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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 |
Please note that these examples were changed to run under Python 3. The Python2orPython3 page provides advice on how to decide which one will best suit your needs. Some differences from Python 2 to Python 3: {{{ # Python 2 syntax print 'Hello, world!' raw_input('What is your name?\n') self.failUnlessEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7) # Unit testing with unittest copy[(size - 1) / 2] # access of a list index without casting to int xrange(BOARD_SIZE) # xrange exists cmp(1.0, 0.0) # cmp function exists iteratable.next() # list iteration itertools.ifilter(prime.__rmod__, iteratable) # itertools.ifilter is filter in Python 3 # Python 3 syntax print('Hello, World!') input('What is your name?\n') self.assertEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7) # Unit testing with unittest, failUnlessEqual deprecated copy[int((size - 1) / 2)] # access of a list index needs casting to int range(BOARD_SIZE) # xrange was renamed to range in Python 3 #cmp needs to be implemented! def cmp(a, b): return (a > b) - (a < b) next(iteratable) # list iteration filter(prime.__rmod__, iteratable) # is itertools.ifilter in Python 2 }}} The examples below will increase in number of lines of code and difficulty: 1 line: Output {{{ print ('Hello, world!') }}} ---- 2 lines: Input, assignment {{{ name = input('What is your name?\n') print ('Hi, %s.' % name) }}} ---- 3 lines: For loop, built-in enumerate function, new style formatting {{{ friends = ['john', 'pat', 'gary', 'michael'] for i, name in enumerate(friends): print ("iteration {iteration} is {name}".format(iteration=i, name=name)) }}} ---- 4 lines: Fibonacci, tuple assignment |
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print 'This generation has %d babies' % babies | print ('This generation has {0} babies'.format(babies)) |
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------ 5 lines: Functions |
---- 5 lines: Functions |
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print 'hello', name | print ('Hello', name) |
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------ 6 lines: Import, regular expressions |
---- 6 lines: Import, regular expressions |
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for test_string in [ '555-1212', 'ILL-EGAL']: if re.match('^\d\d\d-\d\d\d\d$', test_string): print test_string, 'is a valid US local phone number' |
for test_string in ['555-1212', 'ILL-EGAL']: if re.match(r'^\d{3}-\d{4}$', test_string): print (test_string, 'is a valid US local phone number') |
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print test_string, 'rejected' }}} ------ 7 lines: Dictionaries, generator expressions |
print (test_string, 'rejected') }}} ---- 7 lines: Dictionaries, generator expressions |
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print 'I owe the grocer $%.2f' % grocery_bill }}} ------ 8 lines: Command line arguments, exception handling {{{ #!/usr/local/bin/python # This program adds up integers in the command line |
print ('I owe the grocer $%.2f' % grocery_bill) }}} ---- 8 lines: Command line arguments, exception handling {{{ # This program adds up integers that have been passed as arguments in the command line |
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print 'sum =', total | print ('sum =', total) |
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print 'Please supply integer arguments' }}} ------ 9 lines: Opening files |
print ('Please supply integer arguments') }}} ---- 9 lines: Opening files |
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for fn in sorted(python_files): print ' ------' for line in open(fn): print ' ' + line.rstrip() }}} ------ 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 |
for file_name in sorted(python_files): print (' ------' + file_name) with open(file_name) as f: for line in f: print (' ' + line.rstrip()) print() }}} ---- 10 lines: Time, conditionals, from..import, for..else {{{ from time import localtime activities = {8: 'Sleeping', 9: 'Commuting', 17: 'Working', 18: 'Commuting', 20: 'Eating', 22: 'Resting' } time_now = localtime() hour = time_now.tm_hour for activity_time in sorted(activities.keys()): if hour < activity_time: print (activities[activity_time]) break else: print ('Unknown, AFK or sleeping!') }}} ---- 11 lines: Triple-quoted strings, while loop |
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bottles_of_beer = 99 | bottles_of_beer = 9 |
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print REFRAIN % (bottles_of_beer, bottles_of_beer, bottles_of_beer - 1) |
