What do people think about renaming this page to be Python By Immersion? That's really what I think was striving for. It's the analogy to spoken languages, of course.

Here are some example simple programs. Please feel free to contribute, but see notice at bottom, please.

These examples assume version 2.4 or above of Python.

1 line: Output

print 'hello world'

2 lines: Looping

for name in ['peter', 'paul', 'mary']:
    print name

3 lines: Input, comments

# This is a Python comment. \n is a newline
name = raw_input('What is your name?\n')
print 'Hi', name

4 lines: Fibonacci, tuple assignment

parents, babies = (1, 1)
while babies < 100:
    print 'This generation has %d babies' % babies
    parents, babies = (babies, parents + babies)

5 lines: Functions

def greet(name):
    print 'hello', name
greet('Jack')
greet('Jill')
greet('Bob')

6 lines: Import, regular expressions

import re
for test_string in [ '555-1212', 'ILL-EGAL']:
    if re.match('^\d\d\d-\d\d\d\d$', test_string):
        print test_string, 'is a valid US local phone number'
    else:
        print test_string, 'rejected'

7 lines: Dictionaries, generator expressions

prices = {'apple': 0.40, 'banana': 0.50}
my_purchase = {
    'apple': 1,
    'banana': 6}
grocery_bill = sum(prices[fruit] * my_purchase[fruit]
                   for fruit in my_purchase)
print 'I owe the grocer $%.2f' % grocery_bill

8 lines: Command line arguments, exception handling

# This program adds up integers in the command line
import sys
try:
    total = sum(int(arg) for arg in sys.argv[1:])
    print 'sum =', total
except ValueError:
    print 'Please supply integer arguments'

9 lines: Opening files

# indent your Python code to put into an email
import glob
# glob supports Unix style pathname extensions
python_files = glob.glob('*.py')
for fn in sorted(python_files):
    print '    ------'
    for line in open(fn):
        print '    ' + line.rstrip()
    print

10 lines: Time, conditionals

import time
now = time.localtime()
hour = now.tm_hour
if hour < 8: print 'sleeping'
elif hour < 9: print 'commuting'
elif hour < 17: print 'working'
elif hour < 18: print 'commuting'
elif hour < 20: print 'eating'
elif hour < 22: print 'resting'
else: print 'sleeping'

11 lines: Triple-quoted strings, while loop

REFRAIN = '''
%d bottles of beer on the wall,
%d bottles of beer,
take one down, pass it around,
%d bottles of beer on the wall!
'''
bottles_of_beer = 99
while bottles_of_beer > 1:
    print REFRAIN % (bottles_of_beer, bottles_of_beer,
        bottles_of_beer - 1)
    bottles_of_beer -= 1

12 lines: List slicing

def sieve_of_eratosthenes(candidates):
    # calculate prime numbers using simple algorithm
    i = 0
    while True:
        divisor = candidates[i]
        if divisor * divisor > candidates[-1]:
            return candidates
        i += 1
        candidates = candidates[:i] + \
            [num for num in candidates[i:]
                if num % divisor != 0]
print sieve_of_eratosthenes(range(2,100))

13 lines: Unit testing

# Let's write reusable code, and unit test it.
def add_money(amounts):
    # do arithmetic in pennies so as not to accumulate float errors
    pennies = sum([round(int(amount * 100)) for amount in amounts])
    return float(pennies / 100.0)
if __name__ == '__main__':
    import unittest
    class TestAddMoney(unittest.TestCase):
        def test_float_errors(self):
            self.failUnlessEqual(add_money([0.13, 0.02]), 0.15)
            self.failUnlessEqual(add_money([100.01, 99.99]), 200)
            self.failUnlessEqual(add_money([0, -13.00, 13.00]), 0)
    unittest.main()

14 lines: Classes

class BankAccount:
    def __init__(self, initial_balance = 0):
        self.balance = initial_balance
    def deposit(self, amount):
        self.balance += amount
    def withdraw(self, amount):
        self.balance -= amount
    def overdrawn(self):
        return self.balance < 0
my_account = BankAccount()
my_account.deposit(15)
my_account.withdraw(5)
print my_account.balance
print my_account.overdrawn()

15 lines: itertools

import itertools
lines = '''
This is the
first paragraph.

This is the second.
'''.splitlines()
# Use itertools.groupby and bool to return groups of
# consecutive lines that either have content or don't.
for has_chars, frags in itertools.groupby(lines, bool):
    if has_chars:
        print ' '.join(frags)
# PRINTS:
# This is the first paragraph.
# This is the second.

18 lines: 8-Queens Problem (recursion)

BOARD_SIZE = 8

def under_attack(col, queens):
    left = right = col
    for r, c in reversed(queens):
        left, right = left-1, right+1
        if c in (left, col, right):
            return True
    return False

def solve(n):
    if n == 0: return [[]]
    smaller_solutions = solve(n-1)
    return [solution+[(n,i+1)]
        for i in range(BOARD_SIZE)
            for solution in smaller_solutions
                if not under_attack(i+1, solution)]
for answer in solve(BOARD_SIZE): print answer

28 lines: 8-Queens Problem (define your own exceptions)

BOARD_SIZE = 8

class BailOut(Exception):
    pass

def validate(queens):
    left = right = col = queens[-1]
    for r in reversed(queens[:-1]):
        left, right = left-1, right+1
        if r in (left, col, right):
            raise BailOut

def add_queen(queens):
    for i in range(BOARD_SIZE):
        test_queens = queens + [i]
        try:
            validate(test_queens)
            if len(test_queens) == BOARD_SIZE:
                return test_queens
            else:
                return add_queen(test_queens)
        except BailOut:
            pass
    raise BailOut

queens = add_queen([])
print queens
print "\n".join( ". "*q + "Q " + ". "*(BOARD_SIZE-q-1) for q in queens )

Hi, I started this page in May 2007, and I provided the first 10+ or so examples (which may have changed since then). -- SteveHowell

All code on this page is open source, of course, with the standard Python license.

Minor cleanups are welcome, but if you want to do major restructuring of this page, please run them by the folks on the Python mailing list, or if you are impatient for a response, please just make your own copy of this page. Thanks, and I hope this code is useful for you!

Some goals for this page:

• 1) All examples should be simple. 2) There should be a gentle progression through Python concepts.

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