#default_value not being displayed in form
Assuming Drupal 7...

The signature for form functions needs to follow a strict pattern; Drupal will pass along $form and $form_state as the first two args, and anything else as subsequent args.

So assuming $str should be the first custom arg to the form, the signature should look like this:

function myfunc($form, &amp;$form_state, $str) {