Using the Template System

A Django project can be configured with one or several template engines (or even zero if you don’t use templates). Django ships with a built-in backend for its own template system – the Django Template Language (DTL). Django 1.11 also includes support for the popular alternative Jinja2.

If you don’t have a pressing reason to choose another backend, you should use the DTL – especially if you’re writing a pluggable application and you intend to distribute templates. Django’s contrib apps that include templates, like django.contrib.admin, use the DTL. All of the examples in this chapter will use the DTL.

Before we go about implementing Django templates in your view, lets first dig inside the DTL a little so you can see how it works. Here is the most basic way you can use Django’s template system in Python code:

• Create a Template object by providing the raw template code as a string.
• Call the render() method of the Template object with a given set of variables (the context). This returns a fully rendered template as a string, with all of the variables and template tags evaluated according to the context. Using the Django custom Python shell (python manage.py shell), here’s what that looks like:

     >>> from django import template    

     >>> t = template.Template('My name is {{ name }}.')    

    >>> c = template.Context({'name': 'Nige'})    

    >>> print (t.render(c))    

     My name is Nige.    

    >>> c = template.Context({'name': 'Barry'})    

    >>> print (t.render(c))    

    My name is Barry.

If you’ve used Python before, you may be wondering why we’re running python manage.py shell instead of just python (or python3). Both commands will start the interactive interpreter, but the manage.py shell command has one key difference: before starting the interpreter, it tells Django which settings file to use.

Many parts of Django, including the template system, rely on your settings, and you won’t be able to use them unless the framework knows which settings to use.If you’re curious, here’s how it works behind the scenes. Django looks for an environment variable called DJANGO_SETTINGS_MODULE, which should be set to the import path of your settings.py. For example, DJANGO_SETTINGS_MODULE might be set to ‘mysite.settings’, assuming mysite is on your Python path.

When you run python manage.py shell, the command takes care of setting DJANGO_SETTINGS_MODULE for you. You will need to use python manage.py shell in these examples, or Django will throw an exception.

The following sections describe each step in much more detail.

Creating Template Objects

The easiest way to create a Template object is to instantiate it directly. The Template class lives in the django.template module, and the constructor takes one argument, the raw template code. Let’s dip into the Python interactive interpreter to see how this works in code. From the mysite project directory you created earlier, type python manage.py shell to start the interactive interpreter.

Let’s go through some template system basics:

         >>> from django.template import Template

        >>> t = Template('My name is {{ name }}.')

        >>> print (t)

If you’re following along interactively, you’ll see something like this:

      <django.template.base.Template object at 0x030396B0>

That 0x030396B0 will be different every time, and it isn’t relevant; it’s a Python thing (the Python “identity” of the Template object, if you must know).

When you create a Template object, the template system compiles the raw template code into an internal, optimized form, ready for rendering. But if your template code includes any syntax errors, the call to Template() will cause a TemplateSyntaxError exception:

       >>> from django.template import Template

      >>> t = Template('{% notatag %}')

       Traceback (most recent call last):

      File "", line 1, in ?

      ...

django.template.exceptions.TemplateSyntaxError: Invalid block tag on line 1: ‘notatag’. Did you forget to register or load this tag?

The term “block tag” here refers to {% notatag %}. “Block tag” and “template tag” are synonymous. The system raises a TemplateSyntaxError exception for any of the following cases:

• Invalid tags
• Invalid arguments to valid tags
• Invalid filters
• Invalid arguments to valid filters
• Invalid template syntax
• Unclosed tags (for tags that require closing tags)

Rendering a Template

Once you have a Template object, you can pass it data by giving it a context. A context is simply a set of template variable names and their associated values. A template uses this to populate its variables and evaluate its tags. A context is represented in Django by the Context class, which lives in the django.template module. Its constructor takes one optional argument: a dictionary mapping variable names to variable values. Call the Template object’s render() method with the context to “fill” the template:

>> from django.template import Context, Template

>> t = Template('My name is {{ name }}.')

          >>> c = Context({'name': 'Stephane'})

          >>> t.render(c)

          'My name is Stephane.'

Dictionaries and Contexts

A Python dictionary is a mapping between known keys and variable values. A Context is similar to a dictionary, but a Context provides additional functionality.

