Handling a Checkbox in FormEncode
++++++++++++++++++++++++++++++++++++

:Posted: 2007-09-19 22:26
:Tags: Pylons, Python

Checkboxes are tricky in web applications because often you want a value of either ``True`` or ``False`` to be associated with a variable and you want to handle that with a checkbox which is either ticked or not but an unticked checkbox doesn't submit a value at all which can be a pain for validation.

FormEncode can handle this situation like this::

    >>> import formencode
    >>> 
    >>> class MySchema(formencode.Schema):
    ...     ticked = formencode.validators.StringBoolean(if_missing=False)

You can then do::

    >>> MySchema().to_python({})
    {'ticked': False}
    >>> MySchema().to_python({'ticked': 'true'})
    {'ticked': True}

Both these states are handled fine by HTMLFill too::

    >>> import formencode.htmlfill
    >>> formencode.htmlfill.render('''<input type="checkbox" name="ticked" />''', {'ticked':False})
    '<input type="checkbox" name="ticked" />'
    >>> formencode.htmlfill.render('''<input type="checkbox" name="ticked" />''', {'ticked':True})
    '<input type="checkbox" name="ticked" checked="checked" />'

Occasionally you run into a situation where if a checkbox is ticked you want to run some extra validation. For this you can use FormEncode's ``RequireIfMissing`` or ``  RequireIfPresent`` validators `documented here <http://formencode.org/module-formencode.validators.html>`_.