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('invalid email address')
Traceback (most recent call last):
...
ValidationError: [u'Enter a valid email 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('')
Traceback (most recent call last):
...
ValidationError: [u'This field is required.']
>>> f.clean(None)
Traceback (most recent call last):
...
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 email 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 /><span class="helptext">100 characters max.</span></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 email 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" /> <span class="helptext">100 characters max.</span></li>
<li>Message: <input type="text" name="message" /></li>
<li>Sender: <input type="text" name="sender" /> A valid email 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" /> <span class="helptext">100 characters max.</span></p>
<p>Message: <input type="text" name="message" /></p>
<p>Sender: <input type="text" name="sender" /> A valid email 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.
The validators argument lets you provide a list of validation functions for this field.
See the validators documentation for more information.
The localize argument enables the localization of form data, input as well as the rendered output.
See the format localization documentation for more information.
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:
If provided, these arguments ensure that the string is at most or at least the given length.
The invalid_choice error message may contain %(value)s, which will be replaced with the selected choice.
Takes one extra required argument:
An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field. This argument accepts the same formats as the choices argument to a model field. See the model field reference documentation on choices for more details.
Just like a ChoiceField, except TypedChoiceField takes two extra arguments, coerce and empty_value.
Takes extra arguments:
A function that takes one argument and returns a coerced value. Examples include the built-in int, float, bool and other types. Defaults to an identity function.
The value to use to represent “empty.” Defaults to the empty string; None is another common choice here. Note that this value will not be coerced by the function given in the coerce argument, so choose it accordingly.
Takes one optional argument:
A list of formats used to attempt to convert a string to a valid datetime.date object.
If no input_formats argument is provided, the default input formats are:
'%Y-%m-%d', # '2006-10-25'
'%m/%d/%Y', # '10/25/2006'
'%m/%d/%y', # '10/25/06'
Additionally, if you specify USE_L10N=False in your settings, the following will also be included in the default input formats:
'%b %d %Y', # 'Oct 25 2006'
'%b %d, %Y', # 'Oct 25, 2006'
'%d %b %Y', # '25 Oct 2006'
'%d %b, %Y', # '25 Oct, 2006'
'%B %d %Y', # 'October 25 2006'
'%B %d, %Y', # 'October 25, 2006'
'%d %B %Y', # '25 October 2006'
'%d %B, %Y', # '25 October, 2006'
See also format localization.
Takes one optional argument:
A list of formats used to attempt to convert a string to a valid datetime.datetime object.
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'
See also format localization.
The max_value and min_value error messages may contain %(limit_value)s, which will be substituted by the appropriate limit.
Takes four optional arguments:
These control the range of values permitted in the field, and should be given as decimal.Decimal values.
The maximum number of digits (those before the decimal point plus those after the decimal point, with leading zeros stripped) permitted in the value.
The maximum number of decimal places permitted.
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.
Has two optional arguments for validation, max_length and allow_empty_file. If provided, these ensure that the file name is at most the given length, and that validation will succeed even if the file content is empty.
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 max_length error refers to the length of the filename. In the error message for that key, %(max)d will be replaced with the maximum filename length and %(length)d will be replaced with the current filename length.
The field allows choosing from files inside a certain directory. It takes three extra arguments; only path is required:
The absolute path to the directory whose contents you want listed. This directory must exist.
If False (the default) only the direct contents of path will be offered as choices. If True, the directory will be descended into recursively and all descendants will be listed as choices.
A regular expression pattern; only files with names matching this expression will be allowed as choices.
Optional. Either True or False. Default is True. Specifies whether files in the specified location should be included. Either this or allow_folders must be True.
Optional. Either True or False. Default is False. Specifies whether folders in the specified location should be included. Either this or allow_files must be True.
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 (PIL) is installed and supports the image formats you use. If you encounter a corrupt image error when you upload an image, it usually means PIL doesn’t understand its format. To fix this, install the appropriate library and reinstall PIL.
When you use an ImageField on a form, you must also remember to bind the file data to the form.
