Introduction to Python

Variables, the three basic data types, reading input, and the for loop — everything you need to write your first real programs. Run every example right here; finish with four auto-graded exercises.

The Python language

Python is a modern programming language whose popularity keeps growing.

• its author is Guido van Rossum (he created it in 1989)
• it is used at Google, YouTube, Dropbox, Mozilla, Quora, Facebook, Raspberry Pi, …
• many top universities teach it as a first language: MIT, Carnegie Mellon, Berkeley, Cornell, Caltech, Illinois, …
• it runs on Windows, Linux and Mac. It is freeware and open source.

Unlike many other languages that are compiled (e.g. Pascal, C/C++, C#), Python is an interpreter. This means:

• the interpreter does not produce an executable file (e.g. an .exe on Windows)
• to run a program, Python must be installed on the computer
• the interpreter also allows interactive work with the environment

Key properties of Python: very simple, highly readable syntax (which makes it great for teaching); it is dynamically typed, so there are no type declarations; it supports advanced features of modern languages (data structures, object-oriented design); it is a general-purpose language for data analysis, media processing, networking and more; and it has a huge, welcoming community.

Starting Python

From python.org download the latest version and run the installer. During installation we recommend: install for all users, add Python to PATH, and include pip, tkinter and IDLE. The bundled IDLE environment is ideal for a complete beginner and is what we use in this course.

When you launch IDLE you see the version info and, at the start of the line, the three characters >>> — the prompt. After this prompt we type commands for Python:

>>> 12345
    12345
>>> 123 + 456
    579
>>> 1 * 2 * 3 * 4 * 5 * 6
    720

This interactive window is called a shell, and it runs a REP loop: Read the line, Evaluate it, Print the result — and repeat. Try it yourself below (this editor runs real Python, just like the shell — use print() to see results):

The three basic data types

Python provides several types of data. To start, we meet three: integers, floats and strings. Every type has a name:

• int — integers, e.g. 0, 15, -123456789. Their size is limited only by available memory (millions of digits are fine).
• float — decimal numbers, e.g. 3.14159, 33e50. They contain a decimal point or an exponent, and have limited precision (~16–17 significant digits).
• str — strings, written between apostrophes 'text' or quotes "text".

You can ask Python for the type of any value with the type() function, and each type defines its own operations:

Integer operations (both operands must be integers)

String operations

Repeated concatenation is unusual but handy — 10 * ' :-) ' repeats the smiley ten times. Experiment:

Variables and assignment

So far we only worked with values. To remember a value for later we use a variable. Unlike Pascal or C, where a variable is a named, reserved memory slot, in Python a variable is a name attached to an existing value in memory. A variable is created not by declaration but by running an assignment statement:

premenna = hodnota   # name  =  value

The value on the right is computed first, then the name on the left is bound to it. A name can refer to at most one value, but several names may refer to the same value. The classic example is increasing a variable by referring to itself:

>>> ab = 13
>>> ab = ab + 7   # right side 13+7=20 is computed first, then stored back into ab

Programming mode: print(), input() and conversions

In a saved program (a script, usually a .py file), typing a bare expression does nothing — its value is ignored. To show a value you must use print(), which prints its arguments separated by spaces:

a = 17
b = 'abcd'
print(a)
print(b)
print('the result is', 3 + a * 2)

The input() function prints an optional prompt and then waits for the user to type a line ending with Enter; it returns that line as a string. Watch what happens if we forget that it is a string — a "euro → koruna" converter that multiplies by 25:

amount = input('enter euros: ')
koruna = amount * 25
print(amount, 'euro is', koruna, 'koruna')
# input '100'  →  '100' * 25  →  '100100100100…'  (the string repeated 25×!)

Try the fixed converter — change the input box below to feed it different values:

The for loop

The for loop repeats a block of indented statements. The simplest form runs a fixed number of times using range(n), which generates the integers from 0 to n-1:

for i in range(4):
    print(i, 'row')
# prints: 0 row / 1 row / 2 row / 3 row

The loop variable (i) automatically takes the values 0, 1, 2, …, n-1. The block is everything indented by 4 spaces; the first non-indented line runs after the loop ends. With two arguments, range(5, 15) starts at 5 and stops before 15.

total = 0
for i in range(n):
    total = total + i
print(total)

The loop can also walk over listed values (for x in 5, 7, 11:) or over the characters of a string (a string is a sequence of characters). This computes 6! by multiplying — run it and change the numbers: