Colander schemas for query string parsing

schema.py

class ColanderSearchParamsSchema(colander.Schema):
    # Fields that affect search result number and ordering.
    sort = colander.SchemaNode(colander.String(),
                               validator=colander.OneOf(["updated", "created", "id", "group", "user"]),
                               missing="updated")

    order = colander.SchemaNode(colander.String(),
                                validator=colander.OneOf(["asc", "desc"]),
                                missing="desc")

    limit = colander.SchemaNode(colander.Integer(),
                                validator=colander.Range(0, 200),
                                missing=20)

    offset = colander.SchemaNode(colander.Integer(),
                                 validator=colander.Range(0, 2000),
                                 missing=0)

    # Undocumented options
    _separate_replies = colander.SchemaNode(colander.Boolean(), missing=False)

    # Specific fields that can be searched by.
    any = colander.SchemaNode(colander.String(), missing=colander.drop)
    group = colander.SchemaNode(colander.String(),
                                missing=colander.drop)
    quote = colander.SchemaNode(colander.String(), missing=colander.drop)
    references = colander.SchemaNode(colander.String(), missing=colander.drop)
    tag = colander.SchemaNode(colander.String(), missing=colander.drop)
    tags = colander.SchemaNode(colander.String(), missing=colander.drop)
    uri = colander.SchemaNode(colander.String(), missing=colander.drop)
    url = colander.SchemaNode(colander.String(), missing=colander.drop)
    user = colander.SchemaNode(colander.String(), missing=colander.drop)

usage.py

from schemas import ColanderSearchParamsSchema

from webob.multidict import MultiDict

params = MultiDict({"order": "asc", "limit": "5", "unknown": "foobar", "_separate_replies": "true"})

print("Input params: ", params)
schema = ColanderSearchParamsSchema()
parsed_params = schema.deserialize(params)
print("Parsed params: ", parsed_params)

# TODO - Figure out how to handle repeat-able fields ("text", "tag", "quote" etc)