Django Don't Do This - DjangoToronto - January, 2012

DON
DO T
THI
S!
ash.christopher@gmail.com
@ashchristopher
Friday, 20 January, 12

Who am I?


Systems Analyst


Senior Developer
and Team Lead


Senior Developer


What is this
talk about?


Common Mistakes


Common mistakes as a result
of gaining more experience
with Django


Common mistakes as a result
of gaining more experience
with Django

(in no particular order)


Don’t be *this* guy.


Defining Models
without indexes


DON’T forget
your indexes


Models


Models
They define your data.


Models
They don’t define behavior.


Models
They can’t read your mind.


Even seasoned Django
developers forget about
indexes


Remember this?


DO add indexes
to your Models.


Over-Indexing


DON’T index
everything


What are indexes?


What are indexes?
They are pointers to your data


What are indexes?
... in memory


What are indexes?
... in memory
... or on disk!!!


They slow down inserts,
updates and deletes.


As a general rule, you
want an index on
anything you will use
to limit results


As a general rule, you
want an index on
anything you will use
to limit results
... but generalizations break down quickly.


You need to assess your
specific needs.


DO analyze
your QuerySets
to determine
where indexes
are needed.

Silly Shortcuts


DON’T use
locals() to
populate your
context

I PITY THE FOOL...
WHO HAS TO DEBUG
YOUR TEMPLATE


It’s not clever.


It’s just lazy.


Every time you use
locals() to populate your
context, God kills a kitten.

def my_view(request):
! users = User.objects.filter(active=True)
! num_users = len(users)

! message = None
! if num_users > 3:
! ! message = "Three's a crowd."

context = locals()
!
return render_to_response(
'/path/to/template.html', context)



! message = None
! if num_users > 3:

context = locals()
!
return render_to_response(

Thumbs down!

context = {'context': <Recursion on dict with id=175092300>,
'ipdb': <module 'ipdb' from '/home/vagrant/.virtualenvs/wave/lib/python2.6/site-packages/ipdb/
__init__.pyc'>,
'message': "Three's a crowd.",
'num_users': 9,
'pprint': <function pprint at 0x8a88f44>,
'request': <WSGIRequest
GET:<QueryDict: {}>,
POST:<QueryDict: {}>,
COOKIES:{'__utma': '111872281.606618788.1318529036.1326747382.1326750603.150',
'__utmc': '111872281',
'__utmz': '111872281.1318529036.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none)',
'csrftoken': '********************************',
'sessionid': '********************************'},
META:{'CELERY_LOADER': 'djcelery.loaders.DjangoLoader',
'CONTENT_LENGTH': '',
'CONTENT_TYPE': 'text/plain',
'CSRF_COOKIE': '********************************',
'DJANGO_SETTINGS_MODULE': 'waveaccounting.settings',
'GATEWAY_INTERFACE': 'CGI/1.1',
'HOME': '/home/vagrant',
'HTTP_ACCEPT': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',
'HTTP_ACCEPT_CHARSET': 'ISO-8859-1,utf-8;q=0.7,*;q=0.7',
'HTTP_ACCEPT_ENCODING': 'gzip, deflate',
'HTTP_ACCEPT_LANGUAGE': 'en-us,en;q=0.5',
'HTTP_CONNECTION': 'keep-alive',
'HTTP_COOKIE': 'csrftoken=********************************;
__utma=111872281.606618788.1318529036.1326747382.1326750603.150;
__utmz=111872281.1318529036.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none);
sessionid=********************************; __utmc=111872281',
'HTTP_HOST': 'localhost:8000',
'HTTP_USER_AGENT': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:9.0.1) Gecko/20100101 Firefox/9.0.1',
'LANG': 'en_US.UTF-8',
'LESSCLOSE': '/usr/bin/lesspipe %s %s',
'LESSOPEN': '| /usr/bin/lesspipe %s',
'LOGNAME': 'vagrant',


