What Python is and how it runs

Python is a programming language: a precise way of writing instructions that a computer can carry out. You write those instructions as plain text, and a program called the Python interpreter reads them and does what they say, one statement at a time. Python is popular because the text is short and readable, it comes with a large toolbox, and the same language covers web servers, data analysis, automation and AI.

1. Your first program

print shows something on the screen. Text goes in quotes. Everything after # on a line is a comment: a note for humans that Python ignores.

print("Hello, Python!")        # text in quotes is a string
print(2 + 3)                   # Python can calculate
print("2 + 3 =", 2 + 3)        # print accepts several things, separated by commas

A slightly bigger program already uses the ideas this course is built on: names, a list, a loop, and a decision.

prices = {"tea": 120.0, "rice": 85.5, "milk": 32.0}     # a dictionary: item → price
cart = ["tea", "milk", "milk"]                          # a list of items

total = 0
for item in cart:                                       # repeat for each item in the cart
    total = total + prices[item]

print(f"Items: {len(cart)}, total: ₹{total:.2f}")
if total > 150:
    print("Free delivery!")

Notice the indentation. The lines inside the for and the if are pushed right by four spaces. In Python, indentation is not decoration: it is how you tell Python which lines belong together. Lesson 06 covers it properly.

2. What happens when you run a program

Step 1 is why a missing bracket is reported before a single line runs, while a problem like dividing by zero only appears when Python reaches that line. Here is a grammar mistake. The error message points at the problem with a caret (^):

print("this line would run first")
print("missing a closing bracket"

3. Interpreted, compiled, and where Python sits

"Python" usually means CPython, the standard interpreter from python.org, written in C. Others exist (PyPy, MicroPython for small devices), but everything in this course is standard CPython.

4. Two ways to run Python: the REPL and scripts

>>> 7 * 6
42
>>> name = "Asha"
>>> name.upper()
'ASHA'
>>> exit()

5. Which Python are you running?

import sys
import platform

print("Python", platform.python_version())
print("implementation:", platform.python_implementation())
print("is it at least 3.12?", sys.version_info >= (3, 12))

There is only one Python today: Python 3. (Python 2 reached end of life in 2020, so ignore any tutorial that writes print "hi" without brackets.) A new 3.x version is released every October, and this course needs 3.12 or newer.

6. Python's personality

A short poem ships inside Python itself. It explains the style you will see in this course:

import this

"Readability counts" and "There should be one obvious way to do it" explain a lot of Python's design. When two solutions work, prefer the one a colleague understands at first reading.

7. What people build with Python

Recap