How to fix VariableDoesNotExist exception in Django

Posted on November 10, 2010 by Daniel Hepper

When Django greets you with this exception:

TemplateSyntaxError at /

Caught VariableDoesNotExist while rendering: Failed lookup for key [request] in...

you are probably using a template tag that requires the request context processor. Simply add ‘django.core.context_processors.request’ to the TEMPLATE_CONTEXT_PROCESSORS tuple in your settings file.

Howto clear Solr index

Posted on November 1, 2010 by Daniel Hepper

If you want to delete all items from Solr index, use the query

<delete><query>*:*</query></delete>

as described in the FAQ on the Solr Wiki.

But how do you actually send this query? You can use this command:

curl http://<host>:<port>/<solr_base>/update?commit=true -d '<delete><query>*:*</query></delete>'

Just replace the placeholders with the right values, e.g.

curl http://localhost:8983/solr/update?commit=true -d  '<delete><query>*:*</query></delete>'

If Solr responds with something like this:

HTTP Status 400 - missing content stream
Status report
message missing content stream
description The request sent by the client was syntactically incorrect (missing content stream).

then you are probably missing the commit parameter i.e. ?commit=true

How to solve the 36 Cube puzzle – hints & solution

Posted on January 10, 2010 by Daniel Hepper

For Christmas, I got the ThinkFun 36 Cube. It is consists of 36 towers in 6 colors and 6 different sizes and a base plate with 6 by 6 slots to plug in the towers. These slots are of different heights. The goal is to place one towers of every color in each row and column. And the towers must fit to form a level cube.

After some tries, I came to the conclusion that this puzzle is the work of the devil and that I should not waste more brain cycles on solving it. So I wrote a little python script to solve the puzzle for me.

Show sourcecode

My program quickly came up with a correct placement for 34 towers – but it failed to find the complete solution.

[('P', 5), ('Y', 3), ('O', 2), ('B', 1), ('R', 4), ('G', 6)]
[('Y', 4), ('O', 1), ('P', 6), ('R', 2), ('G', 5), ('B', 3)]
[('O', 6), ('B', 5), ('R', 3), ('G', 4), ('P', 1), ('Y', 2)]
[('R', 1), ('G', 2), ('Y', 5), ('P', 3), ('B', 6), ('O', 4)]
[('B', 2), ('P', 4), ('G', 1), ('Y', 6), ('O', 3), ('R', 5)]
[('G', 3), ('R', 6), ('B', 4), ('O', 5), ('X', 2), ('X', 1)]

Legend:
P = Purple, Y = Yellow, O = Orange, B = Blue, R = Red, G = Green, X = Empty
The number is the size of the tower.
As you can see, I didn’t waste much time on making the output pretty

So close and yet so far

After spending lots of time verifying that my program was working correctly, I became impatient and googled for help. I found an answer, but it revealed to much, taking all the fun.

Therefore, I split my solution into multiple hints. If you are stuck, reveal just one of them at a time and try to figure it out by yourself. It is way more rewarding!

Hint #1 (show):

Hint #2 (show):

Hint #3 (show):

Hint #4 (show):

Hint #5 (show):

Even if you uncovered all hints, the puzzle is still far from solved. You can still tinker with it forever.

Spoiler alert: Don’t uncover the solution, unless your are really desperate!

Click to show the 36cube solution.

After I had found the two culprits, I adjusted my program by changing the array and inserting two special conditions. Within seconds I had a complete solution of the 36 Cube:

[('P', 5), ('Y', 3), ('B', 2), ('O', 1), ('R', 4), ('G', 6)]
[('B', 4), ('P', 1), ('Y', 5), ('G', 2), ('O', 5), ('R', 3)]
[('R', 6), ('G', 5), ('P', 3), ('Y', 4), ('B', 1), ('O', 2)]
[('G', 1), ('R', 2), ('O', 6), ('B', 3), ('Y', 6), ('P', 4)]
[('Y', 2), ('O', 4), ('R', 1), ('P', 6), ('G', 3), ('B', 5)]
[('O', 3), ('B', 6), ('G', 4), ('R', 5), ('P', 2), ('Y', 1)]

