Your First Python Program

What is programming?

A computer does exactly what it is told, and nothing else. Programming is writing those instructions in a language the computer can execute. A written program is called source code; the tool that reads and runs it is an interpreter (for Python) or a compiler (for languages like C++).

Why Python?

Python is a high-level, interpreted language designed to read almost like plain English. It is widely used for automation, data analysis, web back-ends and AI, and it is one of the most popular languages for learning algorithms because the code stays short:

• Readable: blocks are defined by indentation, not braces.
• Batteries included: the standard library covers sorting, heaps, big integers, string processing and much more.
• Dynamically typed: you do not declare the type of a variable.
• Big integers by default: no overflow, which removes a whole class of bugs in math problems.

The trade-off is speed: pure Python is slower than C++. The Python for Contests track shows how to stay inside the time limit.

Running a program

Install Python 3 from python.org (or through your system's package manager) and check it works:

$ python3 --version
Python 3.12.3

Create a file hello.py containing:

print("Hello, world!")

and run it:

$ python3 hello.py
Hello, world!

print writes its argument to the standard output, followed by a newline. You can also type python3 alone to open the REPL, an interactive prompt where every line runs immediately. It is a perfectly good calculator:

print(2 + 3 * 4)     # 14: multiplication binds tighter than addition
print((2 + 3) * 4)   # 20
print(7 / 2)         # 3.5 (true division)
print(7 // 2)        # 3   (floor division)
print(7 % 2)         # 1   (remainder)
print(2 ** 10)       # 1024 (power)

Comments

Everything after # on a line is ignored by Python. Use comments to explain why, not what.

# A single-line comment
x = 10  # ...or at the end of a line

"""
A triple-quoted string that is not assigned to anything is
sometimes used as a multi-line comment. Prefer several # lines.
"""

Modules and pip

A module is a file of ready-made code that you can reuse. You load one with import:

import math

print(math.sqrt(144))     # 12.0
print(math.gcd(84, 36))   # 12
print(math.pi)            # 3.141592653589793

There are two kinds of modules:

• Built-in (standard library) modules such as math, random, heapq, collections, itertools. They ship with Python.
• External modules such as numpy or flask, which you install with pip, Python's package manager: pip install numpy.

How PyInfo judges a solution

Every problem on the platform works the same way:

1. Your program is saved as solution.py and run once per test as python3 solution.py < input.txt.
2. It reads the standard input (what you would type on the keyboard) and writes the answer to the standard output with print.
3. The output is compared with the expected one. Leading and trailing blank space of the whole output is ignored; everything else must match exactly, including spaces and letter case.
4. Each test has a time limit. You get a verdict: Accepted, Wrong answer, Runtime error or Time limit exceeded.

A first complete solution therefore looks like this:

name = input()
print("Hello,", name)

input() reads one line. More on reading and converting input in Variables and Data Types.

Exercises

1. Write a program that prints your name on the first line and your favourite number squared on the second.
2. Using the REPL, work out how many minutes there are in a year.
3. Print the greatest common divisor of 1071 and 462 with math.gcd.