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Various programming stuff

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My essential guidelines for better Django development

Introduction

In this article I’d like to present a list of guidelines I follow when I develop Django projects, especially projects that are destined to be used in a production environment for many years. I am using django for more than 10 years as my day to day work tool to develop applications for the public sector organization I work for.

My organization has got a number of different Django projects that cover its needs, with some of them running successfully for a lot of years, since Django 1.4. I have been involved in the development of all of them, and I have learned a lot in the process. I understand that some of these may be controversial but they have served me well over these years.

Model design guidelines

Avoid using choices

Django has the convenient feature of allowing you to define choices for a field by defining a tuple of key-value pairs the field can take. So you’ll define a field like choice_field = models.CharField(choices=CHOICES) with CHOICES being a tuple like

CHOICES = (
    ('CHOICE1', 'Choice 1 description'),
    ('CHOICE2', 'Choice 2 description'),
    ('CHOICE3', 'Choice 3 description'),
)

and your database will contain CHOICE1, CHOICE2 or CHOICE3 as values for the field while your users will see the corresponding description.

This is a great feature for prototyping however I suggest to use it only on toy-prototyping-MVP projects and use normal relations in production projects instead. So the choice field would be a Foreign Key and the choices would be tuples on the referenced table. The reasons for this are:

  • The integrity of the choices is only on the application level. So people can go to the database and change a choice field with a random value.
  • More general, the database design is not normalized; saving CHOICE1 for every row is not ideal.
  • Your users may want to edit the choices (add new ones) or change their descriptions. This is easy with a foreign key through the django-admin but needs a code change with choices.
  • It is almost sure that you will need to add “properties” to your choices. No matter what your current requirements are, they are going to change. For example, you may want to make a choice “obsolete” so it can’t be picked by users. This is trivial when you use a foreign key but not very easy when you use choices.
  • The values of the choices is saved only inside your app. The database has only the 'CHOICE1', 'CHOICE2' etc values, so you’ll need to re-use the descriptions when your app is not used. For example, you may have reports that are generated directly from database queries so you’ll need to add the description of each key to your query using something like CASE.
  • It easier to use the ORM to annotate your queries when you use relations instead of the choices.

The disadvantage of relations is of course that you’ll need to follow the relation to display the values. So you must be careful to use select_related to avoid the n+1 queries problem.

So, in short, I suggest to use choices only for quick prototyping and covert them to normal relations in production projects. If you already are using choices in your project but want to convert them to normal relations, you can use take a look at my Django choices to ForeignKey article.

Always use surrogate keys

A surrogate key is a unique identifier for a database tuple which is used as the primary key. By default Django always adds a surrogate key to your models. However, some people may be tempted to use a natural key as the primary key. Although this is possible and supported in Django, I’d recommend to stick to integer surrogate keys. Why ?

  • Django is more or less build upon having integer primary keys. Although non-integer primary keys are supported in core Django, you can’t be assured that this will be supported by the various addons/packages that you’ll want to use.
  • I understand that your requirements say that “the field X will be unique and should be used to identify the row”. This is never true; this can easily be changed in the future and your primary key may stop being unique! It has happened to me and the solution was not something I’d like to discuss here. If there’s a field in the row that is guaranteed to be unique you can make it unique in the database level by adding unique==True; there’s no reason to also make it a primary key.
  • Relying on all your models having an id integer primary key makes it easier to write your code and other people reading it.
  • Using an auto-increment primary key is the fastest way to insert a new row in the database (when compared to, for example using a random uuid)

An even worse idea is to use composite keys (i.e define a primary key using two fields of your tuple). There’s actually a 17-year an open issue about that in Django! This should be enough for you to understand that you shouldn’t touch that with a 10-foot pole. Even if it is implemented somehow in core django, you’ll have something that can’t be used with all other packages that rely on primary key being a single field.

Now, I understand that some public facing projects may not want to expose the auto-increment primary key since that discloses information about the number of rows in the database, the number of rows that are added between a user’s tuples etc. In this case, you may want to either add a unique uuid field, or a slug field, or even better use a library like hashid to convert your integer ids to hashes. I haven’t used uuids myself, but for a slug field I had used the django-autoslug library and was very happy with it.

Concerning hashids, I’d recommend reading my Django hashids article.

Always use a through model on your m2m relations

To add a many-to-many relation in Django, you’ll usually do something like toppings = models.ManyToManyField(Topping) (for a pizza). This is a very convenient but, similar to the choices I mentioned above, it is not a good practice for production projects. This is because your requirements will change and you’ll need to add properties to your m2m relation. Although this is possible, it definitely is not pretty so it’s better to be safe than sorry.

When you use the ManyToManyField field, django will generate an intermediate table with a name similar to app_model1_model2, i.e for pizza and topping it will be pizzas_pizza_topping. This table will have 3 fields - the primary key, a foreign key to the pizza table and a foreign key to the topping table. This is the default behavior of Django and it is not configurable.

What happens if you want to add a relation to the pizzas_pizza_topping table? For example, the amount of each topping on a pizza. Or the fact that some pizzas used to have that topping but it has been replaced now by another one? This is not possible unless you use a through table. As I said it is possible to fix that but it’s not something that you’ll want to do.

So, my recommendation is to always add a through table when you use a m2m relation. Create a model that will represent the relation and has foreign keys to both tables along with any extra attributes the relation may have.

class PizzaTopping(models.Model):
    pizza = models.ForeignKey(Pizza, on_delete=models.CASCADE)
    topping = models.ForeignKey(Topping, on_delete=models.CASCADE)
    amount = models.IntegerField()

and define your pizza toppings relation like toppings = models.ManyToManyField(Topping, through=PizzaTopping).

If the relation doesn’t have no extra attributes don’t worry: You’ll be prepared when these are requested!

A bonus to that is that now you can query directly the PizzaTopping model and you can also add an admin interface for it.

