Django - Don't Do This!

Presentation created for the January 2012 Django Toronto meetup. It outlines some common mistakes that developers make as they become more familiar with the framework.

Transcript

1. ash.christopher@gmail.com
   @ashchristopher
   DONT
   DO
   THIS!
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2. Who am I?
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3. Systems Analyst
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4. Senior Developer
   and Team Lead
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5. Senior Developer
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6. What is this
   talk about?
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7. Common Mistakes
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8. Common mistakes as a result
   of gaining more experience
   with Django
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9. Common mistakes as a result
   of gaining more experience
   with Django
   (in no particular order)
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11. Don’t be *this* guy.
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12. Defining Models
    without indexes
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13. DON’T forget
    your indexes
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14. Models
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15. Models
    They define your data.
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16. Models
    They don’t define behavior.
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17. Models
    They can’t read your mind.
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18. Even seasoned Django
    developers forget about
    indexes
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19. Remember this?
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20. Remember this?
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21. DO add indexes
    to your Models.
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22. Over-Indexing
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23. DON’T index
    everything
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24. What are indexes?
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25. What are indexes?
    They are pointers to your data
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26. What are indexes?
    They are pointers to your data
    ... in memory
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27. What are indexes?
    They are pointers to your data
    ... in memory
    ... or on disk!!!
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28. They slow down inserts,
    updates and deletes.
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29. As a general rule, you
    want an index on
    anything you will use
    to limit results
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30. As a general rule, you
    want an index on
    anything you will use
    to limit results
    ... but generalizations break down quickly.
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31. You need to assess your
    specific needs.
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32. DO analyze
    your QuerySets
    to determine
    where indexes
    are needed.
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33. Silly Shortcuts
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34. DON’T use
    locals() to
    populate your
    context
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35. I PITY THE FOOL...
    WHO HAS TO DEBUG
    YOUR TEMPLATE
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36. It’s not clever.
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37. It’s just lazy.
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38. Every time you use
    locals() to populate your
    context, God kills a kitten.
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39. 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)
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40. 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!
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41. context = {'context': ,
    'ipdb': __init__.pyc'>,
    'message': "Three's a crowd.",
    'num_users': 9,
    'pprint': ,
    'request': GET:,
    POST:,
    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',
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42. '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: \\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',
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43. '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': ', mode 'w' at 0xb77600d0>,
    'wsgi.file_wrapper': ,
    'wsgi.input': ,
    'wsgi.multiprocess': False,
    'wsgi.multithread': True,
    'wsgi.run_once': False,
    'wsgi.url_scheme': 'http',
    'wsgi.version': (1, 0)}>,
    'users': [, , ,
    , , , ,
    , ]}
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44. Weird stuff happens.
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45. Did you notice?
    'context':
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46. Did you notice?
    'context':
    context
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47. Did you notice?
    'context':
    context [‘context’]
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48. Did you notice?
    'context':
    context [‘context’] [‘context’]
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49. Did you notice?
    'context':
    context [‘context’] [‘context’] [‘context’]
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50. Did you notice?
    'context':
    context [‘context’] [‘context’] [‘context’] ...
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51. 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)
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52. DO define your
    context explicitly
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53. Replacing the .all()
    method on QuerySets
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54. DON’T replace
    get_query_set()
    with something
    unexpected.
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55. The majority of
    developers and apps
    expect objects.all() to
    return ALL objects.
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56. That’s why it’s called .all()
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57. 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)
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58. To get the active users:
    MyModel.objects.active()
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59. What about in the
    admin?
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60. 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
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61. DO explicitly
    define your
    QuerySets
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62. Creating new ways to
    do the same things.
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63. DON’T invent
    new Django
    conventions.
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64. Even the best Django
    developers make this mistake
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65. Even the best Django
    developers make this mistake
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66. are you sure that's
    a good idea?
    are you sure that's
    a good idea?
    are you sure that's
    a good idea?
    are you sure that's
    a good idea?
    Even the best Django
    developers make this mistake
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67. i think its a really good
    usage pattern
    i think its a really good
    usage pattern
    i think its a really good
    usage pattern
    i think its a really good
    usage pattern
    Even the best Django
    developers make this mistake
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68. your POST data doesn't
    necessarily exist.
    your POST data doesn't
    necessarily exist.
    your POST data doesn't
    necessarily exist.
    your POST data doesn't
    necessarily exist.
    Even the best Django
    developers make this mistake
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69. oh... crap
    oh... crap
    oh... crap
    oh... crap
    Even the best Django
    developers make this mistake
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70. pwned you!
    pwned you!
    pwned you!
    pwned you!
    Even the best Django
    developers make this mistake
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71. ...
    ...
    ...
    ...
    Even the best Django
    developers make this mistake
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72. FOLLOW THE
    PATTERN, BITCHES!
    FOLLOW THE
    PATTERN, BITCHES!
    FOLLOW THE
    PATTERN, BITCHES!
    FOLLOW THE
    PATTERN, BITCHES!
    Even the best Django
    developers make this mistake
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73. DO follow the
    conventions
    outlined by the
    Django community
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74. They have more
    experience than you
    do.
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75. They have built
    more projects
    than you have.
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76. They know better
    than you do.
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77. Tell me I’m wrong
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78. Tell me I’m wrong
    ash.christopher@gmail.com
    @ashchristopher
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