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The Best (and Worst) of Django

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The Best (and Worst) of Django

  1. The best(and worst) of Django jacob@jacobian.org http://lanyrd.com/sgbwt/
  2. 1,215,616
  3. Public Enemy #1:Over-engineering.
  4. “Do the simplest thing that could possibly work.” — Simon Willison
  5. Small is beautiful
  6. The monolith monster The “application” is the whole site — no apps, no components, no modules. Makes heavy use of site-wide logic: middleware, context processors and custom plugin-like concepts.
  7. (I blame Rails.)
  8. “It’s a pretty simple site; about fifteen ortwenty thousand lines of code…”
  9. site_of_unspeakable_horrors!""  settings.py!""  urls.py!""  views.py#""  models.py
  10. models.py — 2,800 linesviews.py — 11,900 lines
  11. Big is bad;Small is beautiful.
  12. The Django mindset:Application: some bit of functionality.Site: several applications.Spin off new apps liberally.A suite of apps ready for use.
  13. There’s no such thing as “content.”
  14. XXX screenshot: “content” box
  15. Django is an un-CMS.
  16. The Django mindset:A great and powerful respect for data.Model the data correctly and the restof the site will just fall out of that.Denormalization is all well and good, butnever throw data away.
  17. The“everything-is-a…” anti-pattern
  18. “Everything needs a creation date.”
  19. class  BaseObject(models.Model):        creation_date  =  models.DateField()        …class  Animal(BaseObject):        …class  Vegetable(BaseObject):        …class  Mineral(BaseObject):        …
  20. Without a concrete base class:>>>  Animal.objects.all() SELECT  ...  FROM  animal;With a concrete base class:>>>  Animal.objects.all() SELECT  ...  FROM  "animal"  INNER  JOIN  "baseobject"   ON  ("animal"."baseobject_ptr_id"  =  "baseobject"."id")
  21. What you want:>>>  BaseObject.objects.all()[<Animal:  Frog>,  <Vegetable:  Carrot>,  <Mineral:  Gold>,  ...]What you get:>>>  BaseObject.objects.all()[<BaseObject:  1>,  <BaseObject:  2>,  <BaseObject:  3>,  ...]
  22. So maybe you try something like:def  get_some_of_everything():        qs  =  BaseObject.objects.all()        for  obj  in  qs:                for  cls  in  BaseObject.__subclasses__():                        try:                                obj  =  cls.objects.get(baseobject_ptr=obj)                                break                        except  cls.DoesNotExist:                                continue                yield  obj
  23. “Our site worked fine in development and testing, and was working wonderfully for the first few months.“But we just added a bunch more data, and now our homepage takes 27 seconds to load.”
  24. 1,800 queries
  25. “But… but… everything really does need a creation date!”So give everything a creation date.
  26. “Do the simplest thing that could possibly work.” — Simon Willison
  27. If you must get fancy: Abstract base classes don’t suffer from these performance problems. Denormalize into a UI-ordered auxiliary model. Non-relational databases work particular well here (I like SOLR).
  28. Those who do notunderstand PYTHONPATHare doomed to failure.
  29. >>>  import  sys>>>  sys.path[,  /Library/Python/2.6/site-­‐packages/pip-­‐0.8-­‐py2.6.egg,  /Library/Python/2.6/site-­‐packages/pdbpp-­‐0.7-­‐py2.6.egg,  /Library/Python/2.6/site-­‐packages/Pygments-­‐1.4-­‐py2.6.egg,  /Library/Python/2.6/site-­‐packages/wmctrl-­‐0.1-­‐py2.6.egg,  /Library/Python/2.6/site-­‐packages/pyrepl-­‐0.8.2-­‐py2.6.egg,  ...  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6,  ...  /Users/jacob/.local/lib/python2.6/site-­‐packages,  /Library/Python/2.6/site-­‐packages,  ...]>>>  import  re>>>  re<module  re  from  /System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/re.pyc>
  30. ~/Projects/djdash!""  __init__.py!""  dashboard$      !""  __init__.py$      !""  admin.py$      !""  models.py$      #""  views.py#""  manage.py (https://github.com/jacobian/django-dev-dashboard)
  31. $  ./manage.py  shell>>>  import  dashboard.models>>>  dashboard.models<module  dashboard.models    from  /Users/jacob/Projects/djdash/dashboard/models.pyc>>>>  import  djdash.dashboard.models>>>  djdash.dashboard.models<module  djdash.dashboard.models  from  /Users/jacob/Projects/djdash/../djdash/dashboard/models.pyc>>>>  djdash.dashboard.models.Metric  is  dashboard.models.MetricFalse
  32. You might have an import issue if…“Hey, many-to-many relations don’t show up in the admin.”“What’s up with these import errors when I deploy under mod_wsgi?”“Grrr… assertRaises doesn’t work!”
