Upgrading to Ruby 3.1 causes Psych::DisallowedClass exception when using YAML.load_file

Question

Symbol is also not allowed per default. Therefore, you need to add Symbol to the permitted_classes too when loading a file:

hash = YAML.load_file(
  some_file_name, 
  permitted_classes: [Matrix, OpenStruct, Symbol]
)

See the list of default permitted_classes in Psych.

Or, when using in Ruby on Rails, you can configure what classes Rails should permit globally when using YAML files in your config/application.rb:

config.active_record.yaml_column_permitted_classes += [Matrix, OpenStruct, Symbol]

Note that for internal YAML parsing in Ruby on Rails Symbol is already the default for active_record.yaml_column_permitted_classes.