For text, password, submit, reset,
hidden, and button input field types the expression
in the expr attribute is evaluated and the result is used to
generate the value (5.2.2.11) attribute. If the
result is None
then no value attribute will be
written.
For radio and checkbox input field types the expression in the expr attribute is evaluated and the result is compared with the generated value attribute to determine whether or not the checked (5.2.2.2) attribute should be written.
Refer to the documentation for individual input types for more details.