'LS_COLORS':
'rs=0:di=01;34:ln=01;36:hl=44;37:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43
:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.lzma=01;3
1:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rp
m=01;31:*.jar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.
7z=01;31:*.rz=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;3
5:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mo
v=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vo
b=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.fl
v=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.axv=01;35:*.anx=01;35:*.ogv=01;35:*.ogx=01;35:*.a
ac=00;36:*.au=00;36:*.flac=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;
36:*.wav=00;36:*.axa=00;36:*.oga=00;36:*.spx=00;36:*.xspf=00;36:',
'MAIL': '/var/mail/vagrant',
'OLDPWD': '/home/vagrant',
'PATH': '/home/vagrant/.virtualenvs/wave/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/opt/
ruby/bin/',
'PATH_INFO': u'/en/settings/business/1/',
'PIP_RESPECT_VIRTUALENV': 'true',
'PS1': '(wave)[e]0;u@h: wa]${debian_chroot:+($debian_chroot)}u@h:w$ ',
'PWD': '/home/vagrant/Projects/wa/waveaccounting',
'QUERY_STRING': '',
'REMOTE_ADDR': '10.0.2.2',
'REMOTE_HOST': '',
'REQUEST_METHOD': 'GET',
'RUN_MAIN': 'true',
'SCRIPT_NAME': u'',
'SERVER_NAME': 'lucid32',
'SERVER_PORT': '8000',
'SERVER_PROTOCOL': 'HTTP/1.1',


'SERVER_SOFTWARE': 'WSGIServer/0.1 Python/2.6.5',
'SHELL': '/bin/bash',
'SHLVL': '1',
'SSH_CLIENT': '10.0.2.2 50056 22',
'SSH_CONNECTION': '10.0.2.2 50056 10.0.2.15 22',
'SSH_TTY': '/dev/pts/2',
'TERM': 'xterm-256color',
'TZ': 'America/Toronto',
'USER': 'vagrant',
'VIRTUALENVWRAPPER_HOOK_DIR': '/home/vagrant/.virtualenvs',
'VIRTUALENVWRAPPER_LOG_DIR': '/home/vagrant/.virtualenvs',
'VIRTUAL_ENV': '/home/vagrant/.virtualenvs/wave',
'WORKON_HOME': '/home/vagrant/.virtualenvs',
'_': '/home/vagrant/.virtualenvs/wave/bin/python',
'wsgi.errors': <open file '<stderr>', mode 'w' at 0xb77600d0>,
'wsgi.file_wrapper': <class 'django.core.servers.basehttp.FileWrapper'>,
'wsgi.input': <socket._fileobject object at 0x9c602ec>,
'wsgi.multiprocess': False,
'wsgi.multithread': True,
'wsgi.run_once': False,
'wsgi.url_scheme': 'http',
'wsgi.version': (1, 0)}>,
'users': [<User: ash@waveaccounting.com>, <User: ash+2@waveaccounting.com>, <User: ash+3@waveaccounting.com>,
<User: ehabjan4@wavetest.com>, <User: foo@example.com>, <User: ash@example.com>, <User: ash@wavereceipts.com>,
<User: >, <User: james+compound@waveaccounting.com>]}


Weird stuff happens.


Did you notice?
'context': <Recursion on dict with id=175092300>


Did you notice?

context


Did you notice?

context [‘context’]


Did you notice?

context [‘context’] [‘context’]


Did you notice?

context [‘context’] [‘context’] [‘context’]


Did you notice?

context [‘context’] [‘context’] [‘context’] ...


Please... do it this way!

! message = None
! if num_users > 3:

! context = {
! ! 'message' : message,
! }
! return render_to_response(


DO define your
context explicitly


Replacing the .all()
method on QuerySets


DON’T replace
get_query_set()
with something
unexpected.

The majority of
developers and apps
expect objects.all() to
return ALL objects.


That’s why it’s called .all()


EXAMPLE

class CustomQuerySet(QuerySet):
def active(self):
return self.filter(is_active=True)

class MyManager(models.Manager):
def get_query_set(self):
return CustomQuerySet(self.model)


To get the active users:

MyModel.objects.active()


What about in the
admin?


What about in the
admin?
class CustomModelAdmin(admin.ModelAdmin):

def queryset(self, request):
“”” Only superusers can access all objects in the admin. “””
qs = super(CustomModelAdmin, self).queryset(request)
if not request.user.is_superuser:
qs = qs.active()
return qs


DO explicitly
define your
QuerySets


Creating new ways to
do the same things.


DON’T invent
new Django
conventions.


Even the best Django
developers make this mistake



are you sure that's
a good idea?



i think its a really good
usage pattern



your POST data doesn't
necessarily exist.



oh... crap



pwned you!



...



FOLLOW THE
PATTERN, BITCHES!


DO follow the
conventions
outlined by the
Django community

They have more
experience than you
do.


They have built
more projects
than you have.


They know better
than you do.


Tell me I’m wrong


Tell me I’m wrong
ash.christopher@gmail.com
@ashchristopher


Django Don't Do This - DjangoToronto - January, 2012

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Django Don't Do This - DjangoToronto - January, 2012