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Comment: "generator expression" is the current name, used in the documentation and PEP289
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These Python examples follow the convention that each program gets one line longer than the one before it. Please try to maintain this convention. I also try to introduce at least one new feature in each program. Also, please see note at the bottom. These examples assume version 2.4 or above of Python. {{{ ------ 1 Output print 'hello world' ------ 2 Looping for name in ['peter', 'paul', 'mary']: print name ------ 3 Input, comments # This is a Python comment. \n is a newline name = raw_input('What is your name?\n') print 'Hi', name ------ 4 Fibonacci, tuple assignment parent_rabbits, baby_rabbits = (1, 1) while baby_rabbits < 100: print 'This generation has %d rabbits' % baby_rabbits parent_rabbits, baby_rabbits = (baby_rabbits, parent_rabbits + baby_rabbits) ------ 5 Functions def greet(name): print 'hello', name greet('Jack') greet('Jill') greet('Bob') ------ 6 Import, regular expresssions 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 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 $%.2f' % grocery_bill ------ 8 Command line arguments, exception handling #!/usr/local/bin/python # This program adds up integers in the command line import sys try: total = sum(int(arg) for arg in sys.argv[1:]) print 'sum =', total except ValueError: print 'Please supply integer arguments' ------ 9 Opening files # indent your Python code to put into an email import glob python_files = glob.glob('*.py') python_files.sort() for fn in python_files: print ' ------' for line in open(fn): |
1 line: Output {{{ print 'Hello, world!' }}} ---- 2 lines: Input, assignment {{{ name = raw_input('What is your name?\n') print 'Hi, %s.' % name }}} ---- 3 lines: For loop, built-in enumerate function, new style formatting {{{ friends = ['john', 'pat', 'gary', 'michael'] for i, name in enumerate(friends): print "iteration {iteration} is {name}".format(iteration=i, name=name) }}} ---- 4 lines: Fibonacci, tuple assignment {{{ parents, babies = (1, 1) while babies < 100: print 'This generation has {0} babies'.format(babies) parents, babies = (babies, parents + babies) }}} ---- 5 lines: Functions {{{ def greet(name): print 'Hello', name greet('Jack') greet('Jill') greet('Bob') }}} ---- 6 lines: Import, regular expressions {{{ import re for test_string in ['555-1212', 'ILL-EGAL']: if re.match(r'^\d{3}-\d{4}$', 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 {{{ #!/usr/bin/env python # 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 file_name in sorted(python_files): print ' ------' + file_name with open(file_name) as f: for line in f: |
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print ------ 10 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 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! |
}}} ---- 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 = 99 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(5) print my_account.balance }}} ---- 13 lines: Unit testing with unittest {{{ import unittest def median(pool): copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[(size - 1) / 2] else: return (copy[size/2 - 1] + copy[size/2]) / 2 class TestMedian(unittest.TestCase): def testMedian(self): self.failUnlessEqual(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]) 7 |
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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 List slicing def sieve_of_eratosthenes(candidates): i = 0 while True: divisor = candidates[i] if divisor * divisor > candidates[-1]: return candidates |
copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[(size - 1) / 2] else: return (copy[size/2 - 1] + copy[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 # write stocks data as comma-separated values writer = csv.writer(open('stocks.csv', 'wb', buffering=0)) 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, -1.49) ]) # read stocks data, print status messages stocks = csv.reader(open('stocks.csv', 'rb')) 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 (%s%%)' % (name, status, 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 xrange(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 = numbers.next() yield prime # this code iteratively builds up a chain of # filters...slightly tricky, but ponder it a bit numbers = itertools.ifilter(prime.__rmod__, numbers) for p in iter_primes(): if p > 1000: break print p }}} ---- 21 lines: XML/HTML parsing (using Python 2.5 or third-party library) {{{ 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>''' # In Python 2.5 or 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 |
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i += 1 candidates = candidates[:i] + \ [num for num in candidates[i:] if num % divisor != 0] print sieve_of_eratosthenes(range(2,100)) ------ 13 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 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() }}} 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 contact me through 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! ---- CategoryLanguage CategoryLanguage |
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 = raw_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(raw_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|CategoryDocumentation]] |
1 line: Output
print 'Hello, world!'
2 lines: Input, assignment
name = raw_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 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 = 99 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(5) print my_account.balance
13 lines: Unit testing with unittest
import unittest def median(pool): copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[(size - 1) / 2] else: return (copy[size/2 - 1] + copy[size/2]) / 2 class TestMedian(unittest.TestCase): def testMedian(self): self.failUnlessEqual(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]) 7 ''' copy = sorted(pool) size = len(copy) if size % 2 == 1: return copy[(size - 1) / 2] else: return (copy[size/2 - 1] + copy[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 # write stocks data as comma-separated values writer = csv.writer(open('stocks.csv', 'wb', buffering=0)) 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, -1.49) ]) # read stocks data, print status messages stocks = csv.reader(open('stocks.csv', 'rb')) 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 (%s%%)' % (name, status, 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 xrange(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 = numbers.next() yield prime # this code iteratively builds up a chain of # filters...slightly tricky, but ponder it a bit numbers = itertools.ifilter(prime.__rmod__, numbers) for p in iter_primes(): if p > 1000: break print p
21 lines: XML/HTML parsing (using Python 2.5 or third-party library)
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>''' # In Python 2.5 or 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 = raw_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(raw_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)