What is Python? · Lessons

Language vs runtime

Python is both:

A language — rules for writing instructions (syntax).
A runtime — a program that reads those instructions and carries them out.

You write source code (plain text). The runtime turns it into actions: printing text, calculating values, talking to files, and so on.

print("hello")

Check yourself

What does the Python runtime do with your `.py` file?

It only spell-checks the text It executes the instructions according to Python’s rules It uploads the file to the internet automatically It converts Python into Java forever

Indentation is structure

Many languages use braces { } to group statements. Python uses indentation (leading spaces) to show which lines belong together — for example, the body of an if or a loop.

Mixing tabs and spaces, or guessing the indent, causes IndentationError. Pick 4 spaces per level and stay consistent.

for i in range(3):
    print(i)
print("done")

Check yourself

In Python, indentation is only cosmetic and never changes program meaning.

True False

Check yourself

How many times does `print(1)` run in: `for i in range(3): print(1)` ?

Example: 3 · integer