print (REFRAIN % (bottles_of_beer, bottles_of_beer, bottles_of_beer - 1)) |
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------ 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): |
---- 12 lines: Classes {{{ class BankAccount(object): def __init__(self, initial_balance=0): |
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my_account = BankAccount() my_account.deposit(15) my_account.withdraw(5) print my_account.balance print my_account.overdrawn() }}} ------ 15 lines: itertools {{{ import itertools |
my_account = BankAccount(15) my_account.withdraw(50) print (my_account.balance, my_account.overdrawn()) }}} ---- 13 lines: Unit testing with unittest {{{ import unittest def median(pool): copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[int((size - 1) / 2)] else: return (copy[int(size/2 - 1)] + copy[int(size/2)]) / 2 class TestMedian(unittest.TestCase): def testMedian(self): self.assertEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7) if __name__ == '__main__': unittest.main() }}} ---- 14 lines: Doctest-based testing {{{ def median(pool): '''Statistical median to demonstrate doctest. >>> median([2, 9, 9, 7, 9, 2, 4, 5, 8]) 6 #change to 7 in order to pass the test ''' copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[int((size - 1) / 2)] else: return (copy[int(size/2 - 1)] + copy[int(size/2)]) / 2 if __name__ == '__main__': import doctest doctest.testmod() }}} ---- 15 lines: itertools {{{ from itertools import groupby |
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for has_chars, frags in itertools.groupby(lines, bool): | for has_chars, frags in groupby(lines, bool): |
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print ' '.join(frags) | print (' '.join(frags)) |
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------ 18 lines: 8-Queens Problem (recursion) |
---- 16 lines: csv module, tuple unpacking, cmp() built-in {{{ import csv # need to define cmp function in Python 3 def cmp(a, b): return (a > b) - (a < b) # write stocks data as comma-separated values with open('stocks.csv', 'w', newline='') as stocksFileW: writer = csv.writer(stocksFileW) writer.writerows([ ['GOOG', 'Google, Inc.', 505.24, 0.47, 0.09], ['YHOO', 'Yahoo! Inc.', 27.38, 0.33, 1.22], ['CNET', 'CNET Networks, Inc.', 8.62, -0.13, -1.4901] ]) # read stocks data, print status messages with open('stocks.csv', 'r') as stocksFile: stocks = csv.reader(stocksFile) status_labels = {-1: 'down', 0: 'unchanged', 1: 'up'} for ticker, name, price, change, pct in stocks: status = status_labels[cmp(float(change), 0.0)] print ('%s is %s (%.2f)' % (name, status, float(pct))) }}} ---- 18 lines: 8-Queens Problem (recursion) |
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left, right = left-1, right+1 | left, right = left - 1, right + 1 |
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if n == 0: return [[]] smaller_solutions = solve(n-1) |
if n == 0: return [[]] smaller_solutions = solve(n - 1) |
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for answer in solve(BOARD_SIZE): print answer }}} ------ 28 lines: 8-Queens Problem (define your own exceptions) |
for answer in solve(BOARD_SIZE): print (answer) }}} ---- 20 lines: Prime numbers sieve w/fancy generators {{{ import itertools def iter_primes(): # an iterator of all numbers between 2 and +infinity numbers = itertools.count(2) # generate primes forever while True: # get the first number from the iterator (always a prime) prime = next(numbers) yield prime # this code iteratively builds up a chain of # filters...slightly tricky, but ponder it a bit numbers = filter(prime.__rmod__, numbers) for p in iter_primes(): if p > 1000: break print (p) }}} ---- 21 lines: XML/HTML parsing {{{ dinner_recipe = '''<html><body><table> <tr><th>amt</th><th>unit</th><th>item</th></tr> <tr><td>24</td><td>slices</td><td>baguette</td></tr> <tr><td>2+</td><td>tbsp</td><td>olive oil</td></tr> <tr><td>1</td><td>cup</td><td>tomatoes</td></tr> <tr><td>1</td><td>jar</td><td>pesto</td></tr> </table></body></html>''' # From http://effbot.org/zone/element-index.htm import xml.etree.ElementTree as etree tree = etree.fromstring(dinner_recipe) # For invalid HTML use http://effbot.org/zone/element-soup.htm # import ElementSoup, StringIO # tree = ElementSoup.parse(StringIO.StringIO(dinner_recipe)) pantry = set(['olive oil', 'pesto']) for ingredient in tree.getiterator('tr'): amt, unit, item = ingredient if item.tag == "td" and item.text not in pantry: print ("%s: %s %s" % (item.text, amt.text, unit.text)) }}} ---- 28 lines: 8-Queens Problem (define your own exceptions) |
