Part 5 — Your First Python Code

The Python Interactive Shell (REPL)

Open your terminal and type:

python

(On Mac/Linux, use python3)

You will see:

>>>

This is the REPL — Read, Evaluate, Print, Loop.

• Read — Python reads what you type
• Evaluate — Python executes it
• Print — Python shows the result
• Loop — Python waits for the next input

The REPL is useful for quick experiments and testing small pieces of code. It is not meant for building applications.

Try this:

>>> 2 + 3
5
>>> "hello"
'hello'

To exit the REPL:

>>> exit()

---

print() — Sending Output to the Screen

print() is the most basic way to send data from your program to the screen.

print("Hello, World")

Output:

Hello, World

You can print numbers, text, or results of calculations:

print(10)
print(2 + 3)
print("Python is powerful")

Printing Multiple Values

print("Score:", 95)

Output:

Score: 95

Python automatically adds a space between values when you separate them with commas.

Printing Different Data Types

print(42)            # Output: 42
print(3.14)          # Output: 3.14
print(True)          # Output: True
print(None)          # Output: None
print([1, 2, 3])     # Output: [1, 2, 3]
print({"a": 1})      # Output: {'a': 1}

Why print() Matters Beyond Tutorials

print() is the simplest debugging tool. In real projects, when something behaves unexpectedly, developers add print() statements to check what values variables hold at different points.

Before learning advanced debugging tools, print() is your first line of investigation.

---

The Full Power of print()

print() has four parameters most people never learn:

print(*objects, sep=' ', end='\n', file=sys.stdout, flush=False)

sep — Change What Goes Between Values

print("Python", "Java", "C++")
# Output: Python Java C++

print("Python", "Java", "C++", sep=", ")
# Output: Python, Java, C++

print("Python", "Java", "C++", sep=" | ")
# Output: Python | Java | C++

print(2026, 3, 17, sep="-")
# Output: 2026-3-17

end — Change What Goes After

By default, print() adds a newline. end changes that.

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

print("One", end=" → ")
print("Two", end=" → ")
print("Three")
# Output: One → Two → Three

file — Print to a File Instead of Screen

with open("log.txt", "w") as f:
    print("App started", file=f)
    print("User logged in", file=f)

Nothing appears in terminal — output goes to log.txt.

flush — Force Output Immediately

Python sometimes holds output in a buffer. flush=True forces it out instantly.

import time

print("Step 1...", flush=True)
time.sleep(1)
print("Step 2...", flush=True)
time.sleep(1)
print("Done!", flush=True)

Each line appears one by one, not all at once.

Other Useful print() Tricks

# Empty line for spacing
print()

# Visual separator
print("=" * 40)
# Output: ========================================

# String repetition
print("Ha" * 5)
# Output: HaHaHaHaHa

# Multi-line output
print("""
==========================
   Welcome to Python!
==========================
""")

repr() — Reveal Hidden Characters

When a string "looks right" but behaves wrong, repr() shows what's really inside.

text = "Hello\tWorld\n"

print(text)
# Output:
# Hello	World
#

print(repr(text))
# Output: 'Hello\tWorld\n'

Tabs (\t) and newlines (\n) become visible. Very useful for debugging.

Quick Reference: All print() Combinations

---

print() Is Your First Debugger

print() is not just for displaying output. It is the first debugging tool every developer uses.

Example: Finding a Bug

You write this:

price = input("Enter price: ")
discount = 20
final = price - (price * discount / 100)
print(f"Final price: {final}")

You enter 500 and get:

TypeError: unsupported operand type(s) for -: 'str' and 'str'

Debug with print()

Add prints to check what you actually have:

price = input("Enter price: ")
print(f"DEBUG: price = {price}")
print(f"DEBUG: type = {type(price)}")

Output:

DEBUG: price = 500
DEBUG: type = <class 'str'>

price is a string, not a number. Fix it:

price = int(input("Enter price: "))

The Debugging Pattern

1. Something breaks or gives wrong output
2. Add print() before the broken line — check values and types
3. Compare what you expected vs what you see
4. Fix the root cause
5. Remove debug prints when done

Even senior developers do this daily. Before you learn advanced debuggers, print() is all you need.

When print() Is Not Enough

We will cover these in later parts. For now, print() is your best friend.

---

input() — Receiving Data from the User

input() pauses the program and waits for the user to type something.

name = input("Enter your name: ")
print("Hello,", name)

When this runs:

Enter your name: OnePercentDev
Hello, OnePercentDev

The text inside input() is called a prompt — it tells the user what to type.

Important: input() Always Returns a String

Everything that comes from input() is a string (text), even if the user types a number.

age = input("Enter your age: ")
print(type(age))

Output:

<class 'str'>

Even if you type 25, Python treats it as the text "25", not the number 25.

