[Zope3-dev] default and required analysis

[Jim Fulton]

Based on Martijn's analysis and Steve's comments, I proposed the following:

- If a field is in the schema, objects that implement the schema must
   have a corresponding property implementing the field.

- If a field is required, then all values of properties implementing the field
   must satisfy the field's constraints.

- If a field is not required, then a property implementing a field may have a
   field-defined missing value that does not satisfy the field's constraints.
   The default missing value will be None, but fields may provide different missing
   values.

- Fields may define default values. An application (like a form) can use this
   default value to provide a value when a value is needed and no other value is
   available.  The default value may be the field-defined missing value or
   a value that satisfies the field's constraints.

   If a field is required and the default value does not satisfy the field's
   constraints, then the default value cannot be used to provide a value for a
   property implementing the field.


Note that, when collecting user input, you usually need to know if a user
specified anything. How you do this need not have anything to do with the
field definition.

Jim

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