Two Scoops of Django - Common Patterns for Forms

Two Scoops of Django
Common Patterns for Forms
&
More Things to Know About Forms

Who am I?
• Vic (Shan-Ho Yang)
• Graduate Student from NTNU
• Love Python & Web

Outline
• The Power of Django Forms
• Pattern 1: Simple ModelForm With Default Validators
• Pattern 2: Custom Form Field Validators in ModelForms
• Pattern 3: Overriding the Clean Stage of Validation
• Pattern 4: Hacking Form Fields (2 CBVs, 2 Forms, 1 Model)
• Pattern 5: Reusable Search Mixin View
• More Things to Know About Forms
• Know How Form Validation Works

The Power of Django Forms
• Django forms are powerful, flexible, extensible and
robust.
• Powerful validation features
• Package tips:
• django-floppyforms - http://goo.gl/kTFgu9
• django-crispy-forms - http://goo.gl/JNmx5h
• django-forms-bootstrap - http://goo.gl/nFpmQJ
4

The Power of Django Forms
• Probably using Django forms even if project
doesn’t serve HTML.
• The chapter goes explicitly into one of the best
parts of Django: forms, models and CBVs
working in concert.
5

Pattern 1:
Simple ModelForm With Default Validators
(1/3)
FlavorCreateView FlavorDetailView
(CreateView) (DetailView)
FlavorUpdateView FlavorDetailView
(UpdateView) (DetailView)
(Ch.9, subsection 9.5.1)
6

Pattern 1:
(2/3)
# flavors/views.py
from django.views.generic import CreateView, UpdateView
from braces.views import LoginRequiredMixin
from .models import Flavor
class FlavorCreateView(LoginRequiredMixin, CreateView):
model = Flavor
fields = ('title', 'slug', 'scoops_remaining')
class FlavorUpdateView(LoginRequiredMixin, UpdateView):
model = Flavor
7

Pattern 1:
(3/3)
• FlavorCreateView and FlavorUpdateView are
assigned Flavor as their model.
• Both views auto-generate a ModelForm based on
the Flavor model.
• Those ModelForms rely on the default field
validation rules of the Flavor model.
• Django gives us a lot of great defaults for data
validation, but the defaults are never enough.
8

Pattern 2:
Custom Form Field Validators in
ModelForms (1/9)
• Target: title field across our project’s dessert
app started with the word “Tasty”.
• This can be solved with a simple custom field
validation.
9

Pattern 2:
ModelForms (2/9)
# core/validators.py
from django.core.exception import ValidationError
def validate_tasty(value):
"""Raise a ValidationError if the value doesn't start
with the word 'Tasty'
"""
if not value.startswith(u"Tasty"):
msg = u"Must start with Tasty"
raise ValidationError(msg)
10

Pattern 2:
ModelForms (3/9)
# core/models.py
from django.db import models
from .validators import validate_tasty
class TastyTitleAbstractModel(models.Model):
title = models.CharField(max_length=255, validators=[validate_tasty])
class Meta:
abstract = True
11

Pattern 2:
ModelForms (4/9)
# flavors/models.py
from django.core.urlresolvers import reverse
from core.models import TastyTitleAbstractModel
class Flavor(TastyTitleAbstractModel):
slug = models.SlugField()
scoops_remaining = models.IntegerField(default=0)
12
def get_absolute_url(self):
return reverse("flavor_detail", kwars={"slug": self.slug})

Pattern 2:
ModelForms (5/9)
• Work with any other tasty food-based models
such as a WaffleCone or Cake model.
• Any model that inherits from the
TastyTitleAbstractModel class will throw a
validation error if anyone attempts to save a
model with a title that doesn’t start with ‘Tasty’.
13

Pattern 2:
ModelForms (6/9)
• What if we wanted to use validate_tasty() in just
forms?
• What if we wanted to assign it to other fields
besides the title?
14

Pattern 2:
ModelForms (7/9)
# flavors/forms.py
from django import forms
from core.validators import validate_tasty
class FlavorForm(forms.ModelForm):
def __init__(self, *args, **kwargs):
super(FlavorForm, self).__init__(*args, **kwargs)
self.fields["title"].validators.append(validate_tasty)
self.fields["slug"].validators.append(validate_tasty)
class Meta:
model = Flavor
15
Not change

Pattern 2:
ModelForms (8/9)
# flavors/views.py
from django.contrib import messages
from django.views.generic import CreateView, UpdateView, DetailView
from braces.views import LoginRequiredMixin
from .forms import FlavorForm
class FlavorActionMixin(object):
model = Flavor
@property
def success_msg(self):
return NotImplemented
def form_valid(self, form):
messages.info(self.request, self.success_msg)
return super(FlavorActionMixin, self).form_valid(form)
16