Finally solved

#!/usr/bin/python
 
COLORS = dict(zip((1 &lt;&lt; i for i in range(6)), ('P','Y','O','B','R','G')))
COLORS[False] = 'X'
 
used_towers = [0 for i in range(6)]
 
# definition with (1,2) and (3,2) switched
cube = ((1,3,4,5,2,0),
        (2,5,1,4,1,3),
        (0,1,3,2,5,4),
        (5,4,0,3,0,2),
        (4,2,5,0,3,1),
        (3,0,2,1,4,5))
 
solution = [[False for x in range(6)] for x in range(6)]
used = tuple(set() for i in range(6))
 
def test_color(pos, solution, used_towers, color, size):
 #check if tower of this size and color has already been used
 if color &amp; used_towers[size]:
  return False
 #check if tower has already been used in row or column:
 for i in range(6):
  if color == solution[pos/6][i]: return False
  if color == solution[i][pos%6]: return False
 # special conditions for the two special towers
 if (pos/6, pos%6) == (1,2) and color != 2: return False
 if (pos/6, pos%6) == (3,2) and color != 4: return False
 return True
 
def print_solution(solution):
 for i in range(6):
  print zip((COLORS[c] for c in solution[i]), (6 - s for s in cube[i]))
 
def solve(pos, solution, used_towers, best):
 if pos == 36: return best, solution
 size = cube[pos/6][pos%6]
 for i in range(6):
  color = 1 &lt;&lt; i
  if test_color(pos, solution, used_towers, color, size):
   used_towers[size] = used_towers[size] | color
   solution[pos/6][pos%6] = color
   if pos &gt; best:
    print "#######", pos + 1, "towers placed"
    print_solution(solution)
    best = pos
   best, solution = solve(pos+1, solution, used_towers, best)
   if best &gt;= 35:
    return best, solution
   solution[pos/6][pos%6] = False
   used_towers[size] = used_towers[size] ^ color
 return best, solution
 
solve(0, solution, used_towers, 0)

Automatic plugin update problem in WordPress 2.8.x

Posted on September 26, 2009 by Daniel Hepper

After upgrading WordPress to the current version, I could no longer automatically update plugins. This is the error message I got:

Downloading update from http://downloads.wordpress.org/plugin/xxx.zip.

Unpacking the update.
Warning: unlink(/…/wp-content/upgrade/xxx.zip) [function.unlink]: No such file or directory in /…/wp-admin/includes/class-wp-upgrader.php on line 146

Incompatible archive PCLZIP_ERR_MISSING_FILE (-4) : Missing archive file ‘/…/wp-content/upgrade/wp-syntax.0.9.8.zip’

After digging through the forums I found a solution to the plugin autoupdate problem in the WordPress Support forum.

Open the file wp-config.php in the root directory of your WordPress installation and remove the definition of the WP_TEMP_DIR variable, i.e. a line that looks like this

define('WP_TEMP_DIR', ABSPATH . "wp-content/upgrade");

WLAN an der Uni Karlsruhe (DUKATH) mit Ubuntu 8.10

Posted on December 17, 2008 by Daniel Hepper

Ubuntu hat inzwischen eine ziemlich gute WLAN-Unterstützung. Lange Zeit konnte DUKATH nur[1] mit einem VPN-Client genutzt werden, inzwischen geht das auch per WPA. Laut Anleitung des Microbit [PDF] soll man ein Skript schreiben um den Zugang zu nutzen. Unter Ubuntu geht es aber auch etwas einfacher:

Auf das Symbol des Netzwerk-Manager-Applet klicken:

Das Netzwerk dukath-??x auswählen.

?? hängt vom Standort ab. Beim RZ gibt es eine Übersicht über die verwendeten SSIDs.

Auswahl DUKATH-Accesspoint

Im darauf folgenden Dialog die richtigen Werte eintragen:

Einstellungen DUKATH

Bei Zertifikat sollte das Deutsche Telekom Root CA 2 Zertifikat ausgewählt werden.

Benutzername und Passwort entsprechen denen bei der Verwendung von VPN.

Fertig!

[1] Der Zugang über das DUKATH Web-Interface sollte nur in Notfällen genutzt werden, da die Verbindung nicht verschlüsselt wird und deshalb einfach abgehört werden kann.