code: def isValidFeatureWriter(klass):
      
      if not isclass(klass):
          logger.error("%r is not a class", klass)
          return False
      if not hasattr(klass, "tableTag"):
          logger.error("%r does not have required 'tableTag' attribute", klass)
          return False
      if not hasattr(klass, "write"):
          logger.error("%r does not have a required 'write' method", klass)
          return False
      if (
          getargspec(klass.write).args
          != getargspec(BaseFeatureWriter.write).args
      ):
          logger.error("%r 'write' method has incorrect signature", klass)
          return False
      return True
docs: Return True if 'klass' is a valid feature writer class.
      A valid feature writer class is a class (of type 'type'), that has
      two required attributes:
      1) 'tableTag' (str), which can be "GSUB", "GPOS", or other similar tags.
      2) 'write' (bound method), with the signature matching the same method
         from the BaseFeatureWriter class:
  
             def write(self, font, feaFile, compiler=None)