Wojciech Jamrozy blog » Python

[Python] Synchronization and threading examples

admin — Mon, 10 Aug 2009 16:08:19 +0000

I want to present my code in Python solving Senate Bus Problem from The Little book of Semaphores (page 211). I’ve used several synchronizing objects such as Lock, Thread, Condition and Semaphore.
Here is link to Python threading library.

#!/usr/bin/env python
# Wojciech Jamrozy
# Script written during Operating Systems course VI 2009
# Problem: The Senate Bus Problem
from time import sleep
from threading import Lock, Thread, Condition, Semaphore
from random import random
import sys
waiting_passengers = 0
mutex = Lock()
bus = Condition()
on_board = Semaphore(0)
 
class Configuration:
        NUMBER_OF_BUSES = 15
        NUMBER_OF_PASSENGERS = 100
        BUS_DELAY = 5
        PASSENGER_DELAY = .8
        TIME_OF_BOARDING = 1
        SEATS = 10
 
 
class Bus(Thread):
    def __init__(self, numer):
        Thread.__init__(self)
        self.numer = numer
    def run(self):
        mutex.acquire()
        print "Bus %d arrived" % (self.numer,)
        global waiting_passengers
        n = min(waiting_passengers, Configuration.SEATS)
        for i in range(n):
            bus.acquire()
            bus.notify()
            bus.release()
            on_board.acquire()
            waiting_passengers -= 1
        sleep(Configuration.TIME_OF_BOARDING*random())
        print "Bus %d departed" % (self.numer,)
        mutex.release()
 
 
 
 
class Passenger(Thread):
    def __init__(self, numer):
        Thread.__init__(self)
        self.numer = numer
    def run(self):
        global waiting_passengers
        print "Passenger %d has come" % (self.numer,)
        mutex.acquire()
        waiting_passengers += 1
        mutex.release()
        bus.acquire()
        bus.wait()
        print "Passenger %d is on board" % (self.numer,)
        bus.release()
        on_board.release()
 
 
class CreateBuses(Thread):
    def run(self):
        for i in range(Configuration.NUMBER_OF_BUSES):
            b = Bus(i)
            b.start()
            sleep(random()*Configuration.BUS_DELAY)
        print "Creating buses finished - nacisnij ENTER aby zakonczyc"
 
class CreatePassengers(Thread):
    def run(self):
        for i in range(Configuration.NUMBER_OF_PASSENGERS):
            o = Passenger(i)
            o.start()
            sleep(random()*Configuration.PASSENGER_DELAY)
        print "Creating passengers finished - press ENTER in order to finish"
 
busy = CreateBuses()
passengers = CreatePassengers()
 
busy.start()
passengers.start()
 
raw_input("Press any key in order to finish\n") 
sys.exit()

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[Python] Django FileField problem with polish/non-english letters

admin — Mon, 27 Jul 2009 12:39:09 +0000

I’ve created a Django model using FileField:

class Car(models.Model):
    ...
    car = models.FileField(upload_to='upl/%Y/%m/%d')
    ...

I wanted to upload files named using polish characters (for example: “auto-żółw”). Unfortunately, Django erease polish letters (“auto-żółw” -> “auto-w”).

I’ve find out that the problem is in django.utils.text.get_valid_filename function.
After I’ve changed line:

return re.sub(r'[^-A-Za-z0-9_.]', '', s)

return re.sub('(?u)[^-.\w]', '', s)

Django accepts polish letters, too.

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[Python,Algorithms] Fast Modular exponentiation script

admin — Sat, 06 Sep 2008 19:26:35 +0000

If we want to know the last ten digits of number aⁿ – we have to evaluate expression aⁿ mod 10¹⁰. Using brute force approach, we have to do O(n) ( If you don’t understand big „O” notation, visit: Big O notation – it’s VERY important) multiplications and modulo divisions. When n is greater than 10⁹, it will take a lot of time. Fortunately, we can use more „mathematical” approach, which has O(log n) complexity.
Firstly, we have to notice, that:
(1) a^2c mod r = (a^c)² mod r
(2) a^2c+1 mod r = a*(a^c)² mod r

