a keyword as a name

Words like class, from, and is belong to the language. Adding a trailing underscore is the usual convention when you really want that word.

✗ class = "Physics"

✓ class_name = "Physics"

Expecting output the REPL gave you

In the REPL, typing 2 + 2 shows 4. In a file it computes 4 and throws it away. In a file you have to print() it.

✗ # in hello.py
2 + 2

✓ # in hello.py
print(2 + 2)

Best practices

Use 4 spaces per indentation level. It is what the standard style guide says and what every Python editor defaults to.
Write comments that say why, not what. print("Hello") needs no comment; a magic number does.
Name files in lowercase with underscores - user_service.py, not UserService.py.
Keep the REPL open in a second terminal while you work. Checking something there is faster than adding a print and rerunning.
Read the last line of an error message first. It names the error and usually the fix.

Practice

Write these yourself before opening anything. Getting them wrong first is most of how this sticks.

Write a program that prints your name and the current year on two separate lines.

Show hint

Two print() calls is the simplest answer, and simplest is right here.

Show solution

print("Chandu")
print(2026)

Print the result of 7 divided by 2, then the result of 7 divided by 2 using floor division. Note the difference.

Show hint

/ and // are different operators.

Show solution

print(7 / 2)     # 3.5   - true division, always a float
print(7 // 2)    # 3     - floor division, drops the remainder

Use type() to print the kind of each of these: 5, 5.0, "5", True, None.

Show hint

Five print() calls, or one with five arguments.

Show solution

print(type(5))       # <class 'int'>
print(type(5.0))     # <class 'float'>
print(type("5"))     # <class 'str'>
print(type(True))    # <class 'bool'>
print(type(None))    # <class 'NoneType'>

Write an if statement that prints "Even" when a number is divisible by 2 and "Odd" otherwise. Test it with 4 and 7.

Show hint

The modulo operator % gives the remainder. A remainder of 0 means divisible.

Show solution

number = 7

if number % 2 == 0:
    print("Even")
else:
    print("Odd")

Coding challenge

A greeting card

Write a single program that introduces someone, using only what this lesson covered - variables, print(), comments, an if statement, and correct indentation.

It should

Store a name, an age, and a city in three variables
Print a sentence that uses all three
Print how many characters are in the name, using len()
Print "Adult" or "Minor" depending on the age
Include one comment that explains a decision, not a line

Start here

name = ""
age = 0
city = ""

# your code below

Show one solution

One solution

name = "Chandu"
age = 34
city = "Hyderabad"

print("This is", name, "from", city, "- age", age)
print("The name has", len(name), "characters.")

# 18 is the threshold used here; change it if your rules differ.
if age >= 18:
    print("Adult")
else:
    print("Minor")

Key points

Python is interpreted: CPython reads your file top to bottom and runs each line as it reaches it.
The REPL is for quick questions; a .py file is for programs you keep.
print() is how a program shows you something. In a file, an expression on its own produces nothing visible.
Indentation is syntax. Four spaces, consistently, is the convention.
Every line that opens a block ends in a colon.
Comments start with # and exist for people, not the interpreter.
type() answers "what is this", id() answers "which object is this", and help() answers "how do I use this".
35 keywords are reserved and cannot be used as names.

Quick check before you move on

What actually runs your Python file?

The interpreter - usually CPython - reading it line by line.

Why does indentation matter?

It is how Python knows which lines belong to a block. It is syntax, not formatting.

When does a comment run?

Never. Python ignores everything after # on a line.

What does type() tell you?

Which kind of object a value is - int, str, bool, and so on.

Interview questions

What does it mean that Python is dynamically typed?

A variable has no declared type; the type belongs to the value it currently refers to, and the same name can refer to a string now and an integer later. Types are still checked - at the moment an operation runs, not before.

Python is often called both interpreted and compiled. Why?

CPython compiles your source to bytecode first - the .pyc files in __pycache__ - and then interprets that bytecode on a virtual machine. There is a compile step; it just happens automatically and produces bytecode rather than machine code.

Why does Python use indentation instead of braces?

It removes the possibility of code that is indented one way and grouped another, which is a real source of bugs in brace languages. The trade-off is that whitespace becomes significant, so mixing tabs and spaces breaks things.

What is PEP 8 and why does it come up so often?

It is the style guide for Python code - 4-space indents, snake_case names, line length, import order. It matters because a language this readable loses most of that benefit if every file looks different.

Quiz

1.
Is Python compiled or interpreted, and what does that mean in practice?
2.
What is the difference between the REPL and a .py file?
3.
What happens if you indent with three spaces on one line and four on the next inside the same block?
4.
Why does 2 + 2 show 4 in the REPL but nothing when run in a file?
5.
What is CPython?

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Introduction to Python

What you will be able to do

The idea, in plain English

The REPL and a .py file

Syntax and examples

Built-in functions used in this lesson

Keywords you cannot use as names

Try it yourself

What usually goes wrong

Best practices

Practice

A greeting card

Key points

Quick check before you move on

Interview questions

Quiz

Comments

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