There are no disadvantages to adding the through model (except the 1 minute needed to add the through model minor) since Django will anyway create the intermediate table to represent the relation so you’ll still need to use prefetch_related to get the toppings of a pizza and avoid the n+1 query problem.

Use a custom user model

Using a custom user model when starting a new project is already advised in the Django documentation. This will make it easier to add custom fields to your user model and have better control over it. Also, although you may be able to add a Profile model with an one to one relation with the default django.auth.User model you’ll still need to use a join to retrieve the profile for each user (something that won’t be necessary when the extra fields are on your custom user model).

Another very important reason to use a custom user model is that you’ll be able to easily add custom methods to your user model. For example, there’s the get_full_name method in builtin-Django that returns the first_name plus the last_name, with a space in between so you’re able to call it like {{ user.get_full_name }} in your templates. If you don’t have a custom user model, you’ll need to add template tags for similar functionality; see the discussion about not adding template tags when you can use a method.

There’s no real disadvantage to using a custom user model except the 5 minute it is needed to set it up. I actually recommend create a users app that you’re going to use to keep user related information (see the users app on my cookiecutter project).

Views guidelines

Use class based views

I recommend to prefer using class-based views instead of function-based views. This is because class-based views are easier to reuse and extend. I’ve written an extensive comprehensive Django CBV guide that you can read to learn everything about class based views!

Also, by properly using CBVs people reading your code will use sensible defaults and you be able to understand what you or others are doing much easier. Consider this

class FooDetailView(DetailView):
    model = Foo

vs

def object_detail_view(request, pk):
    foo = get_object_or_404(Foo, pk=pk)
    return render(request, 'foo/foo_detail.html', {'foo': foo})

These are more or less the same. However in the function-based view you need to actually write some logic for retrieving the Foo instance and then define the name of the template and the context object. Also notice that you use the get_object_or_404 function that helps you being DRY. Whereas in the class based view this is already done for you using well-known defaults. So, for example you’ll know which is the name of the template without the need to check the code.

View method overriding guidelines

It is important to know which method you need to override to add functionality to your class based views. You can use the excellent CBV Inspector app to understand how each CBV is working. Also, I’ve got many examples in my comprehensive Django CBV guide.

Some quick guidelines follow:

  • For all methods do not forget to call the parent’s method by super().
  • Override dispatch(self, request, *args, **kwargs) if you want to add functionality that is executed before any other method. For example to add permission checks or add some attribute (self.foo) to your view instance. This method will always run on both HTTP GET/POST or whatever. Must return a Response object (i.e HttpResponse, HttpResponseRedirect, HttpResponseForbidden etc)
  • You should rarely need to override the get or post methods of your CBVs since they are called directly after dispatch so any code should be there.
  • To add extra data in your context (template) override get_context_data(self, **kwargs). This should return a dictionary with the context data.
  • To pass extra data to your form (i.e the current request) override get_form_kwargs(self). This data will be passed on the __init__ of your form, you need to remove it by using something like self.request = kwargs.pop('request') before calling super().__init(*args, **kwargs)
  • To override the initial data of your form override get_form_initial(self). This should return a dictionary with the initial data.
  • You can override get_form(self, form_class=None) to use a configurable form instance or get_form_class(self) to use a configurable form class. The form instance will be generated by self.get_form_class()(**self.get_form_kwargs()) (notice that the kwargs will contain an initial=self.get_form_initial() value)
  • To do stuff after a valid form is submitted you’ll override form_valid(self, form). This should return an HttpResponse object and more specifically an HttpResponseRedirect to avoid double form submission. This is the place where you can also add flash messages to your responses.
  • You can also override form_invalid(self, form) but this is rarely useful. This should return a normal response (not a redirect)
  • Override get_success_url(self) if you only want to set where you’ll be redirected after a valid form submission (notice this is used by form_valid)
  • You can use a different template based on some condition by overriding get_template_names(self). This is useful to return a partial response on an ajax request (for example the same detail view will return a full html view of an object when visited normally but will return a small partial html with the object’s info when called through an ajax call)
  • For views that return 1 or multiple objects (DetailView, ListView, UpdateView etc) you almost always need to override the get_queryset(self) method, not the get_object. I’ll talk about that a little more later.
  • The get_object(self, queryset=None) method will use the queryset returned by get_queryset to get the object based on its pk, slug etc. I’ve observed that this rarely needs to be overridden since most of the time overriding get_queryset will suffice. One possible use case for overriding get_object is for views that don’t care at all about the queryset; for example you may implement a /profile detail view that will pick the current user and display some stuff. This can be implemented by a get_object similar to return self.request.user.

Querying guidelines

Guidelines for the n+1 problem

The most common Django newbie mistake is not considering the n+1 problem when writing your queries.

Because Django automatically follows relations it is very easy to write code that will result in the n+1 queries problem. A simple example is having something like

class Category(models.Model):
    name = models.CharField(max_length=255)

class Product(models.Model):
    name = models.CharField(max_length=255)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)

    def __str__(self):
        return "{0} ({1})".format(self.name, self.category.name)

and doing something like:

for product in Product.objects.all():
    print(product)

or even having products = Product.objects.all() as a context variabile in your template:

{% for product in products %}
    {{ product }}
{% endfor %}

If you’ve got 100 products, the above will run 101 queries to the database: The first one will get all the products and the other 100 will return each product’s category one by one! Consider what may happen if you had thousands of products…

To avoid this problem you should add the select_related, so products = Product.objects.all().select_related('category'). This will do an SQL JOIN between the products and categories table so each product will include its category instance. Now, when you’ve got a many to many relation the situation is a little different. Let’s suppose you’ve got a tags = models.ManyToManyField(Tag) field in your Product model. If you wanted to do something like {{ product.tags.all|join:", " }} to display the product tags you’d also get a n+1 situation because Django will do a query for each product to get its tags. To avoid this you cannot use select_related but should use the prefetch_related method so products = Product.objects.all().prefetch_related('tags'). This will result in 2 queries, one for products and one for their tags, the joining will be done in python.

