When you create a Form class, the most important part is defining the fields of the form. Each field has custom validation logic, along with a few other hooks.
Although the primary way you’ll use Field classes is in Form classes, you can also instantiate them and use them directly to get a better idea of how they work. Each Field instance has a clean() method, which takes a single argument and either raises a django.forms.ValidationError exception or returns the clean value:
>>> from django import forms
>>> f = forms.EmailField()
>>> f.clean('foo@example.com')
u'foo@example.com'
>>> f.clean(u'foo@example.com')
u'foo@example.com'
>>> f.clean('invalid e-mail address')
...
ValidationError: [u'Enter a valid e-mail address.']
Each Field class constructor takes at least these arguments. Some Field classes take additional, field-specific arguments, but the following should always be accepted:
By default, each Field class assumes the value is required, so if you pass an empty value -- either None or the empty string ("") -- then clean() will raise a ValidationError exception:
>>> f = forms.CharField()
>>> f.clean('foo')
u'foo'
>>> f.clean('')
...
ValidationError: [u'This field is required.']
>>> f.clean(None)
...
ValidationError: [u'This field is required.']
>>> f.clean(' ')
u' '
>>> f.clean(0)
u'0'
>>> f.clean(True)
u'True'
>>> f.clean(False)
u'False'
To specify that a field is not required, pass required=False to the Field constructor:
>>> f = forms.CharField(required=False)
>>> f.clean('foo')
u'foo'
>>> f.clean('')
u''
>>> f.clean(None)
u''
>>> f.clean(0)
u'0'
>>> f.clean(True)
u'True'
>>> f.clean(False)
u'False'
If a Field has required=False and you pass clean() an empty value, then clean() will return a normalized empty value rather than raising ValidationError. For CharField, this will be a Unicode empty string. For other Field classes, it might be None. (This varies from field to field.)
The label argument lets you specify the "human-friendly" label for this field. This is used when the Field is displayed in a Form.
As explained in "Outputting forms as HTML" above, the default label for a Field is generated from the field name by converting all underscores to spaces and upper-casing the first letter. Specify label if that default behavior doesn't result in an adequate label.
Here's a full example Form that implements label for two of its fields. We've specified auto_id=False to simplify the output:
>>> class CommentForm(forms.Form):
... name = forms.CharField(label='Your name')
... url = forms.URLField(label='Your Web site', required=False)
... comment = forms.CharField()
>>> f = CommentForm(auto_id=False)
>>> print f
<tr><th>Your name:</th><td><input type="text" name="name" /></td></tr>
<tr><th>Your Web site:</th><td><input type="text" name="url" /></td></tr>
<tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>
The initial argument lets you specify the initial value to use when rendering this Field in an unbound Form.
To specify dynamic initial data, see the Form.initial parameter.
The use-case for this is when you want to display an "empty" form in which a field is initialized to a particular value. For example:
>>> class CommentForm(forms.Form):
... name = forms.CharField(initial='Your name')
... url = forms.URLField(initial='http://')
... comment = forms.CharField()
>>> f = CommentForm(auto_id=False)
>>> print f
<tr><th>Name:</th><td><input type="text" name="name" value="Your name" /></td></tr>
<tr><th>Url:</th><td><input type="text" name="url" value="http://" /></td></tr>
<tr><th>Comment:</th><td><input type="text" name="comment" /></td></tr>
You may be thinking, why not just pass a dictionary of the initial values as data when displaying the form? Well, if you do that, you'll trigger validation, and the HTML output will include any validation errors:
>>> class CommentForm(forms.Form):
... name = forms.CharField()
... url = forms.URLField()
... comment = forms.CharField()
>>> default_data = {'name': 'Your name', 'url': 'http://'}
>>> f = CommentForm(default_data, auto_id=False)
>>> print f
<tr><th>Name:</th><td><input type="text" name="name" value="Your name" /></td></tr>
<tr><th>Url:</th><td><ul class="errorlist"><li>Enter a valid URL.</li></ul><input type="text" name="url" value="http://" /></td></tr>
<tr><th>Comment:</th><td><ul class="errorlist"><li>This field is required.</li></ul><input type="text" name="comment" /></td></tr>
This is why initial values are only displayed for unbound forms. For bound forms, the HTML output will use the bound data.
Also note that initial values are not used as "fallback" data in validation if a particular field's value is not given. initial values are only intended for initial form display:
>>> class CommentForm(forms.Form):
... name = forms.CharField(initial='Your name')
... url = forms.URLField(initial='http://')
... comment = forms.CharField()
>>> data = {'name': '', 'url': '', 'comment': 'Foo'}
>>> f = CommentForm(data)
>>> f.is_valid()
False
# The form does *not* fall back to using the initial values.
>>> f.errors
{'url': [u'This field is required.'], 'name': [u'This field is required.']}
Instead of a constant, you can also pass any callable:
>>> import datetime
>>> class DateForm(forms.Form):
... day = forms.DateField(initial=datetime.date.today)
>>> print DateForm()
<tr><th>Day:</th><td><input type="text" name="day" value="12/23/2008" /><td></tr>
The callable will be evaluated only when the unbound form is displayed, not when it is defined.
The widget argument lets you specify a Widget class to use when rendering this Field. See Widgets for more information.
The help_text argument lets you specify descriptive text for this Field. If you provide help_text, it will be displayed next to the Field when the Field is rendered by one of the convenience Form methods (e.g., as_ul()).
Here's a full example Form that implements help_text for two of its fields. We've specified auto_id=False to simplify the output:
>>> class HelpTextContactForm(forms.Form):
... subject = forms.CharField(max_length=100, help_text='100 characters max.')
... message = forms.CharField()
... sender = forms.EmailField(help_text='A valid e-mail address, please.')
... cc_myself = forms.BooleanField(required=False)
>>> f = HelpTextContactForm(auto_id=False)
>>> print f.as_table()
<tr><th>Subject:</th><td><input type="text" name="subject" maxlength="100" /><br />100 characters max.</td></tr>
<tr><th>Message:</th><td><input type="text" name="message" /></td></tr>
<tr><th>Sender:</th><td><input type="text" name="sender" /><br />A valid e-mail address, please.</td></tr>
<tr><th>Cc myself:</th><td><input type="checkbox" name="cc_myself" /></td></tr>
>>> print f.as_ul()
<li>Subject: <input type="text" name="subject" maxlength="100" /> 100 characters max.</li>
<li>Message: <input type="text" name="message" /></li>
<li>Sender: <input type="text" name="sender" /> A valid e-mail address, please.</li>
<li>Cc myself: <input type="checkbox" name="cc_myself" /></li>
>>> print f.as_p()
<p>Subject: <input type="text" name="subject" maxlength="100" /> 100 characters max.</p>
<p>Message: <input type="text" name="message" /></p>
<p>Sender: <input type="text" name="sender" /> A valid e-mail address, please.</p>
<p>Cc myself: <input type="checkbox" name="cc_myself" /></p>
The error_messages argument lets you override the default messages that the field will raise. Pass in a dictionary with keys matching the error messages you want to override. For example, here is the default error message:
>>> generic = forms.CharField()
>>> generic.clean('')
Traceback (most recent call last):
...
ValidationError: [u'This field is required.']
And here is a custom error message:
>>> name = forms.CharField(error_messages={'required': 'Please enter your name'})
>>> name.clean('')
Traceback (most recent call last):
...
ValidationError: [u'Please enter your name']
In the built-in Field classes section below, each Field defines the error message keys it uses.
Naturally, the forms library comes with a set of Field classes that represent common validation needs. This section documents each built-in field.
For each field, we describe the default widget used if you don't specify widget. We also specify the value returned when you provide an empty value (see the section on required above to understand what that means).
Note
Since all Field subclasses have required=True by default, the validation condition here is important. If you want to include a boolean in your form that can be either True or False (e.g. a checked or unchecked checkbox), you must remember to pass in required=False when creating the BooleanField.
Has two optional arguments for validation:
Takes one extra required argument:
Just like a ChoiceField, except TypedChoiceField takes an extra coerce argument.
Takes extra arguments:
Takes one optional argument:
If no input_formats argument is provided, the default input formats are:
'%Y-%m-%d', '%m/%d/%Y', '%m/%d/%y', # '2006-10-25', '10/25/2006', '10/25/06'
'%b %d %Y', '%b %d, %Y', # 'Oct 25 2006', 'Oct 25, 2006'
'%d %b %Y', '%d %b, %Y', # '25 Oct 2006', '25 Oct, 2006'
'%B %d %Y', '%B %d, %Y', # 'October 25 2006', 'October 25, 2006'
'%d %B %Y', '%d %B, %Y', # '25 October 2006', '25 October, 2006'
Takes one optional argument:
If no input_formats argument is provided, the default input formats are:
'%Y-%m-%d %H:%M:%S', # '2006-10-25 14:30:59'
'%Y-%m-%d %H:%M', # '2006-10-25 14:30'
'%Y-%m-%d', # '2006-10-25'
'%m/%d/%Y %H:%M:%S', # '10/25/2006 14:30:59'
'%m/%d/%Y %H:%M', # '10/25/2006 14:30'
'%m/%d/%Y', # '10/25/2006'
'%m/%d/%y %H:%M:%S', # '10/25/06 14:30:59'
'%m/%d/%y %H:%M', # '10/25/06 14:30'
'%m/%d/%y', # '10/25/06'
Takes four optional arguments:
Has two optional arguments for validation, max_length and min_length. If provided, these arguments ensure that the string is at most or at least the given length.
To learn more about the UploadedFile object, see the file uploads documentation.
When you use a FileField in a form, you must also remember to bind the file data to the form.
The field allows choosing from files inside a certain directory. It takes three extra arguments; only path is required:
Takes two optional arguments for validation, max_value and min_value. These control the range of values permitted in the field.
Using an ImageField requires that the Python Imaging Library is installed.
When you use an ImageField on a form, you must also remember to bind the file data to the form.
Takes two optional arguments for validation:
Takes one extra argument, choices, as for ChoiceField.
Takes one required argument:
Also takes max_length and min_length, which work just as they do for CharField.
The optional argument error_message is also accepted for backwards compatibility. The preferred way to provide an error message is to use the error_messages argument, passing a dictionary with 'invalid' as a key and the error message as the value.
Takes one optional argument:
If no input_formats argument is provided, the default input formats are:
'%H:%M:%S', # '14:30:59'
'%H:%M', # '14:30'
Takes the following optional arguments:
The following are not yet documented.
Takes two optional arguments:
If no input_date_formats argument is provided, the default input formats for DateField are used.
If no input_time_formats argument is provided, the default input formats for TimeField are used.
For representing relationships between models, two fields are provided which can derive their choices from a QuerySet:
These fields place one or more model objects into the cleaned_data dictionary of forms in which they're used. Both of these fields have an additional required argument:
Allows the selection of a single model object, suitable for representing a foreign key.
The __unicode__ method of the model will be called to generate string representations of the objects for use in the field's choices; to provide customized representations, subclass ModelChoiceField and override label_from_instance. This method will receive a model object, and should return a string suitable for representing it. For example:
class MyModelChoiceField(ModelChoiceField):
def label_from_instance(self, obj):
return "My Object #%i" % obj.id
By default the <select> widget used by ModelChoiceField will have a an empty choice at the top of the list. You can change the text of this label (which is "---------" by default) with the empty_label attribute, or you can disable the empty label entirely by setting empty_label to None:
# A custom empty label
field1 = forms.ModelChoiceField(queryset=..., empty_label="(Nothing)")
# No empty label
field2 = forms.ModelChoiceField(queryset=..., empty_label=None)
Note that if a ModelChoiceField is required and has a default initial value, no empty choice is created (regardless of the value of empty_label).
Allows the selection of one or more model objects, suitable for representing a many-to-many relation. As with ModelChoiceField, you can use label_from_instance to customize the object representations.
If the built-in Field classes don't meet your needs, you can easily create custom Field classes. To do this, just create a subclass of django.forms.Field. Its only requirements are that it implement a clean() method and that its __init__() method accept the core arguments mentioned above (required, label, initial, widget, help_text).
Sep 20, 2009