Fixed #14638 -- Clarified model Field.to_python() docs.

    class HandField(with_metaclass(models.SubfieldBase, models.Field)):

        ...

This ensures that the :meth:`.to_python` method, documented below, will always

be called when the attribute is initialized.

This ensures that the :meth:`.to_python` method will always be called when the

attribute is initialized.

ModelForms and custom fields

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``ModelForm``\s and custom fields

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you use :class:`~django.db.models.SubfieldBase`, :meth:`.to_python`

will be called every time an instance of the field is assigned a

value. This means that whenever a value may be assigned to the field,

you need to ensure that it will be of the correct datatype, or that

you handle any exceptions.

If you use :class:`~django.db.models.SubfieldBase`, :meth:`.to_python` will be

called every time an instance of the field is assigned a value (in addition to

its usual call when retrieving the value from the database). This means that

whenever a value may be assigned to the field, you need to ensure that it will

be of the correct datatype, or that you handle any exceptions.

This is especially important if you use :doc:`ModelForms

</topics/forms/modelforms>`. When saving a ModelForm, Django will use