Step 4 · Ubuntu

Python 3

Python is the fastest way to get a working program. There is no compiler to run and nothing to build — you write a file and run it.

Run this

$ curl -fsSL https://aikaryashala.com/system_setup/scripts/install_py.sh | bash

It installs python3 and writes a sample program into ~/python-samples.

What gets installed

python3The interpreter. It reads your file and does what each line says.

That is the whole list. Stopping a program in the middle to see what it is doing is step 12, and installing libraries is step 8. Neither is needed to write a program and run it.

The sample program

This is the same program as sum.c from step 3, written in Python. You will find it at ~/python-samples/sum.py, and you can also download it.

print("To add two numbers.")

num1 = int(input("Enter the first number: "))
num2 = int(input("Enter the second number: "))

num1 = num1 + num2

print(f"The sum of two numbers is {num1}.")

Go to the folder:

$ cd ~/python-samples

And run it:

$ python3 sum.py
To add two numbers.
Enter the first number: 3
Enter the second number: 4
The sum of two numbers is 7.

Compare that with step 3

In C the same program needed two commands: one to build it, one to run it. Here there is only one, and no new file appeared. Nothing was built.

python3 reads your file and carries out each line as it goes. Chapter 7 of the book explains what that costs and what it buys you.

Write your own

Make a new file with nano:

$ nano hello.py

Type this in, then press Ctrl+O and Enter to save, and Ctrl+X to leave:

name = input("What is your name? ")

print(f"Namasthey, {name}!")

$ python3 hello.py

Edit, run, edit, run, with nothing in between. That short loop is why Python is used so widely for learning, and for getting a first version working.

Try one line at a time

Run python3 with no file and it waits for you to type. This is handy for checking what something does before you put it in a program.

$ python3
>>> 2 + 3
5
>>> "hello".upper()
'HELLO'
>>> numbers = [12, 7, 3, 21]
>>> sum(numbers)
43
>>> len(numbers)
4
>>> exit()

Getting out again

Type exit() and press Enter, or press Ctrl+D. The >>> prompt is Python waiting, not your shell — your normal commands will not work while it is showing.

Check it worked

Which Python you have:

$ python3 --version

That your sample arrived:

$ ls ~/python-samples

And the one that really proves Python works — a program that runs and gives the right answer:

$ cd ~/python-samples && python3 sum.py

Where to go for more

To stop a program in the middle and look at its variables, go to step 12.

To install a library such as requests, go to step 8. Do not use pip against the system Python — Ubuntu depends on it, and modern releases refuse the change anyway.

Step 5 is the Java toolchain.