The document discusses various techniques in Django including: - Using model inheritance and mixins to add common fields and functionality to models - Monkey patching the model save method to add additional keyword arguments - Handling null values across deep dictionary lookups - Using locals() to pass additional context when rendering templates - Defining choices as classes to avoid hardcoding in models - Adding operators to choice values to customize display - Conditionally including fields and methods based on settings - Injecting context from other files using execfile

2. Priorities on
model.save()
device = Device.objects.get(sn=‘1234567890’)
device.barcode = ‘46262’
device.save()

3. from django.db import models as db
class Device(db.Model):
name = db.CharField(verbose_name=_("name"), max_length=255)
parent = db.ForeignKey('self', verbose_name=_("parent device"),
on_delete=db.SET_NULL,
null=True, blank=True, default=None, related_name="child_set")
model = db.ForeignKey(DeviceModel, verbose_name=_("model"),
null=True, blank=True, default=None, related_name="device_set",
on_delete=db.SET_NULL)
sn = db.CharField(verbose_name=_("serial number"), max_length=255,
unique=True, null=True, blank=True, default=None)
barcode = db.CharField(verbose_name=_("barcode"), max_length=255,
unique=True, null=True, blank=True, default=None)
remarks = db.TextField(verbose_name=_("remarks"),
help_text=_("Additional information."),
blank=True, default="")
boot_firmware = db.CharField(verbose_name=_("boot firmware"),
null=True, blank=True, max_length=255)
hard_firmware = db.CharField(verbose_name=_("hardware firmware"),
null=True, blank=True, max_length=255)
# ...

4. SNMP
plugin
Puppet SSH
plugin plugin
Manual
data Device …
entry

5. Loss of information
12:01 12:02 12:37
SNMP Puppet SNMP
• Device • Device • Device
has has has
a CPU1 a Xeon a CPU1
E5645 @
2.4 GHz

6. device.save(priority=plugin.priority)
12:01 P= 12:02 P= 12:37 P=
10 20 10
SNMP Puppet SNMP
• Device • Device • Attribute
has has change
a CPU1 a Xeon ignored
E5645 @ • Device
2.4 GHz still has
a Xeon
E5645 @
2.4 GHz

7. Manual data entry by a human
user
P=
100
0

9. from django.db import models as db
class Device(db.Model):
name = db.CharField(verbose_name=_("name"), max_length=255)
parent = db.ForeignKey('self', verbose_name=_("parent device"),
on_delete=db.SET_NULL,
null=True, blank=True, default=None, related_name="child_set")
model = db.ForeignKey(DeviceModel, verbose_name=_("model"),
null=True, blank=True, default=None, related_name="device_set",
on_delete=db.SET_NULL)
sn = db.CharField(verbose_name=_("serial number"), max_length=255,
unique=True, null=True, blank=True, default=None)
barcode = db.CharField(verbose_name=_("barcode"), max_length=255,
unique=True, null=True, blank=True, default=None)
remarks = db.TextField(verbose_name=_("remarks"),
help_text=_("Additional information."),
blank=True, default="")
boot_firmware = db.CharField(verbose_name=_("boot firmware"),
null=True, blank=True, max_length=255)
hard_firmware = db.CharField(verbose_name=_("hardware firmware"),
null=True, blank=True, max_length=255)
# ...

11. Multiple inheritance

12. class Article(Localized, Titled, Slugged,
Categorized, Taggable, AbstractArticle,
TimeTrackable, EditorTrackable,
Publishable, Commentable,
DisplayCounter,
HasShowContent):
class Meta:
verbose_name = _("article")
verbose_name_plural = _("articles")

13. class EditorTrackable(db.Model):
created_by = db.ForeignKey(
EDITOR_TRACKABLE_MODEL,
verbose_name=_("created by"),
null=True, blank=True, default=None,
related_name='+', on_delete=db.SET_NULL,
limit_choices_to={'is_staff’
if EDITOR_TRACKABLE_MODEL is User
else 'user__is_staff': True})
modified_by = db.ForeignKey(
EDITOR_TRACKABLE_MODEL,
verbose_name=_("modified by"),
null=True, blank=True, default=None,
related_name='+', on_delete=db.SET_NULL,
limit_choices_to={'is_staff’
if EDITOR_TRACKABLE_MODEL is User
else 'user__is_staff': True})

15. class EditorTrackable(db.Model):
created_by = db.ForeignKey(
EDITOR_TRACKABLE_MODEL,
verbose_name=_("created by"),
null=True, blank=True, default=None,
related_name='+', on_delete=db.SET_NULL,
limit_choices_to={'is_staff’
if EDITOR_TRACKABLE_MODEL is User
else 'user__is_staff': True})
modified_by = db.ForeignKey(
EDITOR_TRACKABLE_MODEL,
verbose_name=_("modified by"),
null=True, blank=True, default=None,
related_name='+', on_delete=db.SET_NULL,
limit_choices_to={'is_staff’
if EDITOR_TRACKABLE_MODEL is User
else 'user__is_staff': True})