One final comment about the prefetch_related is that you must be very careful to use what you prefetch. Let’s suppose that we had prefeched the tags but we wanted to display them ordered by name: Doing this ", ".join([tag for tag in product.tags.all().order_by('name')]) will not use the prefetched tags but will do a new query for each product to get its tags resulting in the n+1 problem! Django has tag.objects.all() for each product, not tag.objects.all().order_by('name'). To fix that you need to use Prefetch like this:

Product.objects.prefetch_related(Prefetch('tags', queryset=Tag.objects.order_by('name')))

The same is true if you wanted to filter your tags etc.

Now, one thing to understand is that this behavior of Django is intentional. Instead of automatically following the relationships, Django could throw an exception when you tried to follow a relationship that wasn’t in a select_related (this how it works in other frameworks). The disadvantage of this is that it would make Django more difficult to use for new users. Also, there are cases that the n+1 problem isn’t really a big deal, for example you may have a DetailView fetching a single object so in this case the n+1 problem will be 1+1 and wouldn’t really matter. So, at least for Django, it’s a case of premature optimization: Write your queries as good as you can (but keep in mind the n+1 problem), if you miss some cases that actually make your views slow, you can easily optimize them later.

Re-use your queries

You should re-use your queries to avoid re-writing them. You can either put them inside your models (as instance methods) or in a mixin for the queries of your views or even add a new manager for your model. Let’s see some examples:

Let’s suppose I wanted to get the tags of my product: I’d add this method to my Product model:

class Product(models.Model):
    # ...

    def get_tags(self):
        return self.tags.all().order_by('name')

Please notice that if you haven’t used a proper prefetch this will result in the n+1 queries problem. See the discussion above for more info. To get the products with their tags I could add a new manager like:

class ProductWithTagManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().prefetch_related(Prefetch('tags', queryset=Tag.objects.order_by('name')))

class Product(models.Model):
    # ...

    products_with_tags = ProductWithTagManager()

Now I could do [p.get_tags() for p in Product.products_with_tags.all()] and not have a n+1 problem.

Actually, if I knew that I would always wanted to display the product’s tags I could override the default manager like

class Product(models.Model):
    # ...

    objects = ProductWithTagManager()

However I would not recommend that since having a consistent behavior when you run Model.objects is very important. If you are to modify the default manager then you’ll need to always remember what your default manager does. This is very problematic in old projects and when you want to quickly query your database from a shell. Also, even more problematic is if you override your default manager to filter (hide) objects. Don’t do that or you’ll definitely regret it.

The other query re-use option is through a mixin that would override the get_queryset of your models. Let’s suppose that each user can only see his products: I could add a mixin like:

class ProductPermissionMixin:
    def get_queryset(self):
        return super().get_queryset().filter(created_by=self.request.user)

Then I could inherit my ListView, DetailView, UpdateView and DeleteView i.e ProductListView(ProductPermissionMixin, ListView) from that mixin and I’d have a consistent behavior on which products each user can view. More on this can be found on my comprehensive Django CBV guide.

Forms guidelines

Always use django-forms

This is a no-brainer: The django-forms offers some great class-based functionality for your forms. I’ve seen people creating html forms “by hand” and missing all this. Don’t be that guy; use django-forms!

I understand that sometimes the requirements of your forms may be difficult to be implemented with a django form and you prefer to use a custom form. This may seem fine at first but in the long run you’re gonna need (and probably re-implement) most of the django-forms capabilities.

Overriding Form methods guidelines

Your CustomForm inherits from a Django Form so you can override some of its methods. Which ones should you override?

  • The most usual method for overriding is clean(self). This is used to add your own server-side checks to the form. I’ll talk a bit more about overriding clean later.
  • The second most usual to override is __init__(self, *args, **kwargs). You should override it to “pop” any extra kwargs from the kwargs dict before calling super().__init__(*args, **kwargs). See the view method overriding guidelines for more info. Also you’ll use it to change.
  • I usually avoid overriding the form’s save() method. The save() is almost always called from the view’s form_valid method so I prefer to do any extra stuff from the view. This is mainly a personal preference in order to avoid having to hop between the form and view modules; by knowing that the form’s save is always the default the behavior will be consistent. This is personal preference though.

There shouldn’t be a need to override any other method of a Form or ModelForm. However please notice that you can easily use mixins to add extra functionality to your forms. For example, if you had a particular check that would be called from many forms, you could add a

class CustomFormMixin:
    def clean(self):
        super().clean() # Not really needed here but I recommend to add it to keep the inheritance chain
        # The common checks that does the mixin

class CustomForm(CustomFormMixin, Form):
    # Other stuff

    def clean(self):
        super().clean() # This will run the mixin's clean
        # Any checks that only this form needs to do

Proper cleaning

When you override the clean(self) method of a Form you should always use the self.cleaned_data to check the data of the form. The common way to mark errors is to use the self.add_error method, for example, if you have a date_from and date_to and date_from is after the date_to you can do your clean something like this:

def clean(self):

    date_from = self.cleaned_data.get("date_from")
    date_to = self.cleaned_data.get("date_to")

    if date_from and date_to and date_from > date_to:
        error_str = "Date from cannot be after date to"
        self.add_error("date_from", error_str)
        self.add_error("date_from", error_str)

Please notice above that I am checking that both date_from and date_to are not null (or else it will try to compare null dates and will throw). Then I am adding the same error message to both fields. Django will see that the form has errors and run form_invalid on the view and re-display the form with the errors.

Beyond the self.add_error method that adds the error to the field there’s a possibility to add an error to the “whole” form using:

from django.core.exceptions import ValidationError

def clean(self):
    if form_has_error:
        raise ValidationError(u"The form has an error!")

This kind of error won’t be correlated with a field. You can use this approach when an error is correlated to multiple fields instead of adding the same error to multiple fields.