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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. ---- CategoryLanguage |
print (queens) print ("\n".join(". "*q + "Q " + ". "*(BOARD_SIZE-q-1) for q in queens)) }}} ---- 33 lines: "Guess the Number" Game (edited) from http://inventwithpython.com {{{ import random guesses_made = 0 name = input('Hello! What is your name?\n') number = random.randint(1, 20) print ('Well, {0}, I am thinking of a number between 1 and 20.'.format(name)) while guesses_made < 6: guess = int(input('Take a guess: ')) guesses_made += 1 if guess < number: print ('Your guess is too low.') if guess > number: print ('Your guess is too high.') if guess == number: break if guess == number: print ('Good job, {0}! You guessed my number in {1} guesses!'.format(name, guesses_made)) else: print ('Nope. The number I was thinking of was {0}'.format(number)) }}} ---- CategoryDocumentation |
Please note that these examples were changed to run under Python 3. The Python2orPython3 page provides advice on how to decide which one will best suit your needs.
Some differences from Python 2 to Python 3:
# Python 2 syntax print 'Hello, world!' raw_input('What is your name?\n') self.failUnlessEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7) # Unit testing with unittest copy[(size - 1) / 2] # access of a list index without casting to int xrange(BOARD_SIZE) # xrange exists cmp(1.0, 0.0) # cmp function exists iteratable.next() # list iteration itertools.ifilter(prime.__rmod__, iteratable) # itertools.ifilter is filter in Python 3 # Python 3 syntax print('Hello, World!') input('What is your name?\n') self.assertEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7) # Unit testing with unittest, failUnlessEqual deprecated copy[int((size - 1) / 2)] # access of a list index needs casting to int range(BOARD_SIZE) # xrange was renamed to range in Python 3 #cmp needs to be implemented! def cmp(a, b): return (a > b) - (a < b) next(iteratable) # list iteration filter(prime.__rmod__, iteratable) # is itertools.ifilter in Python 2
The examples below will increase in number of lines of code and difficulty:
1 line: Output
print ('Hello, world!')
2 lines: Input, assignment
name = input('What is your name?\n') print ('Hi, %s.' % name)
3 lines: For loop, built-in enumerate function, new style formatting
friends = ['john', 'pat', 'gary', 'michael'] for i, name in enumerate(friends): print ("iteration {iteration} is {name}".format(iteration=i, name=name))
4 lines: Fibonacci, tuple assignment
parents, babies = (1, 1) while babies < 100: print ('This generation has {0} babies'.format(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(r'^\d{3}-\d{4}$', 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 that have been passed as arguments 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 file_name in sorted(python_files): print (' ------' + file_name) with open(file_name) as f: for line in f: print (' ' + line.rstrip()) print()
10 lines: Time, conditionals, from..import, for..else
from time import localtime activities = {8: 'Sleeping', 9: 'Commuting', 17: 'Working', 18: 'Commuting', 20: 'Eating', 22: 'Resting' } time_now = localtime() hour = time_now.tm_hour for activity_time in sorted(activities.keys()): if hour < activity_time: print (activities[activity_time]) break else: print ('Unknown, AFK or 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 = 9 while bottles_of_beer > 1: print (REFRAIN % (bottles_of_beer, bottles_of_beer, bottles_of_beer - 1)) bottles_of_beer -= 1
12 lines: Classes
class BankAccount(object): 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(15) my_account.withdraw(50) print (my_account.balance, my_account.overdrawn())
13 lines: Unit testing with unittest
import unittest def median(pool): copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[int((size - 1) / 2)] else: return (copy[int(size/2 - 1)] + copy[int(size/2)]) / 2 class TestMedian(unittest.TestCase): def testMedian(self): self.assertEqual(median([2, 9, 9, 7, 9, 2, 4, 5, 8]), 7) if __name__ == '__main__': unittest.main()
14 lines: Doctest-based testing
def median(pool): '''Statistical median to demonstrate doctest. >>> median([2, 9, 9, 7, 9, 2, 4, 5, 8]) 6 #change to 7 in order to pass the test ''' copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[int((size - 1) / 2)] else: return (copy[int(size/2 - 1)] + copy[int(size/2)]) / 2 if __name__ == '__main__': import doctest doctest.testmod()
15 lines: itertools
from itertools import groupby 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 groupby(lines, bool): if has_chars: print (' '.join(frags)) # PRINTS: # This is the first paragraph. # This is the second.
