Schema with required and optional fields?

I want my schema to require that certain fields be present in the
input, with other specified fields optional and all other fields
disallowed. Is this possible? By default a schema seems to expect
*all* declared fields to be present in the input, but setting
ignore_key_missing appears to have the result that *no* fields are
treated as required  (and a NotEmpty validator for a field won't make
it required, as it doesn't appear to get called if the field is
missing from the input).

Hamish Lawson

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