Python Basics: Hello World, Data Types, Control Flow, and File I/O

This article provides a step‑by‑step introduction to Python programming for beginners.

Hello World print "Hello World!" Comments start with # and are ignored by the interpreter.

# This is a comment
print "This text will be printed because the print statement is executed."
# This line is a comment and will not run
print "But the comment finished with the end of the line."
print "So the 4th and 5th line of the code are executed again."

Data Types

Numeric: integers and floats.

Text: strings.

Composite: tuples, lists, sets, dictionaries.

Integer Example

rectangle_side_a = 10
rectangle_side_b = 5
rectangle_area = rectangle_side_a * rectangle_side_b
rectangle_perimeter = 2*(rectangle_side_a + rectangle_side_b)
print "Let there be a rectangle with the sides of lengths:", rectangle_side_a, "and", rectangle_side_b, "cm."
print "Then the area of the rectangle is", rectangle_area, "cm squared."
print "The perimeter of the rectangle is", rectangle_perimeter, "cm."

Float Example

pi = 3.14159
circle_radius = 10.2
circle_perimeter = 2 * pi * circle_radius
circle_area = pi * circle_radius * circle_radius
print "Let there be a circle with the radius", circle_radius, "cm."
print "Then the perimeter of the circle is", circle_perimeter, "cm."
print "The area of the circle is", circle_area, "cm squared."

String Example

first_name = "Satoshi"
last_name = "Nakamoto"
full_name = first_name + " " + last_name
print "The inventor of Bitcoin is", full_name, "."

Tuple Example

import math
point_a = (1.2,2.5)
point_b = (5.7,4.8)
segment_length = math.sqrt(
    math.pow(point_a[0] - point_b[0], 2) +
    math.pow(point_a[1] - point_b[1], 2))
print "Let the point A have the coordinates", point_a, "cm."
print "Let the point B have the coordinates", point_b, "cm."
print "Then the length of the line segment AB is", segment_length, "cm."

List Example

some_primes = [2, 3]
some_primes.append(5)
some_primes.append(7)
print "The primes less than 10 are:", some_primes

Set Example

from sets import Set
boys = Set(['Adam', 'Samuel', 'Benjamin'])
girls = Set(['Eva', 'Mary'])
teenagers = Set(['Samuel', 'Benjamin', 'Mary'])
print 'Adam' in boys
print 'Jane' in girls
girls.add('Jane')
print 'Jane' in girls
teenage_girls = teenagers & girls  # intersection
mixed = boys | girls                     # union
non_teenage_girls = girls - teenage_girls # difference
print teenage_girls
print mixed
print non_teenage_girls

Dictionary Example

dictionary_names_heights = {}
dictionary_names_heights['Adam'] = 180.
dictionary_names_heights['Benjamin'] = 187
dictionary_names_heights['Eva'] = 169
print 'The height of Eva is', dictionary_names_heights['Eva'], 'cm.'

Control Flow

x = 10
if x == 10:
    print 'The variable x is equal to 10.'
if x > 20:
    print 'The variable x is greater than 20.'
else:
    print 'The variable x is not greater than 20.'
if x > 10:
    print 'The variable x is greater than 10.'
elif x > 5:
    print 'The variable x is not greater than 10, but greater ' + 'than 5.'
else:
    print 'The variable x is not greater than 5 or 10.'

For‑Loop with range

print "The first 5 positive integers are:"
for i in range(1,6):
    print i

For‑Loop over a list

primes = [2, 3, 5, 7, 11, 13]
print 'The first', len(primes), 'primes are:'
for prime in primes:
    print prime

Break and Continue

for i in range(0,10):
    if i % 2 == 1:
        continue
    print 'The number', i, 'is divisible by 2.'
for j in range(20,100):
    print j
    if j > 22:
        break

Function Definition

def rectangle_perimeter(a, b):
    return 2 * (a + b)
print 'Let a rectangle have its sides 2 and 3 units long.'
print 'Then its perimeter is', rectangle_perimeter(2, 3), 'units.'
print 'Let a rectangle have its sides 4 and 5 units long.'
print 'Then its perimeter is', rectangle_perimeter(4, 5), 'units.'

Command‑Line Arguments

import sys
print 'The number of the arguments given is', len(sys.argv), 'arguments.'
print 'The argument list is ', sys.argv, '.'

File Read/Write

# Write to file "test.txt"
file = open("test.txt","w")
file.write("first line
")
file.write("second line")
file.close()
# Read the file
file = open("test.txt","r")
print file.read()

The article concludes with a copyright notice and a reference to the original source.