  33. Django is wrong! (I’m sorry.)
  34. Fixing import madness1. Use non-project-relative imports (import  app.models, not import project.app.models).2. Use relative imports (from  .  import  x) where possible (see http://bit.ly/pep328).3. Stop using manage.py.
  35. Delete manage.py?$  django-­‐admin.py  shell  -­‐-­‐pythonpath=`pwd`  -­‐-­‐settings=settings.local>>>  import  dashboard.models>>>  import  djdash.dashboard.modelsTraceback  (most  recent  call  last)...ImportError:  No  module  named  djdash.dashboard.models
  36. For virtualenv users:$  add2virtualenv  ~/Projects/djdash$  echo  "export  DJANGO_SETTINGS_MODULE=settings.local"      >>  $VIRTUAL_ENV/bin/postactivate$  echo  "unset  DJANGO_SETTINGS_MODULE"      >>  $VIRTUAL_ENV/bin/postdeactivate$  django-­‐admin.py  shell
  37. Keep(your settings) simple.
  38. Don’t do this …INSTALLED_APPS  +=  [p  for  p  in  os.listdir(BASE)                                      if  os.path.isdir(p)]
  39. … or this …urlpatterns  =  patterns(,  ...)for  app  in  settings.INSTALLED_APPS:        if  not  app.startswith(django):                p  =  url(^%s/  %  app,  include(%s.urls)  %  app)                urlpatterns  +=  patterns(,  p)
  40. … or this.MIDDLEWARE_CLASSES  =  [...]def  callback(arg,  dirname,  fnames):        if  middleware.py  in  fnames:                m  =  %s.middleware  %  os.path.split(dirname)[-­‐1])                MIDDLEWARE_CLASSES.append(m)os.path.walk(BASE,  callback,  None)
  41. Python’s design is predicated on the proposition that code is more often read than written.
  42. INSTALLED_APPS  =  (        django.contrib.auth,        django.contrib.contenttypes,        django.contrib.sessions, MIDDLEWARE_CLASSES  =  (        django.contrib.sites,        django.middleware.common.CommonMiddleware,        django.contrib.messages,        django.contrib.sessions.middleware.SessionMiddleware,        django.contrib.staticfiles,        django.middleware.csrf.CsrfViewMiddleware,        django.contrib.admin,        django.contrib.auth.middleware.AuthenticationMiddleware,        django.contrib.messages.middleware.MessageMiddleware,        django.contrib.flatpages,          django.contrib.flatpages.middleware.FlatpageFallbackMiddleware,        django_extensions,          debug_toolbar.middleware.DebugToolbarMiddleware,        debug_toolbar,   )        south,   urlpatterns  =  patterns(,        rs.users,          url(r^admin/,  include(admin.site.urls)),        rs.orgs,          url(r^signup/,  include(rs.signup.urls)),        rs.signup,        url(r^org/,  include(rs.orgs.urls)),          rs.clients,        url(r^clients/,  include(rs.clients.urls)),        rs.timezones,        url(r^caregivers/,  include(rs.caregivers.urls)),        rs.caregivers,        url(r^account/,  include(rs.users.urls)),        rs.dashboard,        url(r^dashboard/,  include(rs.dashboard.urls)),        rs.scripts,        url(r^reminders/,  include(rs.reminders.urls)),        rs.reminders,        url(r^calls/,  include(rs.calls.urls)),        rs.billing,        url(r^scripts/,  include(rs.scripts.urls)),        rs.calls,        url(r^contact/,  include(contact_form.urls)),          chunks,        url(r^login/,  django.contrib.auth.views.login,  {},  login),          contact_form,        url(r^logout/$,  django.contrib.auth.views.logout,  {},  logout,),)        url(r^changepassword/$,  django.contrib.auth.views.password_change) )
  43. Multiple settings files
  44. The localsettings anti-patternAt the bottom of your settings file:try:        from  local_settings  import  *except  ImportError:        pass
  45. “It’s simple: just create a local_settings.py, throw overridden settings in there, and then never check the file into source control.”
  46. “It’s simple: just create a local_settings.py, throw overridden settings in there, and then never check the file into source control.”
  47. Handling multiple settings files1. Don’t. Why is your staging environment different from production?2. Use DJANGO_SETTINGS_MODULE.
  48. The one true waysettings #  base.py!""  __init__.py INSTALLED_APPS  =  [...]!""  base.py!""  staging.py #  local.py!""  production.py from  settings.base  import  *#""  local.py INSTALLED_APPS  +=  [debug_toolbar]$  django-­‐admin.py  shell  -­‐-­‐settings=settings.local#  deploy.wsgios.environ[DJANGO_SETTINGS_MODULE]  =  settings.deploy
  49. “Do the simplest thing that could possibly work.” — Simon Willison
  50. Thank you!jacob@jacobian.org http://lanyrd.com/sgbwt/

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