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frequently ask question python
Creation: 30 aout 2009
Mise à jour: 26 septembre 2009
Version: 1.1
Author: Jean-Louis Bicquelet-Salaün
Location: http://jlbicquelet.free.fr
Copyright: (c) 2009 Jean-Louis BICQUELET-SALAÜN
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frequently ask question python
Creation: 30 aout 2009
Mise à jour: 26 septembre 2009
Version: 1.1
Author: Jean-Louis Bicquelet-Salaün
Location: http://jlbicquelet.free.fr
Copyright: (c) 2009 Jean-Louis BICQUELET-SALAÜN
Python Frequently-Asked Questions
I have been using python since 2000. And I am still surprises by the power of this language. That's the reason why, I decided to open a Python FAQ, containing some tips I have found.
For more information about this faq, please do contact Jean-Louis BICQUELET
FAQ Revised: Monday 28 September 2009 14:40:49
Table of Contents
- 1. syntax
- 2. snippets
- 3. data
- 4. lists
- 5. strings
- 6. regular expression
- 7. files
- 8. system
- 9. sound
- 10. threads
- 11. net
1. syntax
- 1.1. How can I suppress the Non-ASCII character error message ?
When the python script starts you will get the following message :
sys:1: DeprecationWarning: Non-ASCII character '\xe0' in file D:\jlb_2008\pytho \regexp\exemple.py on line 15, but no encoding declared; see http://www.python. rg/peps/pep-0263.html for details
If you want to suppress this message, just add at the beginning of your script this line:
# -*- coding: latin-1 -*-
For more information please read python site.
2. snippets
- 2.1. How can I throw a 6 faces dice ?
from random import * print randint(1,6)
- 2.2. How can I use an output stream in a c++ way ?
- You have to create a class stream. Then you overload the << operator.
The name of the associated function is __lshift__.
You have to redefine constants cout, cerr and nl and you can use it.
import sys class ostream: def __init__(self, file): self.file = file def __lshift__(self, obj): self.file.write(str(obj)); return self cout = ostream(sys.stdout) cerr = ostream(sys.stderr) nl = '\n' x=20 cout << x << nl - 2.3. How can I display the time and the date?
- One can use datetime
import locale import datetime locale.setlocale(locale.LC_ALL, 'fr_FR') now = datetime.datetime.now() now.strftime("%A %d %B %Y, %H:%M")display:
'lundi 28 septembre 2009, 07:12'
You can also use the time module.
from time import strftime strftime(" %H:%M:%S %d/%m/%Y")display:
'07:33:09 26/09/2009'
3. data
- 3.1. How can I create a stack in python ?
- It's really easy. You just have to define a list and use the append() function to push data in the stack,
and the pop() function to get data out of the stack.
stack=[] stack.append(5) stack.append(9) print stack.pop()+stack.pop() 14 print stack []
- 3.2. How can I create a complex nimber class ?
- It is useless! Python knows how to manage complex numbers.
print (2+1j)*2 (4+2j) print (2+1j)+(3+3j) (5+4j) a=2+4j print a.real 2.0 print a.imag 4.0
- 3.3. How can I define set of items ?
- You have to use the keyword set with an objects list. Operators are:
- in the first but not in the second | union & intersection ^ in one set but not in both sets a=set(['chat','chien','poisson']) b=set(['elephant','girafe']) print a - b set(['chien', 'chat', 'poisson']) print a | b set(['chien', 'girafe', 'chat', 'poisson', 'elephant']) print a & b set([]) print a ^ b set(['chien', 'chat', 'poisson', 'girafe', 'elephant'])
4. lists
- 4.1. How can I manipulate a list?
>>>lines=["ligne1","ligne2"] >>>print lines ['ligne1', 'ligne2'] >>> lines=["ligne1","ligne2"] >>> for i in lines: ... print i ligne1 ligne2
- 4.2. How can I know the size of a list?