Pattern 2:
ModelForms (9/9)
class FlavorCreateView(LoginRequiredMixin, FlavorActionMixin,
CreateView):
success_msg = "created"
# Explicitly attach the FlavorFrom class
form_class = FlavorForm
class FlavorUpdateView(LoginRequiredMixin, FlavorActionMixin,
UpdateView):
success_msg = "updated"
# Explicitly attach the FlavorFrom class
form_class = FlavorForm
class FlavorDetailView(DetailView):
model = Flavor
17

Pattern 3:
Overriding the Clean Stage of Validation
(1/6)
• Some interesting use cases:
• Multi-field validation
• Validation involving existing data from the
database that has already been validated.
• Django provides a second stage and process
for validating incoming data.
18

Pattern 3:
(2/6)
• Why Django provides more hooks for validation?
• The clean() method is the place to validate two
or more fields against each other, since it’s not
specific to any one particular field.
• The clean validation stage is a better place to
attach validation against persistent data. Since
the data already has some validation, you won’t
waste as many database cycles on needless
queries.

Pattern 3:
(3/6)
# flavors/forms.py
from flavors.models import Flavor
class IceCreamOrderForm(forms.Form):
slug = forms.ChoiceField("Flavor")
toppings = forms.CharField()
super(IceCreamOrderForm, self).__init__(*args,
**kwargs)
self.fields["slug"].choices = [
(x.slug, x.title) for x in Flavor.objects.all()
]
def clean_slug(self):
slug = self.cleaned_data["slug"]
if Flavor.objects.get(slug=slug).scoops_remaining <= 0:
msg = u"Sorry we are out of that flavor."
raise forms.ValidationError(msg)
return slug
20

Pattern 3:
(4/6)
• For HTML-powered views, the clean_slug()
method in our example, upon throwing an error,
will attach a “Sorry, we are out of that flavor”
message to the flavor HTML input field.
• This is a great shortcut for writing HTML forms!

Pattern 3:
(5/6)
# flavors/forms.py
from flavors.models import Flavor
class IceCreamOrderForm(forms.Form):
# ...
def clean(self):
cleaned_data = super(IceCreamOrderForm, self).clean()
slug = cleaned_data.get("slug", "")
toppings = cleaned_data.get("toppings", "")
# Silly "too much chocolate" validation example
if u"chocolate" in slug.lower() and
u"chocolate" in toppings.lower():
msg = u"Your order has too much chocolate."
raise forms.ValidationError(msg)
return cleaned_data
22

Pattern 3:
(6/6)
• Tip: Common Fields Used In Multi-Field Validation
• Strength of the submitted password.
• If the email model field isn’t set to
unique=True, whether or not the email is
unique.
23

Pattern 4:
Hacking From Fields
(2 CBVs, 2Forms, 1 Model) (1/6)
• This pattern covers a situation where two views/
forms correspond to one model.
• An example might be a list of stores, where we
want each store entered into the system as fast
as possible, but want to add more data such as
phone number and description later.
24

Pattern 4:
Hacking From Fields
# stores/models.py
from django.core.urlresolvers import reverse
class IceCreamStore(models.Model):
title = models.CharField(max_length=100)
block_address = models.TextField()
phone = models.CharField(max_length=20, blank=True)
description = models.TextField(blank=True)
def get_absolute_url(self):
return reverse("store_detail", kwargs={"pk": self.pk})
25

Pattern 4:
Hacking From Fields
# stores/forms.py
from .models import IceCreamStore
class IceCreamStoreUpdateForm(forms.ModelForm):
phone = forms.CharField(required=True)
description = forms.TextField(required=True)
class Meta:
model = IceCreamStore
26
Duplicated

Pattern 4:
Hacking From Fields
# stores/forms.py
# Call phone and description from the self.fields dict-like object
class IceCreamStoreUpdateForm(forms.ModelForm):
class Meta:
# Call the original __init__ method before assigning
# field overloads
super(IceCreamStoreUpdateForm, self).__init__(*args,
**kwargs)
self.fields["phone"].required = True
self.fields["description"].required = True
27

Pattern 4:
Hacking From Fields
# stores/forms.py
class IceCreamStoreCreateForm(forms.ModelForm):
class Meta:
fields = ("title", "block_address")
class IceCreamStoreCreateForm(forms.ModelForm):
super(IceCreamStoreUpdateForm, self).__init__(*args,
**kwargs)
self.fields["phone"].required = True
self.fields["description"].required = True
class Meta(IceCreamStoreCreateForm):
fields = ("title", "block_address", "phone",
"description")
28

