Forms

Emmett provides the Form class to let you easily create forms for your application.

Let's see how to use it with an example:

from emmett import Field, Form

# create a form
@app.route('/form')
async def a():
    simple_form = await Form({
        'name': Field(),
        'number': Field.int(),
        'type': Field(
            validation={'in': ['type1', 'type2']}
        )
    })
    if simple_form.accepted:
        inserted_number = form.params.number
        #do something
    return dict(form=simple_form)

As you can see, the Form class accepts a dict of Field objects as input, and those are described in the database chapter of the documentation. Forms validate the input of the clients using their fields' validation: when the input passes the validation, the accepted attribute is set to True. The example above shows you that you can use this attribute to do things when clients submit the form, and the submitted values are stored in form.params.

Forms with database entities

Forms are quite handy for inserting or editing data in your database, for this purpose Emmett provides another class: ModelForm. The usage is the same of the form, except that you call it directly from your model:

# create a form for Post model
@app.route('/postform')
async def b():
    form = await Post.form()
    if form.accepted:
        #do something
    return dict(form=form)

where, obviously, the form() method of the models is a shortcut for the ModelForm class.

– Wait, what if I need to edit a record?

You can pass the record as an argument in Model.form():

record = db.Post(id=1)
form = Post.form(record)

If you prefer, you can also use a record id:

form = Post.form(record_id=1)

Here is the complete list of parameters accepted by Form class:

parameter	default	description
_action	`None`	allows you to set the HTML `action` tag of the form
_method	`'POST'`	set the form submit method (GET or POST)
_enctype	`'multipart/form-data'`	allows you to change the encoding type for the submitted data
submit	`'Submit'`	the text to show in the submit button
formstyle	`FormStyle`	the class used to style the form
csrf	`'auto'`	Cross-Site Request Forgery protection
keepvalues	`False`	set if the form should keep the values in the input fields after submit
id_prefix	`None`	allows you to set a prefix for the id of the form fields
onvalidation	`None`	set an additional validation for the form
upload	`None`	define a URL for download uploaded fields

ModelForm class add some parameters to the Form ones:

parameter	description
record	as we seen above, set a record to edit
record_id	alternative to `record` using id
fields	list of fields (names) to show in the record
exclude_fields	list of fields (names) not to be included in the form

Note: the fields and exclude_fields parameters should not be used together. If you need to hide just a few fields, you'd better using the exclude_fields, and you should use fields if you have to show only few table fields. The advantages of these parameters are lost if you use both.

Uploads with forms

As we saw above, the upload parameter of forms needs an URL for download. Let's focus a bit on uploads and see an example to completely understand this requirement.

Let's say you want to handle the upload of avatar images from your user. So, in your model you would have an upload field:

avatar = Field.upload()

and the forms produced by Emmett will handle uploads for you. How would you display this image in your template? You need a streaming function like this:

from emmett.helpers import stream_dbfile 

@app.route("/download/<str:filename>")
async def download(filename):
    stream_dbfile(db, filename)

and then, in your template, you can create an img tag pointing to the download function you've just exposed:

<img src="{{ =url('download', record.avatar) }}" />

The upload parameter of Form class has the same purpose: when you edit an existent record the form will display the image or file link for the existing one uploaded. In this example you would do:

record = db.Post(id=someid)
form = Post.form(record, upload=url('download'))

Custom validation

The onvalidation parameter of forms allows you to add custom validation logic to your form. You can pass a callable function, and it will be invoked after the form has processed the fields validators. This means that your function will be invoked only if there weren't errors with the fields validators.

Let's see what we're talking about with an example:

@app.route("/myform")
async def myform():
    def process_form(form):
        if form.params.double != form.params.number*2:
            form.errors.double = "Double is incorrect!"

    form = await Form(
        number=Field.int(), 
        double=Field.int(),
        onvalidation=process_form
    )
    return dict(form=form)

where the form checks if the second number is double the first and returns an error if the input is wrong.

You've just learned how to use the onvalidation parameter and that you can store errors in form.errors, which is a sdict object like form.params.

Also, you understood that Form also accepts Field objects as arguments.

Customizing forms

Good applications also need good styles. This is why Emmett forms allows you to set a specific style with the formstyle attribute. But how should you edit the style of your form?

Well, in Emmett, the style of a form is decided by the FormStyle class.

Creating your style

sub-section under development

Custom widgets

sub-section under development

Forms with database entities
Custom validation
Customizing forms