The Warning module contains a single method named warn, and the module extends itself, making Warning.warn available. Warning.warn is called for all warnings issued by Ruby. By default, warnings are printed to $stderr.

Changing the behavior of Warning.warn is useful to customize how warnings are handled by Ruby, for instance by filtering some warnings, and/or outputting warnings somewhere other than $stderr.

If you want to change the behavior of Warning.warn you should use +Warning.extend(MyNewModuleWithWarnMethod)+ and you can use `super` to get the default behavior of printing the warning to $stderr.


module MyWarningFilter
  def warn(message, category: nil, **kwargs)
    if /some warning I want to ignore/.match?(message)
      # ignore
Warning.extend MyWarningFilter

You should never redefine Warning#warn (the instance method), as that will then no longer provide a way to use the default behavior.

The warning gem provides convenient ways to customize Warning.warn.

Class Methods

Returns the flag to show the warning messages for category. Supported categories are:


deprecation warnings

  • assignment of non-nil value to $, and $;

  • keyword arguments

  • proc/lambda without block



experimental features

  • Pattern matching

Sets the warning flags for category. See Warning.[] for the categories.

Instance Methods

Writes warning message msg to $stderr. This method is called by Ruby for all emitted warnings. A category may be included with the warning.

See the documentation of the Warning module for how to customize this.