Pattern 4:
Hacking From Fields
# stores/views.py
from django.views.generic import CreateView, UpdateView
from .forms import IceCreamStoreCreateForm
from .forms import IceCreamStoreUpdateForm
class IceCreamCreateView(CreateView):
form_class = IceCreamStoreCreateForm
class IceCreamUpdateView(UpdateView):
form_class = IceCreamStoreUpdateForm
29

Pattern 5:
Reusable Search Mixin View
(1/5)
• We’re going to cover how to reuse a search form
in two views that correspond to two different
models.
• This example will demonstrate how a single CBV
can be used to provide simple search
functionality on both the Flavor and
IceCreamStore models.
30

Pattern 5:
(2/5)
# core/views.py
class TitleSearchMixin(object):
def get_queryset(self):
# Fetch the queryset from the parent's get_queryset
queryset = super(TitleSearchMixin, self).get_queryset()
# Get the q GET parameter
q = self.request.GET.get("q")
if q:
# return a filtered queryset
return queryset.filter(title__icontains=q)
# No q is specified so we return queryset
return queryset
31

Pattern 5:
(3/5)
# add to flavors/views.py
from django.views.generic import ListView
from core.views import TitleSearchMixin
class FlavorListView(TitleSearchMixin, ListView):
model = Flavor
# add to stores/views.py
from django.views.generic import ListView
from core.views import TitleSearchMixin
class IceCreamStoreListView(TitleSearchMixin, ListView):
32

Pattern 5:
(4/5)
{# form to go into stores/store_list.html template #}
<form action="" method="GET">
<input type="text" name="q">
<button type="submit">search</button>
</form>
{# form to go into flavors/flavor_list.html template #}
<form action="" method="GET">
<input type="text" name="q">
<button type="submit">search</button>
33
</form>

Pattern 5:
(5/5)
• Mixin are a good way to reuse code, but using
too many mixins in a single class makes for very
hard-to-maintain code.
• Try to keep our code as simple as possible.
34

More Things to Know About
Forms
• Django forms are really powerful, but there are
edge cases that can cause a bit of anguish.
• If you understand the structure of how forms are
composed and how to call them, most edge
cases can be readily overcome.
• Don’t disable Django’s CSRF protection.
• https://docs.djangoproject.com/en/1.6/ref/contrib/csrf/
35

Know How Form Validation
Works
• Form validation is one of those areas of Django
where knowing the inner working will drastically
improve your code.
• When we call form.is_valid(), a lot of things
happen behind the scenes.
36

Works
1. If the form has bound data, form.is_valid() calls the
form.full_clean() method.
2. form.full_clean() iterates through the form fields and
each field validates itself:
A. Data coming into this field is coerced into Python via the to_python()
method or raises a ValidationError.
B. Data is validated against field-specific rules, including custom
validators. Failure raises a ValidationError.
C. If there are any custom clean_<field>() methods in the form, they
are called at this time.

Works
3. form.full_clean() executes the form.clean() method.
4. If it’s a ModelForm instance, form._post_clean() does
the following:
A. Sets ModelForm data to the Model instance,
regardless of whether form.is_valid() is True or
False.
B. Calls the model’s clean() method. For reference,
saving a model instance through the ORM does
not call the model’s clean() method.

Works
# core/models.py
class ModelFormFailureHistory(models.Model):
form_data = models.TextField()
model_data = models.TextField()
39

Works
# flavors/models.py
import json
from django.contrib import messages
from django.core import serializers
from core.models import ModelFormFailureHistory
class FlavorActionMixin(object):
@property
def success_msg(self):
return NotImplemented
def form_valid(self, form):
messages.info(self.request, self.success_msg)
return super(FlavorActionMixin, self).form_valid(form)
def form_invalid(self, form):
"""Save invalid form and model data for later reference."""
form_data = json.dumps(form.cleaned_data)
model_data = serializers.serialize("json",
[form.instance])[1:-1]
ModelFormFailureHistory.objects.create(
form_data=form_data,
model_data= model_data
)
return super(FlavorActionMixin,
self).form_invalid(form)
40

Works
• form_invalid() is called after failed validation of a
form with bad data.
• When it called here in this example, both the
cleaned form data and the final data saved to
the database are saved as a
ModelFormFailureHistory record.
41

Reference
• Two Scoops of Django - Best Practice for Django 1.6
• By Daniel Greenfeld and Audrey Roy
42

Two Scoops of Django - Common Patterns for Forms

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