You must be very careful because if you are using a non-standard form layout method (i.e you enumerate the fields) you also need to display the {{ form.errors }} in your template or else you’ll get a rejected form without any errors! This is a very common mistake.

Another thing to notice is that when your clean method raises it will display only the first such error. So if you’ve got multiple checks like:

def clean(self):
    if form_has_error:
        raise ValidationError(u"The form has an error!")
    if form_has_another_error:
        raise ValidationError(u"The form has another error!")

and your form has both errors only the 1st one will be displayed to the user. Then after he fixes it he’ll also see the 2nd one. When you use self.add_error the user will get both at the same time.

Overriding the __init__

You can override the __init__ method of your forms for three main reasons:

1. Override some field attributes on a ModelForm. A Django ModelForm will automatically create a field for each model field. Some times you may want to override some of the attributes of the field. For example, you may want to change the label of the field or make a field required. To do that, you can do something like:

def __init__(self, *args, **kwargs):
    super().__init__(*args, **kwargs)
    self.fields["my_field"].label = "My custom label" # Change the label
    self.fields["my_field"].help_text = "My custom label" # Change the help text
    self.fields["my_field"].required = True # change the required attribute
    self.fields["my_field"].queryset = Model.objects.filter(is_active=True) # Only allow specific objects for the forein key

Please notice that we need to use self.fields["my_field"] after we call super().__init__(*args, **kwargs).

2. Retrieve parameters (usually the request or user) from the view. A view (either a function-based or a CBV through get_form_kwargs) can pass parameters to the form’s constructor. You need to override __init__ to handle these parameters:

def __init__(self, *args, **kwargs):
    self.request = kwargs.pop("request", None)
    super().__init__(*args, **kwargs)

Please notice that we must pop the request from the kwargs dict before calling super().__init__ or else we’ll get an exception since the Form.__init__ method accepts only specific kwargs.

3. Add functionality related to the current user/request. For example, you may want to add a field that is only editable if the user is superuser:

def __init__(self, *args, **kwargs):
    self.request = kwargs.pop("request", None)
    super().__init__(*args, **kwargs)
    if not self.request.user.is_superuser:
        self.fields["my_field"].widget.attrs['readonly'] = True

or you may want to allow some custom validation logic only for non - superusers:

def clean(self):
    if not self.request.user.is_superuser:
        if not cleaned_data['my_field']:
            self.add_error("my_field", "Please field this field")

Laying out forms

To lay out the forms I recommend using a library like django-crispy-forms. This integrates your forms properly with your front-end engine and helps you have proper styling. I’ve got some more info on form layout post.

Please notice that the django-crispy-forms supports specific front-end frameworks like bootstrap or tailwind (see its docs for all available options). If you’re using a non-supported front-end framework you can create a custom template pack. This seems like a lot of work but I recommend to do it. Also you don’t need to implement everything, only the functionality you’re going to need, when you need it.

Improve the formset functionality

Beyond simple forms, Django allows you to use a functionality it calls formsets. A formset is a collection of forms that can be used to edit multiple objects at the same time. This is usually used in combination with inlines which are a way to edit models on the same page as a parent model. For example you may have something like this:

class Pizza(models.Model):
    name = models.CharField(max_length=128)
    toppings = models.ManyToManyField('Topping', through='PizzaTopping')

class Topping(models.Model):
    name = models.CharField(max_length=128)

class PizzaTopping(models.Model):
    amount = models.PositiveIntegerField()
    pizza = models.ForeignKey('Pizza')
    topping = models.ForeignKey('Topping')

Now we’d like to have a form that allows us to edit a pizza by both changing the pizza name and the toppings of the pizza along with their amounts. The pizza form will be the main form and the topping/amount will be the inline form. Notice that we won’t also create/edit the topping name, we’ll just select it from the existing toppings (we’re gonna have a completely different view for adding/editing individual toppings).

First of all, to create a class based view that includes a formset we can use the django-extra-views package (this isn’t supported by built-in django CBVs unless we implement the functionality ourselves). Then we’d do something like:

from extra_views import CreateWithInlinesView, InlineFormSetFactory


class ToppingInline(InlineFormSetFactory):
    model = Topping
    fields = ['topping', 'amount']


class CreatePizzaView(CreateWithInlinesView):
    model = Pizza
    inlines = [ToppingInline]
    fields = ['name']

This will create a form that will allow us to create a pizza and add toppings to it. Now, to display the formset we’d modify our template to be similar to:

<form method="post">
...
{{ form }}

{% for formset in inlines %}
    {{ formset }}
{% endfor %}
...
<input type="submit" value="Submit" />
</form>

This works however it will be very ugly. The default behavior is to display the Pizza form and three empty Topping forms. If we want to add more toppings we’ll have to submit that form so it will be saved and then edit it. But once again we’ll get our existing toppings and three more. I am not fond of this behavior.

That’s why my recommendation is to follow the instructions on my better django inlines article that allows you to sprinkle some javascript on your template and get a much better, dynamic behavior. I.e you’ll get an “add more” button to add extra toppings without the need t submit the form every time.

Template guidelines

Stick to the built-in Django template backend

Django has its own built-in template engine but it also allows you to use the Jinja template engine or even use a completely different one! The django template backend is considered “too restrictive” by some people mainly because you can only call functions without parameters from it.

My opinion is to just stick to the builtin Django template. Its restriction is actually a strength, enabling you to create re-usable custom template tags (or object methods) instead of calling business logic from the template. Also, using a completely custom backend means that you’ll add dependencies to your project; please see my the guideline about the selection of using external packages. Finally, don’t forget that any packages you’ll use that provide templates would be for the Django template backend, so you’ll need to convert/re-write these templates to be used with a different engine.

I would consider the Jinja engine only if I already had a bunch of Jinja templates from a different project and wanted to quickly use them on my project.