Variable names must begin with a letter (A-Z or a-z) and may contain more letters, digits, underscores, and dots. (Dots are a special case we’ll get to in a moment.) Variable names are case sensitive. Here’s an example of template compilation and rendering, using a template similar to the example in the beginning of this chapter:

>>> from django.template import Template, Context
>>> raw_template = """<p>Dear {{ person_name }},</p>
...
... <p> Thanks for placing an order from {{ company }}. It's scheduled to
... ship on {{ ship_date|date:"F j, Y" }}.</p>
...
... {% if ordered_warranty %}
... <p>Your warranty information will be included in the packaging.</p>
... {% else %}
... <p> You didn't order a warranty, so you're on your own when
... the products inevitably stop working.</p>
... {% endif %}
...
... <p>Sincerely,<br />{{ company }}</p>"""
>>> t = Template(raw_template)
>>> import datetime
>>> c = Context({'person_name': 'John Smith',
...     'company': 'Outdoor Equipment',
...     'ship_date': datetime.date(2017, 7, 2),
...     'ordered_warranty': False})
>>> t.render(c)
"<p>Dear John Smith,</p>\n\n<p>Thanks for placing an order from Outdoor Equipment. It\
's scheduled to\nship on July 2,2017.</p>\n\n\n<p>You didn't order a warranty, so you\
're on your own when\nthe products inevitably stop working.</p>\n\n\n<p>Sincerely,<br\
 />Outdoor Equipment</p>"

• First, we import the classes Template and Context, which both live in the module django.template.
• We save the raw text of our template into the variable raw_template. Note that we use triple quote marks to designate the string, because it wraps over multiple lines; in contrast, strings within single quote marks cannot be wrapped over multiple lines.
• Next, we create a template object, t, by passing raw_template to the Template class constructor.
• We import the datetime module from Python’s standard library, because we’ll need it in the following statement.
• Then, we create a Context object, c. The Context constructor takes a Python dictionary, which maps variable names to values. Here, for example, we specify that the person_name is “John Smith”, company is “Outdoor Equipment”, and so forth.
• Finally, we call the render() method on our template object, passing it the context. This returns the rendered template – i.e., it replaces template variables with the actual values of the variables, and it executes any template tags. Note that the “You didn’t order a warranty” paragraph was displayed because the ordered_warranty variable evaluated to False. Also note the date, July 2, 2017, which is displayed according to the format string “F j, Y“. (I’ll explain format strings for the date filter in a little while.)

If you’re new to Python, you may wonder why this output includes newline characters (“\n“) rather than displaying the line breaks. That’s happening because of a subtlety in the Python interactive interpreter: the call to t.render(c) returns a string, and by default the interactive interpreter displays the representation of the string, rather than the printed value of the string. If you want to see the string with line breaks displayed as true line breaks rather than “\n” characters, use the print function: print (t.render(c)).

Those are the fundamentals of using the Django template system: just write a template string, create a Template object, create a Context, and call the render() method.

Multiple Contexts, Same Template

Once you have a Template object, you can render multiple contexts through it. For example:

>>> from django.template import Template, Context
>>> t = Template('Hello, {{ name }}')
>>> print (t.render(Context({'name': 'John'})))
Hello, John
>>> print (t.render(Context({'name': 'Julie'})))
Hello, Julie
>>> print (t.render(Context({'name': 'Pat'})))
Hello, Pat

Whenever you’re using the same template source to render multiple contexts like this, it’s more efficient to create the Template object once, and then call render() on it multiple times:

# Bad
for name in ('John', 'Julie', 'Pat'):
t = Template('Hello, {{ name }}')
print (t.render(Context({'name': name})))

# Good
t = Template('Hello, {{ name }}')
for name in ('John', 'Julie', 'Pat'):
print (t.render(Context({'name': name})))

Django’s template parsing is quite fast. Behind the scenes, most of the parsing happens via a call to a single regular expression. This is in stark contrast to XML-based template engines, which incur the overhead of an XML parser and tend to be orders of magnitude slower than Django’s template rendering engine.

Context Variable Lookup

In the examples so far, we’ve passed simple values in the contexts – mostly strings, plus a datetime.date example. However, the template system elegantly handles more complex data structures, such as lists, dictionaries, and custom objects. The key to traversing complex data structures in Django templates is the dot character (“.”).

Use a dot to access dictionary keys, attributes, methods, or indices of an object. This is best illustrated with a few examples. For instance, suppose you’re passing a Python dictionary to a template. To access the values of that dictionary by dictionary key, use a dot:

>>> from django.template import Template, Context
>>> person = {'name': 'Sally', 'age': '43'}
>>> t = Template('{{ person.name }} is {{ person.age }} years old.')
>>> c = Context({'person': person})
>>> t.render(c)
'Sally is 43 years old.'

Similarly, dots also allow access to object attributes. For example, a Python datetime.date object has year, month, and day attributes, and you can use a dot to access those attributes in a Django template:

>>> from django.template import Template, Context
>>> import datetime
>>> d = datetime.date(2017, 5, 2)
>>> d.year
2017
>>> d.month
5
>>> d.day
2
>>> t = Template('The month is {{ date.month }} and the year is {{ date.year }}.')
>>> c = Context({'date': d})
>>> t.render(c)
'The month is 5 and the year is 2017.'