The max_value and min_value error messages may contain %(limit_value)s, which will be substituted by the appropriate limit.
Takes two optional arguments for validation:
These control the range of values permitted in the field.
A field containing either an IPv4 or an IPv6 address.
The IPv6 address normalization follows RFC 4291 section 2.2, including using the IPv4 format suggested in paragraph 3 of that section, like ::ffff:192.0.2.0. For example, 2001:0::0:01 would be normalized to 2001::1, and ::ffff:0a0a:0a0a to ::ffff:10.10.10.10. All characters are converted to lowercase.
Takes two optional arguments:
Limits valid inputs to the specified protocol. Accepted values are both (default), IPv4 or IPv6. Matching is case insensitive.
Unpacks IPv4 mapped addresses like ::ffff:192.0.2.1. If this option is enabled that address would be unpacked to 192.0.2.1. Default is disabled. Can only be used when protocol is set to 'both'.
The invalid_choice error message may contain %(value)s, which will be replaced with the selected choice.
Takes one extra required argument, choices, as for ChoiceField.
Just like a MultipleChoiceField, except TypedMultipleChoiceField takes two extra arguments, coerce and empty_value.
The invalid_choice error message may contain %(value)s, which will be replaced with the selected choice.
Takes two extra arguments, coerce and empty_value, as for TypedChoiceField.
Takes one required argument:
A regular expression specified either as a string or a compiled regular expression object.
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.
This field is intended for use in representing a model SlugField in forms.
Takes one optional argument:
A list of formats used to attempt to convert a string to a valid datetime.time object.
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:
These are the same as CharField.max_length and CharField.min_length.
Takes one extra required argument:
The list of fields that should be used to validate the field’s value (in the order in which they are provided).
>>> f = ComboField(fields=[CharField(max_length=20), EmailField()])
>>> f.clean('test@example.com')
u'test@example.com'
>>> f.clean('longemailaddress@example.com')
Traceback (most recent call last):
...
ValidationError: [u'Ensure this value has at most 20 characters (it has 28).']
Aggregates the logic of multiple fields that together produce a single value.
This field is abstract and must be subclassed. In contrast with the single-value fields, subclasses of MultiValueField must not implement clean() but instead - implement compress().
Takes one extra required argument:
A tuple of fields whose values are cleaned and subsequently combined into a single value. Each value of the field is cleaned by the corresponding field in fields – the first value is cleaned by the first field, the second value is cleaned by the second field, etc. Once all fields are cleaned, the list of clean values is combined into a single value by compress().
Must be a subclass of django.forms.MultiWidget. Default value is TextInput, which probably is not very useful in this case.
Takes a list of valid values and returns a “compressed” version of those values – in a single value. For example, SplitDateTimeField is a subclass which combines a time field and a date field into a datetime object.
This method must be implemented in the subclasses.
Takes two optional arguments:
A list of formats used to attempt to convert a string to a valid datetime.date object.
If no input_date_formats argument is provided, the default input formats for DateField are used.
A list of formats used to attempt to convert a string to a valid datetime.time object.
If no input_time_formats argument is provided, the default input formats for TimeField are used.
Two fields are available for representing relationships between models: ModelChoiceField and ModelMultipleChoiceField. Both of these fields require a single queryset parameter that is used to create the choices for the field. Upon form validation, these fields will place either one model object (in the case of ModelChoiceField) or multiple model objects (in the case of ModelMultipleChoiceField) into the cleaned_data dictionary of the form.
Allows the selection of a single model object, suitable for representing a foreign key. Note that the default widget for ModelChoiceField becomes impractical when the number of entries increases. You should avoid using it for more than 100 items.
A single argument is required:
A QuerySet of model objects from which the choices for the field will be derived, and which will be used to validate the user’s selection.
ModelChoiceField also takes one optional argument:
By default the <select> widget used by ModelChoiceField will have 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).
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
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, and queryset is a required parameter:
A QuerySet of model objects from which the choices for the field will be derived, and which will be used to validate the user’s selection.
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).
Dec 23, 2012