Converting Input to a Number

To work with numbers from input, convert using int() or float():

age = int(input("Enter your age: "))
print(type(age))

Output:

<class 'int'>

Now age is an actual number and you can do math with it.

---

The Truth About input() in Real Applications

Do We Use input() in Real Applications?

Honest answer: In web apps and APIs — no.

In real applications, data comes from different sources:

None of these use input().

So Why Learn input()?

Because input() teaches you the concept of receiving external data. The concept is the same everywhere — only the source changes:

# Terminal input
name = input("Enter name: ")

# Web API input
name = request.json["name"]

# Command-line input
import sys
name = sys.argv[1]

# Environment variable input
import os
name = os.environ.get("APP_NAME")

Same idea: data comes in from outside your program. What you learn with input() applies to all of these.

Where input() IS Actually Used

Preview: Other Ways to Receive Input

Command-line arguments — data comes with the command, no pausing:

import sys
# Run as: python greet.py OnePercentDev
name = sys.argv[1]
print(f"Hello, {name}")

Environment variables — how real apps load secrets:

import os
api_key = os.environ.get("API_KEY")
print("Key loaded" if api_key else "Warning: no API key")

---

Your First Interactive Programs

Program 1 — Greeting

name = input("What is your name? ")
print("Welcome to OnePercentDev,", name)

Program 2 — Simple Calculator

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))

print("Sum:", a + b)
print("Difference:", a - b)
print("Product:", a * b)

This program takes two numbers, performs basic math, and shows the results.

---

f-strings — Clean Output Formatting

Python provides a clean way to embed values inside strings using f-strings (formatted string literals).

name = "OnePercentDev"
age = 28
print(f"My name is {name} and I am {age} years old")

Output:

My name is OnePercentDev and I am 28 years old

The f before the quotes tells Python to look for {} placeholders and replace them with actual values.

f-strings are the modern, preferred way to format output in Python. You will use them constantly.

f-string Formatting Tricks

print(f"{1400000000:,}")
# Output: 1,400,000,000

print(f"{0.9534:.1%}")
# Output: 95.3%

print(f"{0.85:.0%}")
# Output: 85%

print(f"{'Python':*^20}")
# Output: *******Python*******

---

The Fundamental Program Model

Every program — from a simple calculator to a complex AI system — follows the same core model:

Input → Process → Output

This model applies everywhere:

• A web API receives a request (input), processes it, and returns a response (output)
• An AI model receives text (input), processes it through the model, and generates a response (output)
• A data pipeline reads raw data (input), cleans and transforms it (process), and stores the result (output)

Every system you will ever build follows this pattern.

---

REPL vs Script File

There are two ways to run Python code:

REPL (Interactive Shell)

• Type python in terminal
• Execute code line by line
• Results appear immediately
• Good for quick experiments and testing

Script File (.py)

• Write code in a file (e.g., main.py)
• Run with python main.py
• Executes the entire file top to bottom
• Used for real programs and projects

When to use which:

In professional work, scripts are the standard. The REPL is a tool for exploration.

---

Comments

Comments are lines that Python ignores. They are notes for humans reading the code.

# This calculates the user's age
birth_year = int(input("Enter your birth year: "))
current_year = 2026
age = current_year - birth_year
print(f"You are {age} years old")

The # symbol marks a comment. Everything after # on that line is ignored by Python.

Use comments to explain why something is done, not what is done. The code itself should show what is happening. Comments should explain the reasoning behind decisions.

---

Fun Things to Try

# Emoji — Python 3 supports Unicode natively
print("Python is fun! 🐍🔥")
print("Status: ✅ Complete")

# Walrus operator (Python 3.8+) — assign and print in one line
print(length := len("OnePercentDev"))
# Output: 13
print(f"The name has {length} characters")
# Output: The name has 13 characters

---

Where This Applies in Real Work

---

Practice Assignments

Assignment 1 — About Me

Build a program in a file called about_me.py that:

1. Asks for the user's name
2. Asks for their birth year
3. Calculates their age (use 2026 as the current year)
4. Prints a formatted message using an f-string:

Hello [name], you are [age] years old. Welcome to your 1% developer journey!

Assignment 2 — Unit Converter

Build a program in a file called converter.py that:

1. Asks for a temperature in Celsius
2. Converts to Fahrenheit: F = C * 9/5 + 32
3. Converts to Kelvin: K = C + 273.15
4. Prints a clean formatted output:

=============================
  Temperature Converter
=============================
  Celsius:    37.0
  Fahrenheit: 98.6
  Kelvin:     310.15
=============================

Use float() for conversion, f-strings with :.1f for formatting, and "=" * 29 for the separator.

Save both in your src/ folder and run with python src/about_me.py and python src/converter.py.

---

Next: Part 6 — How Python actually runs your code. What happens between typing python main.py and seeing the output.