This example uses a custom class, demonstrating that variable dots also allow attribute access on arbitrary objects:

>>> from django.template import Template, Context
>>> class Person(object):
... def __init__(self, first_name, last_name):
... self.first_name, self.last_name = first_name, last_name
>>> t = Template('Hello, {{ person.first_name }} {{ person.last_name }}.')
>>> c = Context({'person': Person('John', 'Smith')})
>>> t.render(c)
'Hello, John Smith.'

Dots can also refer to methods of objects. For example, each Python string has the methods upper() and isdigit(), and you can call those in Django templates using the same dot syntax:

>>> from django.template import Template, Context
>>> t = Template('{{ var }} -- {{ var.upper }} -- {{ var.isdigit }}')
>>> t.render(Context({'var': 'hello'}))
'hello -- HELLO -- False'
>>> t.render(Context({'var': '123'}))
'123 -- 123 -- True'

Note that you do not include parentheses in the method calls. Also, it’s not possible to pass arguments to the methods; you can only call methods that have no required arguments. Finally, dots are also used to access list indices, for example:

>>> from django.template import Template, Context
>>> t = Template('Item 2 is {{ items.2 }}.')
>>> c = Context({'items': ['apples', 'bananas', 'carrots']})
>>> t.render(c)
'Item 2 is carrots.'

Negative list indices are not allowed. For example, the template variable {{ items.-1 }} would cause a TemplateSyntaxError. Python lists have 0-based indices. The first item is at index 0, the second is at index 1, and so on.

Dot lookups can be summarized like this: when the template system encounters a dot in a variable name, it tries the following lookups, in this order:

• Dictionary lookup (e.g., foo[“bar”])
• Attribute lookup (e.g., foo.bar)
• Method call (e.g., foo.bar())
• List-index lookup (e.g., foo[2])

The system uses the first lookup type that works. It’s short-circuit logic. Dot lookups can be nested multiple levels deep. For instance, the following example uses {{ person.name.upper }}, which translates into a dictionary lookup (person[‘name’]) and then a method call (upper()):

>>> from django.template import Template, Context
>>> person = {'name': 'Sally', 'age': '43'}
>>> t = Template('{{ person.name.upper }} is {{ person.age }} years old.')
>>> c = Context({'person': person})
>>> t.render(c)
'SALLY is 43 years old.'

Method Call Behavior

Method calls are slightly more complex than the other lookup types. Here are some things to keep in mind:

• If, during the method lookup, a method raises an exception, the exception will be propagated, unless the exception has an attribute silent_variable_failure whose value is True. If the exception does have a silent_variable_failure attribute, the variable will render as the value of the engine’s string_if_invalid configuration option (an empty string, by default). For example:

>>> t = Template("My name is {{ person.first_name }}.")
>>> class PersonClass3:
... def first_name(self):
... raise AssertionError("foo")
>>> p = PersonClass3()
>>> t.render(Context({"person": p}))
Traceback (most recent call last):
...
AssertionError: foo

>>> class SilentAssertionError(Exception):
... silent_variable_failure = True
>>> class PersonClass4:
... def first_name(self):
... raise SilentAssertionError
>>> p = PersonClass4()
>>> t.render(Context({"person": p}))
'My name is .'

• A method call will only work if the method has no required arguments. Otherwise, the system will move to the next lookup type (list-index lookup).
• By design, Django intentionally limits the amount of logic processing available in the template, so it’s not possible to pass arguments to method calls accessed from within templates. Data should be calculated in views and then passed to templates for display.
• Obviously, some methods have side effects, and it would be foolish at best, and possibly even a security hole, to allow the template system to access them.
• Say, for instance, you have a BankAccount object that has a delete() method. If a template includes something like {{ account.delete }}, where account is a BankAccount object, the object would be deleted when the template is rendered! To prevent this, set the function attribute alters_data on the method:

            def delete(self):
                # Delete the account
                delete.alters_data = True

• The template system won’t execute any method marked in this way. Continuing the above example, if a template includes {{ account.delete }} and the delete() method has the alters_data=True, then the delete() method will not be executed when the template is rendered, the engine will instead replace the variable with string_if_invalid.

The dynamically-generated delete() and save() methods on Django model objects get alters_data=true set automatically.

How Invalid Variables Are Handled

Generally, if a variable doesn’t exist, the template system inserts the value of the engine’s string_if_invalid configuration option, which is an empty string by default. For example:

>>> from django.template import Template, Context
>>> t = Template('Your name is {{ name }}.')
>>> t.render(Context())
'Your name is .'
>>> t.render(Context({'var': 'hello'}))
'Your name is .'
>>> t.render(Context({'NAME': 'hello'}))
'Your name is .'
>>> t.render(Context({'Name': 'hello'}))
'Your name is .'

This behaviour is better than raising an exception because it’s intended to be resilient to human error. In this case, all of the lookups failed because variable names have the wrong case or name. In the real world, it’s unacceptable for a web site to become inaccessible due to a small template syntax error.

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