17. Monkey patching
Traceback (most recent call last):
...
TypeError: save() got an unexpected keyword
argument 'priority'

18. from django.db import models
models.Model._lck_save = models.Model.save
models.Model.save = (lambda self,
force_insert=False, force_update=False,
using=None, *args, **kwargs:
self._lck_save(force_insert,
force_update, using)
)

20. Null
data['INFRA2']['MANAGERS']['MANAGER'][0]['POWERLEVEL']

21. # bad solution 1
if 'INFRA2' in d:
if 'MANAGERS' in d['INFRA2’]:
if 'MANAGER' in d['INFRA2']['MANAGERS']:
if len(d['INFRA2']['MANAGERS']):
if 'POWERLEVEL' in d['INFRA2']['MANAGERS']
['MANAGER'][0]:
return d['INFRA2']['MANAGERS']['MANAGER']
[0]['POWERLEVEL’]
return None
# bad solution 2
data.get('INFRA2', {}).get('MANAGERS',
{}).get('MANAGER', {}).get(0, []).get('POWERLEVEL’)
# bad solution 3
try:
return data['INFRA2']['MANAGERS']['MANAGER'][0]
['POWERLEVEL']
except (KeyError, IndexError):
return None

22. >>> from null import Null
>>> Null.any_attribute
Null
>>> Null['any_key’]
Null
>>> Null[123]
Null
>>> Null.any_method()
Null
>>> bool(Null)
False
# solution 4
>>> from null import nullify
>>> data = nullify(data)
>>> data['INFRA2']['MANAGERS']['MANAGER'][0]
['POWERLEVEL’]
Null

24. locals()

25. def messages(request):
template='messages/list.html'
user = request.user
message_list = Message.objects.filter(
owner=user)
return render_to_response(
template,
{'message_list': message_list,
'user': user},
context_instance=RequestContext(request))

26. def messages(request):
template='messages/list.html'
user = request.user
message_list = Message.objects.filter(
owner=user)
return render_to_response(
template,
{'message_list': message_list,
'user': user},
context_instance=RequestContext(request))
@view
def messages(request):
template = 'messages/list.html’
user = request.user
message_list = Message.objects.filter(
owner=user)
return locals()

28. Class context
GENDER_MALE = 0
GENDER_FEMALE = 1
GENDER_NOT_SPECIFIED = 2
GENDER_CHOICES = (
(GENDER_MALE, _('male')),
(GENDER_FEMALE, _('female')),
(GENDER_NOT_SPECIFIED, _('not specified')),
)
class User(db.Model):
gender = db.IntegerField(_("gender"),
choices=GENDER_CHOICES)

29. Class context
class Gender(Choices):
male = Choice(_("male"))
female = Choice(_("female"))
not_specified = Choice(_("not specified"))
class User(db.Model):
gender = ChoiceField(_("gender"), choices=Gender,
default=Gender.not_specified)

30. Class context
class Gender(Choices):
_ = Choices.Choice
male = _("male")
female = _("female")
not_specified = _("not specified")
class User(db.Model):
gender = ChoiceField(_("gender"), choices=Gender,
default=Gender.not_specified)

32. Operators
class User(db.Model):
gender = ChoiceField(_("gender"), choices=Gender,
default=Gender.not_specified)
def greet(self):
if self.gender == Gender.male:
return "Hi, boy.”
elif self.gender == Gender.female:
return "Hello, girl.”
else:
return "Hey there, user!"

33. Operators
class Gender(Choices):
male = _("male") << {'hello': 'Hi, boy.’}
female = _("female") << {'hello': 'Hello, girl.’}
not_specified = _("not specified") << {
'hello': 'Hey there, user!’}
class User(models.Model):
gender = ChoiceField(choices=Gender,
default=Gender.not_specified)
def greet(self):
return self.gender.hello

35. I can do this all day

36. Conditional fields/methods
class ArticlePage(DisplayCounter, HasShowContent):
article = db.ForeignKey(Article,
verbose_name=_("article"))
page_number = db.PositiveIntegerField(
verbose_name=_("page number"),
default=None, null=True, blank=True)
#...
if settings.CUSTOMER_PAID_FOR_PRINTING_SUPPORT:
def print(self):
...

37. Injecting context by
execfile
# settings.py
from lck.django import current_dir_support
execfile(current_dir_support)
# ...
STATIC_ROOT = CURRENT_DIR + 'static'