Require min length only if field is present

Hi,

I have a seemingly simple problem, but I can't quite get my head around it:

I have a user account editing form, where the user should be able to change the
password if he enters the old one and the new one plus confirmation. The form
is validated through a schema (via the turbogears @validate decorator). This is
the schema:

class ExistingUserSchema(validators.Schema):
    email = validators.All(
        validators.Email(not_empty=True, max=255),
        UniqueEmail)
    old_password = validators.All(
        validators.UnicodeString(max=40),
        ValidPassword)
    password1 = validators.UnicodeString(min=passwd_minlen, minmax=40)
    password2 = validators.UnicodeString()
    chained_validators = [validators.All(
        validators.FieldsMatch('password1', 'password2'),
        validators.RequireIfPresent(required='old_password',
	  present='password1')
    )]

This doesn't work as expected, since the 'min' argument to the 'password1'
validator effectively requires this field. How can I require a field to have a
certain minimum length *only* if it is not empty? Can I accomplish this with
existing validators or do I have to write a custom one (which is the workaround
I'm using now)?

Or more generally, is there a validator, that applies another validator only if
a field is not empty?

Chris

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