>>> print len(lines) 2
- 4.3. How can I loop into a list by indexes ?
- HYou just have to combine the xrange and len functions.
For example, if you want to loop from the first element until the one before the last one you will type:
>>> for i in xrange(len(lines)-1): ... print lines[i] ... ligne1
- 4.4. How can I append an item to the end of the list?
- Use the function append.
>>> lines.append("-------") >>> for i in xrange(len(lines)): ... print lines[i] ... ligne1 ligne2 -------
5. strings
- 5.1. How can replace multiples expressions ?
- Use a list that will have an enumeration of the string to replace and the new value. Then make a loop.
from string import repl_car=(('à','à'),('À','À'),('â','â'),('Â','Â'),('ä','ä'),('ã','ã'), ('é','é'),('É','É'),('è','è'),('È','È'),('ê','ê'),('Ê','Ê'),('ë','ë') ); m="Liste à modifier." for i in repl_car: [val,by]=i m=replace(m,val,by) - 5.2. how can I transcode characters (like the command tr) ?
# -*- coding: iso-8859-1 -*- import string french = "je n'aime pas les accents é,è,à et je veux les faire disparaître." char_from = "éêàîêâ" char_to = "eeaiea" print french.traslate(string.maketrans(char_from,char_to))
- 5.3. How can I suppress html tags ?
Create a class that uses sgmllib module that way:
import sgmllib class Stripper(sgmllib.SGMLParser): def __init__(self): sgmllib.SGMLParser.__init__(self) def strip(self, some_html): self.theString = "" self.feed(some_html) self.close() return self.theString def handle_data(self, data): self.theString += data stripper = Stripper() print stripper.strip("""titre
Le retrait des tag est possible!
""")
6. regular expression
- 6.1. How can I seek a regular expression and to substitute it?
One wishes to change all the texts between * and * by boldfacing HTML metatags and all those between _ and _ by italic HTML . One builds a regular expression scanning for a word (\w+) between two *. For a * one must use a backslash because it is a character used to define the regular expressions. The method sub is built to carry out substitution. In the case of an underscore (_), you don' need a backslash.
import re p = re.compile(r'\*(\w+)\*') m = p.sub(r'\1',"le texte est *important* lorsqu'il est en *gras* ou en _italique_.") p = re.compile(r'_(\w+)_') m = p.sub(r'\1',m) print m
le texte est important lorsqu'il est en gras ou en italique.
- 6.2. How can I seek forr a sentence ?
- One wishes to seek an expression of the type ...[mot1 mot2]...............
Found words will be affected to variables to be re-used thereafter.
. have to be backslashed. Word are detected by the match method. The group method returns founded elements.
import re m='...[test ok]............' p = re.compile(r'\.\.\.\[(.*) (.*)]\.\.\.(.*)') r=p.match(m) if r: arg1=r.group(1) arg2=r.group(2) - 6.3. How can I suppress html tags in a text?
import re def remove_tags(s): return re.compile('<[^>]+>',re.S|re.I).sub('',s) s="""texte
<h1>texte</h1>
Un vrai texte <b>html</b> avec des <i\>effets\</i> de style. Et il n'en reste rien! """ print remove_tags(s)
7. files
- 7.1. How can I read a text file ?
- There are 3 ways to read a text file in python:
- reading all lines once:
f = open('myfile.txt') lines = f.readlines() f.close() for line in lines: print line, - reading file line by line. You can then read big files :
f = open('myfile.txt') line = f.readline() while line: print line, line = f.readline() f.close() - reading using an iterator:
f = open('myfile.txt') for line in iter(f): print line, f.close()
- reading all lines once:
8. system
- 8.1. How can I list the files of a directory ?
import os rep='.' print files
result:
['file.py', 'fileutil.py','fileutil.pyc', 'MEF.PL', 'mef2html.py', 'resub.py', 'test.html', 'test.mef', 'util.py']
If you want to format the output :
for i in files: print i file.py fileutil.py fileutil.pyc MEF.PL mef2html.py resub.py test.html test.mef util.py
- 8.2. How can I get the resultat of a system command ?
