Form Assets (the Media class)¶
Rendering an attractive and easy-to-use Web form requires more than just HTML - it also requires CSS stylesheets, and if you want to use fancy “Web2.0” widgets, you may also need to include some JavaScript on each page. The exact combination of CSS and JavaScript that is required for any given page will depend upon the widgets that are in use on that page.
This is where asset definitions come in. Django allows you to associate different files – like stylesheets and scripts – with the forms and widgets that require those assets. For example, if you want to use a calendar to render DateFields, you can define a custom Calendar widget. This widget can then be associated with the CSS and JavaScript that is required to render the calendar. When the Calendar widget is used on a form, Django is able to identify the CSS and JavaScript files that are required, and provide the list of file names in a form suitable for inclusion on your Web page.
Assets and Django Admin
The Django Admin application defines a number of customized widgets for calendars, filtered selections, and so on. These widgets define asset requirements, and the Django Admin uses the custom widgets in place of the Django defaults. The Admin templates will only include those files that are required to render the widgets on any given page.
If you like the widgets that the Django Admin application uses,
feel free to use them in your own application! They’re all stored
in django.contrib.admin.widgets.
Which JavaScript toolkit?
Many JavaScript toolkits exist, and many of them include widgets (such as calendar widgets) that can be used to enhance your application. Django has deliberately avoided blessing any one JavaScript toolkit. Each toolkit has its own relative strengths and weaknesses - use whichever toolkit suits your requirements. Django is able to integrate with any JavaScript toolkit.
Assets as a static definition¶
The easiest way to define assets is as a static definition. Using this
method, the declaration is an inner Media class. The properties of the
inner class define the requirements.
Here’s an example:
from django import forms
class CalendarWidget(forms.TextInput):
class Media:
css = {
'all': ('pretty.css',)
}
js = ('animations.js', 'actions.js')
This code defines a CalendarWidget, which will be based on TextInput.
Every time the CalendarWidget is used on a form, that form will be directed
to include the CSS file pretty.css, and the JavaScript files
animations.js and actions.js.
This static definition is converted at runtime into a widget property
named media. The list of assets for a CalendarWidget instance
can be retrieved through this property:
>>> w = CalendarWidget()
>>> print(w.media)
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://static.example.com/actions.js"></script>
Here’s a list of all possible Media options. There are no required options.
css¶
A dictionary describing the CSS files required for various forms of output media.
The values in the dictionary should be a tuple/list of file names. See the section on paths for details of how to specify paths to these files.
The keys in the dictionary are the output media types. These are the same types accepted by CSS files in media declarations: ‘all’, ‘aural’, ‘braille’, ‘embossed’, ‘handheld’, ‘print’, ‘projection’, ‘screen’, ‘tty’ and ‘tv’. If you need to have different stylesheets for different media types, provide a list of CSS files for each output medium. The following example would provide two CSS options – one for the screen, and one for print:
class Media:
css = {
'screen': ('pretty.css',),
'print': ('newspaper.css',)
}
If a group of CSS files are appropriate for multiple output media types, the dictionary key can be a comma separated list of output media types. In the following example, TV’s and projectors will have the same media requirements:
class Media:
css = {
'screen': ('pretty.css',),
'tv,projector': ('lo_res.css',),
'print': ('newspaper.css',)
}
If this last CSS definition were to be rendered, it would become the following HTML:
<link href="http://static.example.com/pretty.css" type="text/css" media="screen" rel="stylesheet">
<link href="http://static.example.com/lo_res.css" type="text/css" media="tv,projector" rel="stylesheet">
<link href="http://static.example.com/newspaper.css" type="text/css" media="print" rel="stylesheet">
js¶
A tuple describing the required JavaScript files. See the section on paths for details of how to specify paths to these files.
extend¶
A boolean defining inheritance behavior for Media declarations.
By default, any object using a static Media definition will
inherit all the assets associated with the parent widget. This occurs
regardless of how the parent defines its own requirements. For
example, if we were to extend our basic Calendar widget from the
example above:
>>> class FancyCalendarWidget(CalendarWidget):
... class Media:
... css = {
... 'all': ('fancy.css',)
... }
... js = ('whizbang.js',)
>>> w = FancyCalendarWidget()
>>> print(w.media)
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
<link href="http://static.example.com/fancy.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://static.example.com/actions.js"></script>
<script src="http://static.example.com/whizbang.js"></script>
The FancyCalendar widget inherits all the assets from its parent
widget. If you don’t want Media to be inherited in this way, add
an extend=False declaration to the Media declaration:
>>> class FancyCalendarWidget(CalendarWidget):
... class Media:
... extend = False
... css = {
... 'all': ('fancy.css',)
... }
... js = ('whizbang.js',)
>>> w = FancyCalendarWidget()
>>> print(w.media)
<link href="http://static.example.com/fancy.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/whizbang.js"></script>
If you require even more control over inheritance, define your assets using a dynamic property. Dynamic properties give you complete control over which files are inherited, and which are not.
Media as a dynamic property¶
If you need to perform some more sophisticated manipulation of asset
requirements, you can define the media property directly. This is
done by defining a widget property that returns an instance of
forms.Media. The constructor for forms.Media accepts css
and js keyword arguments in the same format as that used in a
static media definition.
