Differences between revisions 38 and 155 (spanning 117 versions)
Revision 38 as of 2007-06-09 18:14:58
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Editor: c-68-55-114-201
Comment: another 8 queens, counting lines
Revision 155 as of 2019-11-09 23:29:53
Size: 9662
Comment: Remove Python 2-specific information, leaving a link to previous revision for accessibility
Deletions are marked like this. Additions are marked like this.
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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
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
Line 29: Line 29:
    print 'This generation has %d babies' % babies     print ('This generation has {0} babies'.format(babies))
Line 32: Line 32:

    
------ 5 lines: Functions
----
5 lines: Functions
Line 36: Line 37:
    print 'hello', name     print ('Hello', name)
Line 41: Line 43:

    
------ 6 lines: Import, regular expressions
----
6 lines: Import, regular expressions
Line 45: Line 48:
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')
Line 49: Line 52:
        print test_string, 'rejected'
}}}

    
------ 7 lines: Dictionaries, generator expressions
        print (test_string, 'rejected')
}}}
----
7 lines: Dictionaries, generator expressions
Line 60: Line 64:
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
Line 71: Line 74:
    print 'sum =', total     print ('sum =', total)
Line 73: Line 76:
    print 'Please supply integer arguments'
}}}


    
------ 9 lines: Opening files
    print ('Please supply integer arguments')
}}}
----
9 lines: Opening files
Line 83: Line 86:
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
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
Line 112: Line 128:
bottles_of_beer = 99 bottles_of_beer = 9
Line 114: Line 130:
    print REFRAIN % (bottles_of_beer, bottles_of_beer,
        bottles_of_beer - 1)
    print (REFRAIN % (bottles_of_beer, bottles_of_beer,
        bottles_of_beer - 1))
Line 118: Line 134:

    ------ 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):
Line 163: Line 147:
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
Line 181: Line 201:
for has_chars, frags in itertools.groupby(lines, bool): for has_chars, frags in groupby(lines, bool):
Line 183: Line 203:
        print ' '.join(frags)         print (' '.join(frags))
Line 188: Line 208:

    ------ 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)
Line 195: Line 244:
Line 196: Line 246:
        left, right = left-1, right+1         left, right = left - 1, right + 1
Line 202: Line 253:
    if n == 0: return [[]]
    smaller_solutions = solve(n-1)
    if n == 0:
       
return [[]]

    smaller_solutions = solve(n - 1)
Line 208: Line 262:
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)
Line 239: Line 346:
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))
}}}

----
These all run under Python 3. If you are interested in differences between Python 2 and Python 3, please take a look at the [[https://wiki.python.org/moin/SimplePrograms?action=recall&rev=154|previous version of this page]].
----
CategoryDocumentation

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))


These all run under Python 3. If you are interested in differences between Python 2 and Python 3, please take a look at the previous version of this page.


CategoryDocumentation

SimplePrograms (last edited 2019-11-09 23:29:53 by FrancesHocutt)

Unable to edit the page? See the FrontPage for instructions.