Don’t add template tags when you can use a method

Continuing from the discussion on the previous guideline, I recommend you to add methods to your models instead of adding template tags. For example, let’s suppose that we want to get our pizza toppings order by their name. We could add a template tag that would do that like:

def get_pizza_toppings(context, pizza):
    return pizza.toppings.all().order_by('name')

and use it like {% get_pizza_toppings pizza as pizza_toppings %} in our template. Notice that if you don’t care about the ordering you could instead do {{ pizza.toppings.all }} but you need to use the order_by and pass a parameter so you can’t call the method.

Instead of adding the template tag that I recommend adding a method to your pizza model like:

def get_toppings(self):
    return self.toppings.all().order_by('name')

and then call it like {{ pizza.get_toppings }} in your template. This is much cleaner and easier to understand.

Please notice that this guideline is not a proposal towards the “fat models” approach. You can add 1 line methods to your models that would only call the corresponding service methods if needed.

Re-use templates with partials

When you have a part of a template that will be used in multiple places you can use partials to avoid repeating yourself. For example, let’s suppose you like to display your pizza details. These details would be displayed in the list of pizzas, in the cart page, in the receipt page etc. So can create an html page named _pizza_details.html under a partial folder (or whatever name you want but I recommend having a way to quickly check your partials) with contents similar to:

<div class='pizza-details'>
    <h3>{{ pizza.name }}</h3>
    {% if show_photo %}
        <img src='{{ pizza.photo.url }}'>
    {% endif %}
    <p>Toppings: {{ pizza.get_toppings|join:", " }}</p>
</div>

and then include it in your templates like {% inlude "pizzas/partials/_pizza_details.html" %} to display the info without photo or {% inlude "pizzas/partials/_pizza_details.html" with show_photo=True %} to display the photo. Also notice that you can override the {{ pizza }} context variable so, if you want to display two pizzas in a template you’ll write something like

{% inlude "partials/_pizza_details.html" with show_photo=True pizza=pizza1 %}
{% inlude "partials/_pizza_details.html" with show_photo=True pizza=pizza2 %}

Settings guidelines

Use a package instead of module

This is a well known guideline but I’d like to mention it here. When you create a new project, Django will create a settings.py file. This file is a python module. I recommend to create a settings folder next to the settings.py and put in it the settings.py renamed as base.py and an __init__.py file so the settings folder will be a python package. So instead of project\settings.py you’ll have project\settings\base.py and project\settings\__init__.py.

Now, you’ll add an extra module inside settings for each kind of environment you are gonna use your app on. For example, you’ll have something like * project\settings\dev.py for your development environment * project\settings\uat.py for the UAT environment * project\settings\prod.py for the production environment

Each of these files will import the base.py file and override the settings that are different from the base settings, i.e these files will start like:

from .base import *

# And now all options that are different from the base settings

All these files will be put in your version control. You won’t put any secrets in these files. We’ll see how to handle secrets later.

When Django starts, it will by default look for the project/settings.py module. So, if you try to run python manage.py now it will complain. To fix that, you have to set the DJANGO_SETTINGS_MODULE environment variable to point to the correct settings module you wanna use. For example, in the dev env you’ll do DJANGO_SETTINGS_MODULE=project.settings.dev.

To avoid doing that every time I recommend creating a script that will initiate the project’s virtual environment and set the settings module. For example, in my projects I have a file named dovenv.bat (I use windows) with the following contents:

Handle secrets properly

You should never put secrets (i.e your database password or API KEYS) on your version control. There are two ways that can be used to handle secrets in Django:

  • Use a settings/local.py file that contains all your secrets for the current environment and is not under version control.
  • Use environment variables.

For the settings/local.py solution, you’ll add the following code at the end of each one of your settings environment modules (i.e you should put it at the end of dev.py, uat.py, prod.py etc):

try:
    from .local import *
except ImportError:
    pass

The above will try to read a module named local.py and if it exists it will import it. If it doesn’t exist it will just ignore it. Because this file is at the end of the corresponding settings module, it will override any settings that are already defined. The above file should be excluded from version control so you’ll add the line local.py to your .gitignore.

Notice that the same solution to store secrets can be used if you don’tt use the settings package approach but you have a settings.py module. Create a settings_local.py module and import from that at the end of your settings module instead. However I strongly recommend to use the settings package approach.

To catalogue my secrets, I will usually add a local.py.template file that has all the settings that I need to override in my local.py with empty values. I.e it will may be similar to:

API_TOKEN=''
ANOTHER_API_TOKEN=''
DATABASES_U = {
    'default': {
        'ENGINE': 'django.db.backends.postgresql_psycopg2',
        'NAME': '',
        'USER': '',
        'PASSWORD': '',
        'HOST': '',
        'PORT': '',
    }
}

Then I’ll copy over local.py.template to local.py when I initialize my project and fill in the values.

Before continuing, it is important to understand the priority of the settings modules. So let’s suppose we are on production. We should have a DJANGO_SETTINGS_MODULE=project.settings.prod. The players will be base.py, prod.py and local.py. The priority will be

  1. local.py
  2. prod.py
  3. base.py

So any settings defined in prod.py will override the settings of base.py. And any settings defined in local.py will override any settings defined either in prod.py or base.py. Please notice that I mention any setting, not just secrets.

To use the environment variables approach, you’ll have to read the values of the secrets from your environment. A simple way to do that is to use ths os.getenv function, for example in your prod.py you may have something like:

import os

API_TOKEN = os.getenv('API_TOKEN')

This will set API_TOKEN setting to None if the API_TOKEN env var is not found. You can do something like os.environ["API_TOKEN"] instead to throw an exception. Also, there are libraries that will help you with this like python-dotenv, However I can’t really recommend them because I haven’t used them.

Now, which one to use? My recommendation (and what I always do) is to use the first approach (local.py) unless you need to use environment variables to configure your project. For example, if you are using a PaaS like Heroku, you’ll have to use environment variables because of the way you deploy so you can’t really choose. However using the local.py is much simpler, does not have any dependencies and you can quickly understand which settings are overriden. Also you can use it to override any setting by putting it in your local.py, not just secrets.

