Null Values

In some applications, and especially in database applications, data variables may alternate between "good" and "null" or "missing" values. A format that is used for good values may be inappropriate for null values. For this reason, the null attribute can be used to specify text to be inserted for null values. Null values are defined as values which:

• Cannot be formatted with the specified format, and
• Are either the special Python value None or are false and yield an empty string when converted to a string.

For example, when showing a monetary value retrieved from a database that is either an invalid number or a missing value, the following variable insertion might be used:

<dtml-var cost fmt="$%.2d" null='n/a'>