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

[email protected]@ashchristopher

DONT DO THIS!

Friday, 20 January, 12

mailto:[email protected]


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


ModelsThey define your data.


ModelsThey don’t define behavior.


ModelsThey 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



... in memory



... 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.Friday, 20 January, 12

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.Friday, 20 January, 12

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)


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)

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': <WSGIRequestGET:<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;31:*.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:*.rpm=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;35:*.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:*.mov=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:*.vob=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:*.flv=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:*.aac=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: \\w\\a\\]${debian_chroot:+($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: [email protected]>, <User: [email protected]>, <User: [email protected]>, <User: [email protected]>, <User: [email protected]>, <User: [email protected]>, <User: [email protected]>, <User: >, <User: [email protected]>]}


















Weird stuff happens.


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



context



context [‘context’]



context [‘context’] [‘context’]



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



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


Please... do it this way!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 = {! ! 'message' : message,! }! return render_to_response( '/path/to/template.html', context)


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'sa good idea?are you sure that'sa good idea?are you sure that'sa good idea?are you sure that'sa good idea?



i think its a really goodusage patterni think its a really goodusage patterni think its a really goodusage patterni think its a really goodusage pattern



your POST data doesn'tnecessarily exist.your POST data doesn'tnecessarily exist.your POST data doesn'tnecessarily exist.your POST data doesn'tnecessarily exist.



oh... crapoh... crapoh... crapoh... crap



pwned you!pwned you!pwned you!pwned you!



............



FOLLOW THE PATTERN, BITCHES!FOLLOW THE PATTERN, BITCHES!FOLLOW THE PATTERN, BITCHES!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

[email protected]

@ashchristopher






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

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