Static and media guidelines

Use ManifestStaticFilesStorage

Django has a STATICFILES_STORAGE setting that can be used to specify the storage engine that will be used to store the static files. By default, Django uses the StaticFilesStorage engine which stores the files in the file system under the STATIC_ROOT folder and with a STATIC_URL url.

For example if you’ve got a STATIC_ROOT=/static_root and a STATIC_URL=/static_url/ and you’ve got a file named styles.css which you include with {% static "styles.css" %}. When you run python manage.py collectstatic the styles.css will be copied to /static_root/styles.css and you’ll be able to access it with /static_url/styles.css.

Please notice that the above should be configured in your web server (i.e nginx). Thus, you need to configure your web server so as to publish the files under /static_root on the /static_url url. This should work without Django, i.e if you have configured the web server properly you’ll be able to visit example.com/static_url/styles.css even if your Django app isn’t running. For more info see how to deploy static files.

Now, the problem with the StaticFilesStorage is that if you change the styles.css there won’t be any way for the user’s browser to understand that the file has been changed so it will keep using the cached version.

This is why I recommend using the ManifestStaticFilesStorage instead. This storage will append the md5 has of each static file when copying it so the styles.css will be copied to /static_root/styles.fb2be32168f5.css and the url will be /static_url/styles.fb2be32168f5.css. When the styles.css is changed, its hash will also be changed so the users are guaranteed to pick the correct file each time.

Organize your media files

When you upload a file to your app, Django will store it in the MEDIA_ROOT folder and serve it through MEDIA_URL similar to the static files as I explained before. The problem with this approach is that you’ll end up with a lot of files in the same folder. This is why I recommend creating a folder structure for your media files. To create this structure you should set the upload_to attribute of FileField.

So instead of having file = models.FileField or image = models.ImageField you’d do something like file = models.FileField(upload_to='%Y/%m/files') or image = models.ImageField(upload_to='%Y/%m/images') to upload these files to their corresponding folder organized by year/month.

Notice that instead of a string you can also pass a function to the upload_to attribute. This function will need to return a string that will contain the path of the uploaded file including the filename. For example, an upload_to function can be similar to this:

def custom_upload_path(instance, filename):
    dt_str = instance.created_on.strftime("%Y/%m/%d")
    fname, ext = os.path.splitext(filename)
    slug_fn = slugify(anyascii.anyascii(fname))
    if ext:
        slug_fn += "" + ext
    return "protected/{0}/{1}/{2}".format(dt_str, instance.id, slug_fn)

The above code will convert the filename to an ascii slug (i.e a file named δοκιμή.pdf will be converted to dokime.pdf) and will store it in a folder after the created date year/month/day and id of the object instance the file belongs to. So if for example the file δοκιμή.pdf belongs to the object with id 3242 and created date 2022-09-30 will be stored on the directory protected/2022/09/30/3242/dokime.pdf.

The above code is just an example. You can use it as a starting point and modify it to fit your needs. Having the media files in separate folders will enable you to easily navigate the folder structure and for example back up only a portion of the files.

Do not serve media through your application server

This is important. The media files of your app have to be served through your web server (i.e nginx) and not your application server (i.e gunicorn). This is because the application server has a limited number of workers and if you serve the media files through them, it will be a bottleneck for your app. Thus you need to configure your web server to serve the media files by publishing the MEDIA_ROOT folder under the MEDIA_URL url similar to the static files as described above.

Notice that by default Django will only serve your media files for development by using the following at the end of your urls.py file:

if settings.DEBUG:
    urlpatterns += static(settings.MEDIA_URL, document_root=settings.MEDIA_ROOT)

Under no circumstances you should use this when settings.DEBUG = False (i.e on production).

Secure your media properly

Continuing from the above, if you are not allowed to serve your media files through your application then how are you supposed to secure them? For example you may want to allow a user to upload files to your app but you want only that particular user to be able to download them and not anybody else. So you’ll need to check somehow that the user that tries to download the file is the same user that uploaded it. How can you do that?

The answer is to use a functionality offered by most web servers called X SendFile. First of all I’d like to explain how this works:

  1. A user wants to download a file with id 1234 so he clicks the “download” button for that file
  2. The browser of the user will then visit a normal django view for example /download/1234
  3. This view will check if the user is allowed to download the file by doing any permissions checks it needs to do, all in Django code
  4. If the user is not allowed to download, it will return a 403 (forbidden) or 404 (not-found) response
  5. However if the user is allowed to download the Django view will return an http response that will not contain the file but will have a special header with the path of the file to download (which is the path that file 1234 is saved on)
  6. When the web server (i.e nginx) receives the http response it will check if the response has the special header and if it does it will serve the response it got along with the file, directly from the file system without going through the application server (i.e gunicorn)

The above gives us the best of both worlds: We are allowed to do any checks we want in Django and the file is served through nginx.

A library that implements this functionality is django-sendfile2 which is a fork of the non-maintained anymore django-sendfile. To use it you’ll need to follow the instructions provided and depend on your web server. However, let’s see a quick example for nginx from one production project:

# nginx conf

server {
    # other stuff

    location /media_project/protected/ {
        internal;
        alias /home/files/project/media/protected/;
    }

    location /media_project/ {
        alias /home/files/project/media/;
    }


}

For nginx we add a new location block that will serve the files under the /media_project/protected/ url. The internal; directive will prevent the client from going directly to the URI, so visiting example.com/media_project/protected/file.pdf directly will not work. We also have a /media_project/ location that serves the files under /media that are not protected. Please notice that nginx matches the most specific path first so all files under protected will be matched with the correct, internal location.

