Python SQLite parameter substitution with wildcards in LIKE

The quotes protect either ? or :name from being taken as a place-holder -- they're taken literally. You need to place the percent signs around the string you're passing, and use the plain placeholder without quotes. I.e.:

self.cursor.execute(
  "select string from stringtable where string like ? and type = ?",
  ('%'+searchstr+'%', type))

Note that neither ? is in quotes -- and that's exactly as it should be for them to be taken as placeholders.