For example, the static definition for our Calendar Widget could also be defined in a dynamic fashion:
class CalendarWidget(forms.TextInput):
@property
def media(self):
return forms.Media(css={'all': ('pretty.css',)},
js=('animations.js', 'actions.js'))
See the section on Media objects for more details on how to construct
return values for dynamic media properties.
Paths in asset definitions¶
Paths used to specify assets can be either relative or absolute. If a
path starts with /, http:// or https://, it will be
interpreted as an absolute path, and left as-is. All other paths will
be prepended with the value of the appropriate prefix. If the
django.contrib.staticfiles app is installed, it will be used to serve
assets.
Whether or not you use django.contrib.staticfiles, the
STATIC_URL and STATIC_ROOT settings are required to
render a complete web page.
To find the appropriate prefix to use, Django will check if the
STATIC_URL setting is not None and automatically fall back
to using MEDIA_URL. For example, if the MEDIA_URL for
your site was 'http://uploads.example.com/' and STATIC_URL
was None:
>>> from django import forms
>>> class CalendarWidget(forms.TextInput):
... class Media:
... css = {
... 'all': ('/css/pretty.css',),
... }
... js = ('animations.js', 'http://othersite.com/actions.js')
>>> w = CalendarWidget()
>>> print(w.media)
<link href="/css/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="http://uploads.example.com/animations.js"></script>
<script src="http://othersite.com/actions.js"></script>
But if STATIC_URL is 'http://static.example.com/':
>>> w = CalendarWidget()
>>> print(w.media)
<link href="/css/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://othersite.com/actions.js"></script>
Or if staticfiles is configured using the
ManifestStaticFilesStorage:
>>> w = CalendarWidget()
>>> print(w.media)
<link href="/css/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="https://static.example.com/animations.27e20196a850.js"></script>
<script src="http://othersite.com/actions.js"></script>
Media objects¶
When you interrogate the media attribute of a widget or form, the
value that is returned is a forms.Media object. As we have already
seen, the string representation of a Media object is the HTML
required to include the relevant files in the <head> block of your
HTML page.
However, Media objects have some other interesting properties.
Subsets of assets¶
If you only want files of a particular type, you can use the subscript operator to filter out a medium of interest. For example:
>>> w = CalendarWidget()
>>> print(w.media)
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://static.example.com/actions.js"></script>
>>> print(w.media['css'])
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
When you use the subscript operator, the value that is returned is a
new Media object – but one that only contains the media of interest.
Combining Media objects¶
Media objects can also be added together. When two Media objects are
added, the resulting Media object contains the union of the assets
specified by both:
>>> from django import forms
>>> class CalendarWidget(forms.TextInput):
... class Media:
... css = {
... 'all': ('pretty.css',)
... }
... js = ('animations.js', 'actions.js')
>>> class OtherWidget(forms.TextInput):
... class Media:
... js = ('whizbang.js',)
>>> w1 = CalendarWidget()
>>> w2 = OtherWidget()
>>> print(w1.media + w2.media)
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://static.example.com/actions.js"></script>
<script src="http://static.example.com/whizbang.js"></script>
Order of assets¶
The order in which assets are inserted into the DOM is often important. For
example, you may have a script that depends on jQuery. Therefore, combining
Media objects attempts to preserve the relative order in which assets are
defined in each Media class.
For example:
>>> from django import forms
>>> class CalendarWidget(forms.TextInput):
... class Media:
... js = ('jQuery.js', 'calendar.js', 'noConflict.js')
>>> class TimeWidget(forms.TextInput):
... class Media:
... js = ('jQuery.js', 'time.js', 'noConflict.js')
>>> w1 = CalendarWidget()
>>> w2 = TimeWidget()
>>> print(w1.media + w2.media)
<script src="http://static.example.com/jQuery.js"></script>
<script src="http://static.example.com/calendar.js"></script>
<script src="http://static.example.com/time.js"></script>
<script src="http://static.example.com/noConflict.js"></script>
Combining Media objects with assets in a conflicting order results in a
MediaOrderConflictWarning.
Media on Forms¶
Widgets aren’t the only objects that can have media definitions –
forms can also define media. The rules for media definitions
on forms are the same as the rules for widgets: declarations can be
static or dynamic; path and inheritance rules for those declarations
are exactly the same.
Regardless of whether you define a media declaration, all Form
objects have a media property. The default value for this property
is the result of adding the media definitions for all widgets that
are part of the form:
>>> from django import forms
>>> class ContactForm(forms.Form):
... date = DateField(widget=CalendarWidget)
... name = CharField(max_length=40, widget=OtherWidget)
>>> f = ContactForm()
>>> f.media
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://static.example.com/actions.js"></script>
<script src="http://static.example.com/whizbang.js"></script>
If you want to associate additional assets with a form – for example,
CSS for form layout – add a Media declaration to the form:
>>> class ContactForm(forms.Form):
... date = DateField(widget=CalendarWidget)
... name = CharField(max_length=40, widget=OtherWidget)
...
... class Media:
... css = {
... 'all': ('layout.css',)
... }
>>> f = ContactForm()
>>> f.media
<link href="http://static.example.com/pretty.css" type="text/css" media="all" rel="stylesheet">
<link href="http://static.example.com/layout.css" type="text/css" media="all" rel="stylesheet">
<script src="http://static.example.com/animations.js"></script>
<script src="http://static.example.com/actions.js"></script>
<script src="http://static.example.com/whizbang.js"></script>