# django settings
MEDIA_ROOT = "/home/files/project/media"
SENDFILE_ROOT = "/home/files/project/media/protected"

MEDIA_URL = "/media_project/"
SENDFILE_URL = "/media_project/protected"
SENDFILE_BACKEND = "sendfile.backends.nginx"

Notice the difference between the MEDIA_ROOT (that contains all our media files - some are not protected) and SENDFILE_ROOT and same for MEDIA_URL and SENDFILE_URL

# django view

def get_document(request, doc_id):
    from django_sendfile import sendfile

    doc = get_object_or_404(Document, pk=doc_id)
    rules_light.require(request.user, "apps.app.read_docs", doc.app)
    return sendfile(request, doc.file.path, attachment=True)

So this view first gets the Document instance from its id and checks to see if the current user can read it. Finally, it returns the sendfile response that will serve the file directly from the file system passing the path of that file. This function view will have a url like path("get_doc/<int:doc_id>/", login_required(views.get_document), name="get_document", ),

A final comment is that for your dev environment you probably want to use the SENDFILE_BACKEND = "django_sendfile.backends.development" (please see the settings package guideline on how to override settings per env).

Handle stale media

Django does never delete your media files. For example if you have an object that has a file field and the object is deleted, the file that this file field refers to will not be deleted. The same is true if you upload a new file on that file field, the old file will also be kept there!

This is very problematic in some cases, resulting to GB of unused files in your disk. To handle that, there are two solutions:

  • Add a signal in your models that checks if they are deleted or a file field is updated and delete the non-used file. This is implemented by the django-cleanup package.
  • Use a management command that will periodically check for stale files and delete them. This is implemented by the django-unused-media package.

I’ve used both packages in various projects and they work great. I’d recommend the django-cleanup on greenfield projects so as to avoid stale files from the beginning.

Debugging guidelines

Be careful when using django-debug-toolbar

The django-debug-toolbar is a great and very popular library that can help you debug your Django applications and identify slow views and n+1 query problems. However I have observed that it makes your development app much slower. For some views I am seeing like 10x decrease in speed i.e instead of 500 ms we’ll need more than 5 seconds to display that view! Since Django development (at least for me) is based on a very quick feedback loop, this is a huge problem.

Thus, I recommend to keep it disabled when you are doing normal development and only enable it when you need it, for example to identify n+1 query problems.

Use the Werkzeug debugger

Instead of using the traditional runserver to run your app in development I recommend installing the django-extensions package so as to be able to use the Werkzeug debugger. This will enable you to get a python prompt whenever your code throws an exception or even to add your own breakpoints by throwing exceptions.

In a nutshell, you’ll something like aa+=1 (aa should not be an integer) somewhere in your code (in a view, or a model method etc) and python will throw an exception. You’ll be able to get a python shell and inspect the state of your app inside that particular point, so you can see what variables are available and their values, run code etc. This is a superpower that after you start using it you’ll never want to go back to the traditional runserver.

More info on my Django Werkzeug debugger article.

General guidelines

Consider using a cookiecutter project template

If you are working on a Django shop so you need to create frequenctly new Django apps I’d recommend to consider creating (or use an existing) cookiecutter project template. You can use my own cookiecutter to create your projects or as an inspiration to create your own. It follows all the conventions I mention in this post and it is very simple to use.

Be careful on your selection of packages/addons

Django, because of its popularity, has an abundance of packages/addons that can help you do almost anything. However, my experience has taught me that you should be very careful and do your research before adding a new package to your project. I’ve been left many times with projects that I was not able to upgrade because they heavily relied on functionality from an external package that was abandoned by its creator. I also have lost many hours trying to debug a problem that was caused by a package that was not compatible with the latest version of Django.

So my guidelines before using an external Django addon are:

  • Make sure that it has been upgraded recently. There are no finished Django addons. Django is constantly evolving by releasing new versions and that must be true for the addons. Even if the addons are compatible with the new Django version they need to denote that in their README so as to know that their maintainers care.
  • Avoid using very new packages. I’ve seen many packages that are not yet mature and they are not yet ready for production. If you really need to use such a package make sure that you understand what it does and you can fix problems with the package if needed.
  • Avoid using packages that rely heavily on Javascript; this is usually better to do on your own.
  • Try to understand, at least at a high level, what the package does. If you don’t understand it, you will not be able to debug if it breaks.
  • Make sure that the package is well documented and that it has a good test coverage.
  • Don’t use very simple packages that you can easily implement yourself. Don’t be a left-pad developer.

I already propose some packages in this article but I also like to point you out to my Django essential package list. This list was compiled 5 years ago and I’m happy to still recommend all of these packages with the following minor changes:

  • Nowadays I recommend using wkhtmltopdf for creating PDFs from Django instead of xhtml2pdf. Please see my PDFs in Django like it’s 2022 article for more info. Notice that there’s nothing wrong with the xhtml2pdf package, it still works great and is supported but my personal preference is to use the wwhtmltopdf.
  • The django-sendfile is no longer supported so you need to use django-sendfile2 instead. This is a drop-in replacement from django-sendfile2. See the point about media securing for more info.
  • django-auth-ldap uses github now (nothing changed, it just uses github instead of bitbucket).

The fact that from a list of ~30 packages only one (django-sendfile) is no longer supported (and the fact that even for that there’s a drop-in replacement) is a testament to the quality of the Django ecosystem (and to my choosing capabilities).

In addition to the packages of my list, this article already contains a bunch of packages that I’ve used in my projects and I am happy with them so I’d also recommend them to you.

Don’t be afraid to use threadlocals

One controversial aspect if Django is that it avoids using the threadlocals functionality. The thread-local data is a way to store data that is specific to the current running thread. This, combined with the fact that each one of the requests to your Django app will be served by the same thread (worker) gives you a super powerful way to store and then access data that is specific to the current request and would be very difficult (if at all possible) to do it otherwise.

The usual way to work with thread locals in Django is to add a middleware that sets the current request in the thread local data. Then you can access this data from wherever you want in your code, like a global. You can either create that middleware yourself but I’d recommend using the django-tools library for adding this functionality. You’ll add the 'django_tools.middlewares.ThreadLocal.ThreadLocalMiddleware' to your list of middleware (at the end of the listt unless you want to use the current user from another middleware) and then you’ll use it like this:

from django_tools.middlewares import ThreadLocal

# Get the current request object:
request = ThreadLocal.get_current_request()
# You can get the current user directly with:
user = ThreadLocal.get_current_user()

Please notice that Django recommends avoiding this technique because it hides the request/user dependency and makes testing more difficult. However I’d like to respectfully disagree with their rationale.