#! /usr/bin/env python
# Author: Wojtek Jamrozy (www.wojtekrj.net)
 
def modexp(a, n, m):
	bits = []
	while n:
		bits.append(n%2)
		n /= 2
	solution = 1
	bits.reverse()
	for x in bits:
		solution = (solution*solution)%m
		if x:
			solution = (solution*a)%m
	return solution
 
print modexp(2,34, pow(10,10))
print modexp(2,pow(10,20), pow(10,10))

output:

7179869184
1787109376

Function pow(a,b) is Python’s function, which computes a^b
Function modexp(a, n, m) computes aⁿ mod m. Here is example, which shows running of the function a=2, n=34, m=10¹⁰
Firstly, we want to have bits of binary notation of n in list bits.
Before reversion, bits have following content: [0, 1, 0, 0, 0, 1] (2¹ + 2⁵ = 33)
After reversion of list: bits= [1, 0, 0, 0, 1, 0]
Bits of binary notation of n are in order from “bigger” powers to “lower” ones.
I will use (b) for binary notation of number: 33 = 100001(b)
At the beggining, solution = 2⁰ mod 10¹⁰ = 1. Then, during execution of for loop, solution has following values: 2^1(b) mod 10¹⁰, 2^10(b) mod 10¹⁰, 2^100(b) mod 10¹⁰, 2^1000(b) mod 10¹⁰, 2^10001(b) mod 10¹⁰ and finally 2^100010(b) mod 10¹⁰.
Using (1) and (2) we can calcualte every solution’s value if we know previous one.
Loop for is executed for each element of bits. Bits size is log₂n, so the complexity of whole alghorithm is O(log n)

You can download code here:
Note: There is a file embedded within this post, please visit this post to download the file.

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[Python] Updater/archiver/synchronizer of directories

admin — Wed, 30 Jul 2008 08:29:14 +0000

I have directory with data (music, photos, ebooks etc.) on my two computers. I wanted to have exactly same files in my data directory on both machines, but sometimes I changed content of the directory on one of computers. I’ve decided to write program, which would help me to update data directory on another machine.

Firstly, I’ve installed Samba, a good tool for managing it – gsambad and smbfs (mount and umount commands for the smbfs). You can use NFS instead of samba if you want. Then, I’ve made my “changed” data directory accessible for another computer.
Lets assume, that /data is “unchanged” data directory on second computer, and /datarem is mounted “changed” data directory from first computer. You can mount this directory using for example:

sudo mount -t smbfs -o iocharset=utf8,username="TYPE_SMB_USER",password="TYPE_PASS" //192.168.2.15/data /datarem

Then you can use my script mov.py with following usage:

./mov.py changed_directory directory_to_update

for example:

./mov.py /datarem /data

The script will scan /datarem recursively for files and directories. If corresponding file or directory doesn’t exist in /data, it will be created. If size of file in /datarem is different to a size of corresponding file in /data, the file in /data will be replaced. If sizes are equal, file in /data is keep unchanged (it doesn’t matter for me, but for some usages it may be a problem).

Here is code:

#! /usr/bin/env python
# Created by Wojtek Jamrozy (www.wojtekrj.net)
import os, sys, shutil
 
def update(start, end):
	if os.path.isdir(start):
		for f in os.listdir(start):
			s_ = os.path.join(start, f)
			e_ = os.path.join(end, f)
			if os.path.isdir(s_) and not os.path.exists(e_):
				os.mkdir(e_)
				print "Directory " +e_+" has been created\n"
			update(s_,e_)
	else:
		if os.path.exists(end):
			if os.path.getsize(start) != os.path.getsize(end):
				shutil.copyfile(start,end)
				print "File " +end+" has been updated\n"
		else:
			shutil.copyfile(start,end)
			print "File " +end+" has been created\n"
st = sys.argv[1]
en = sys.argv[2]
if not os.path.exists(st):
	print "Source directory does not exist!"
	sys.exit()
if os.path.isdir(st) and not os.path.exists(en):
	os.mkdir(en)
update(st,en)

You can download it here:
Note: There is a file embedded within this post, please visit this post to download the file.

Most functions used in this script are easy to understand. You can find further information here:
http://docs.python.org/lib/module-os.path.html
http://www.python.org/doc/current/lib/module-shutil.html
http://docs.python.org/lib/module-sys.html

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