16 lines: csv module, tuple unpacking, cmp() built-in
import csv # need to define cmp function in Python 3 def cmp(a, b): return (a > b) - (a < b) # write stocks data as comma-separated values with open('stocks.csv', 'w', newline='') as stocksFileW: writer = csv.writer(stocksFileW) writer.writerows([ ['GOOG', 'Google, Inc.', 505.24, 0.47, 0.09], ['YHOO', 'Yahoo! Inc.', 27.38, 0.33, 1.22], ['CNET', 'CNET Networks, Inc.', 8.62, -0.13, -1.4901] ]) # read stocks data, print status messages with open('stocks.csv', 'r') as stocksFile: stocks = csv.reader(stocksFile) status_labels = {-1: 'down', 0: 'unchanged', 1: 'up'} for ticker, name, price, change, pct in stocks: status = status_labels[cmp(float(change), 0.0)] print ('%s is %s (%.2f)' % (name, status, float(pct)))
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)
20 lines: Prime numbers sieve w/fancy generators
import itertools def iter_primes(): # an iterator of all numbers between 2 and +infinity numbers = itertools.count(2) # generate primes forever while True: # get the first number from the iterator (always a prime) prime = next(numbers) yield prime # this code iteratively builds up a chain of # filters...slightly tricky, but ponder it a bit numbers = filter(prime.__rmod__, numbers) for p in iter_primes(): if p > 1000: break print (p)
21 lines: XML/HTML parsing
dinner_recipe = '''<html><body><table> <tr><th>amt</th><th>unit</th><th>item</th></tr> <tr><td>24</td><td>slices</td><td>baguette</td></tr> <tr><td>2+</td><td>tbsp</td><td>olive oil</td></tr> <tr><td>1</td><td>cup</td><td>tomatoes</td></tr> <tr><td>1</td><td>jar</td><td>pesto</td></tr> </table></body></html>''' # From http://effbot.org/zone/element-index.htm import xml.etree.ElementTree as etree tree = etree.fromstring(dinner_recipe) # For invalid HTML use http://effbot.org/zone/element-soup.htm # import ElementSoup, StringIO # tree = ElementSoup.parse(StringIO.StringIO(dinner_recipe)) pantry = set(['olive oil', 'pesto']) for ingredient in tree.getiterator('tr'): amt, unit, item = ingredient if item.tag == "td" and item.text not in pantry: print ("%s: %s %s" % (item.text, amt.text, unit.text))
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))
33 lines: "Guess the Number" Game (edited) from http://inventwithpython.com
import random guesses_made = 0 name = input('Hello! What is your name?\n') number = random.randint(1, 20) print ('Well, {0}, I am thinking of a number between 1 and 20.'.format(name)) while guesses_made < 6: guess = int(input('Take a guess: ')) guesses_made += 1 if guess < number: print ('Your guess is too low.') if guess > number: print ('Your guess is too high.') if guess == number: break if guess == number: print ('Good job, {0}! You guessed my number in {1} guesses!'.format(name, guesses_made)) else: print ('Nope. The number I was thinking of was {0}'.format(number))