- You have to use popen3 after having imported popen2 module.
import popen2 (stdout, stdin, stderr) = popen2.popen3("dir c:") print stdout.readlines() [" Le volume dans le lecteur C s'appelle SYSTEM\n", ' Le num\x82ro de s\x82rie d u volume est 7CAB-1080\n', '\n', ' R\x82pertoire de C:\\\n', '\n', '16/01/2008 13:47Admin\n', '16/01/2009 13:08 Applicati on Data\n', '04/04/2008 10:17 40 AUTOEXEC.BAT\n', '16/01/2008 1 3:26 BMC\n', '05/11/2007 14:21 0 CONFIG.SYS\n ', '25/09/2009 10:13 cygnus\n', '09/07/2009 12:11 cygwin\n', '09/07/2009 14:05 cygwin_neo\n', '24/08/20 09 13:54 95 DEBUG.TXT\n', '16/01/2008 15:46 D ocuments and Settings\n', '17/04/2009 17:41 DownloadDirector\ n', '07/05/2008 16:34 Downloads\n', '14/01/2008 11:54 DRIVERS\n', '18/07/2008 15:05 103 DSHELTMP.BAT\n', '0 ... - 8.3. How can I list environment variables ?
- environment variables are in the os.environ dictionnary.
import os for param in os.environ.keys(): print "%20s %s" % (param,os.environ[param])sous Windows cela donne
GROUPBUR C:\Documents and Settings\Default User SYSTEMROOT C:\WINNT COMSPEC C:\WINNT\system32\cmd.exe TEMP C:\DOCUME~1\WBICQ\LOCALS~1\Temp PROCESSOR_ARCHITECTURE x86 ENVDON TR SIT_NOMHOTE techno NWUSERNAME WBICQ ALLUSERSPROFILE C:\Documents and Settings\All Users ENVIR WINNT SESSIONNAME Console HOMEPATH \ NWLANGUAGE English USER_NAME WBICQ USERNAME WBICQ SIT_NUMPORT 2000 MACHINE %login_name OS_VERSION V5.01 PROMPT $P$G LOGONSERVER \\C920001074 PATHEXT .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH LOCAL c:\usr\local FP_NO_HOST_CHECK NO WINDIR C:\WINNT APPDATA C:\Documents and Settings\WBICQ\Application Data HOMEDRIVE C: TYPP S SYSTEMDRIVE C: NOM WBICQ NUMBER_OF_PROCESSORS 2 STATION 001D091A1100 OS WINNT PROGRAMFILES C:\Program Files
9. sound
- 9.1. How can I play a soud under windows ?
import winsound winsound.PlaySound("c:\winnt\media\tada.wav",winsound.SND_FILENAME)
10. threads
- 10.1. How can I launch multiple threads in python ?
- Import the module thread. Use the method start_new with a pointer on the function and its parameters.
Par exemple pour Lancer 3 compteurs le code est:
import thread def counter(myid, count): for i in range(count): print '[%s] => %s' % (myid, i) for i in range(3): print thread.start_new(counter, (i, 30)) import time time.sleep(3) print 'exit Main thread'Un autre exemple:
import thread, time def counter(myid, count): for i in range(count): time.sleep(1) print 'thread number %d reporting in at %d...' % (myid, time.clock()) print time.clock() thread.start_new(counter, (1, 3)) thread.start_new(counter, (2, 8)) time.sleep(10) print 'Exit main thread'
11. net
- 11.1. How can I get my IP adress?
import socket print socket.gethostbyname(socket.gethostname())
display:
192.168.60.4
Copyright (c) 2001-2009 Jean-Louis BICQUELET
This list of questions and answers was generated by makefaq.