  • First of all, please notice that this is exactly how Flask works when you access the current request. It stores the request in the thread locals and then you can access it from anywhere in your code.
  • Second, there are things that are very difficult (or even not possible) without using the threadlocals. I’ll give you an example in a little.
  • Third, you can be careful to use the thread locals functionality properly. After all it is a very simple concept. The fact that you are using thread locals can be integrated to your tests.

One example of why thread locals are so useful is this abstract class that I use in almost all my projects and models:

class UserDateAbstractModel(models.Model):
    created_on = models.DateTimeField(auto_now_add=True, )
    modified_on = models.DateTimeField(auto_now=True)

    created_by = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.PROTECT,
        related_name="%(class)s_created",
    )
    modified_by = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.PROTECT,
        related_name="%(class)s_modified",
    )

    class Meta:
        abstract = True

    def save(self, *args, **kwargs):
        user = ThreadLocal.get_current_user()
        if user:
            if not self.pk:
                self.created_by = user

            self.modified_by = user
        super(UserDateAbstractModel, self).save(*args, **kwargs)

Models that override this abstract model will automatically set the created_by and modified_by fields to the current user. This works the same no matter if I edit the object from the admin, or from a view. To use that functionality all I need to do is to inherit from that model i.e class MyModel(UserDateAbstractModel) and that’s it.

What would I need to do if I didn’t use the thread locals? I’d need to create a mixin from which all my views (that modify an object) would inherit! This mixin would pick the current user from the request and set it up. Please consider the difference between these two approaches; using the model based approach with the thread locals I can be assured that no matter where I modify an object, the created_by and modified_by will be set properly (unless of course I modify it through the database or django shell — actually, I could make save throw if the current use hasn’t been setup so it wouldn’t be possible to modify from the shell). If I use the mixin approach, I need to make sure that all my views inherit from that mixin and that I don’t forget to do it. Also other people that add code to my project will also need to remember that. This is a lot more error prone and difficult to maintain.

The above is a simple example. I have seen many more cases where without the use of thread locals I’d need to replicate 3-4 classes from an external library (this library was django-allauth for anybody interested) in order to be able to pass through the current user to where I needed to use this. This is a lot of code duplication and a maintenance hell.

One final comment: I’m not recommending to do it like Flask, i.e use thread locals anywhere. For example, in your views and forms it is easy to get the current request, there’s no need to use thread locals there. However, in places where there’s no simple path for accessing the current user then definitely use thread locals and don’t feel bad about it!

The async tasks situation

It is very common for new projects to add support for async tasks, either using celery, or django-rq or various other solutions. I’ve already written a bunch. of. posts. about this topic.

First of all let’s understand why we need async tasks: When you serve Djagno (or any Python web app) in production, you’ll start a number of worker processes that will be used to serve the users. These usually are normal OS processes. The guidelines are to start a finite amount of such processes, equal to 2-4 times the number of the CPU cores of your server. So with 2 cores you’ll have like 8 workers. This means your Django app can handle up to 8 concurrent requests at the same time. If we have a view that takes too long to response (e.g. because it runs a slow query), we’ll have many workers “stuck” on that view, resulting in delays for the other users since the number of workers always stays the same.

To resolve that issue we can use async tasks to offload the work to the “background”. I.e instead of the view waiting for the slow query to finish, it will now add a task in a queue and return immediately. The tasks in the queue will then be run by the async worker one after another. The other way to resolve that is to increase the number of workers, but that is not a good idea since each worker takes a certain amount of memory and resources and we still can’t be positive that we’ll be able to handle all traffic peaks to our slow views.

So, although I believe that async tasks are essential for some situations, my recommendation here is to be very careful and think twice before adding support for async tasks for your project. Because of how python works, the only way to have support for async tasks is to have one or more extra moving parts to your project.

These moving parts will be always a task worker process (that would pick the async tasks from the queue and execute them asynchronously) and probably an external process that would store your queue. Actually the queue process may be redis if you already use it for caching or even the database but also there are projects that use a separate application for the queue like Rabbitmq.

This may look like a small thing but in a production environment this means that instead of running 1 thing for your django app (a gunicorn or uwsgi app server) you need to add at least another thing (the worker). This results tp

  • Make sure that the worker sees and handles your tasks
  • Monitoring the worker (getting alerts when the worker stops, make sure it runs etc)
  • Start the worker when your server starts (i.e when your server reboots)
  • Re-start the worker when you deploy changes (this is critical and easily missed; your worker won’t pick any changes to your app when you deploy it, if you don’t re-start it it will run stale code)
  • Handle exceptions to your async tasks properly
  • Make sure you have some kind of logging and exception tracking for the worker

All this adds up especially if you need to do for every new app.

Taking this into account, I’d recommend to think twice before adding support for async tasks in you Django app. If you really need it, then of course, you’ll need to bite the bullet and add it. But lately my understanding is that people tend to add support for async tasks even though they don’t really need it. Let’s see some examples:

  • I’ve got a view that sends mails and it is too slow. If the view opens a connection to an SMTP server and sends the email then probably it will be slow. However, before using the async task situation, consider using a service like sendgrid or mailgun that will send your mails for you and will be much faster.
  • I need to run slow queries. Well… no you don’t. Your queries should not be slow. You should try to optimize your queries to run faster. If you have done all optimizations and still your queries are slow then you should consider de-normalizing your data to increase performance. This (at least in my book) is preferable over adding async tasks.
  • I need to do bulk operations. This probably is a reason to run async tasks. But before biting the bullet, consider: how many of your users are going to run such bulk operations at the same time?

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