In Ruby, how do I check if method “foo=()” is defined?

The problem is that the foo= method is designed to be used in assignments. You can use defined? in the following way to see what’s going on:

defined?(self.foo=())
#=> nil
defined?(self.foo = "bar")
#=> nil

def foo=(bar)
end

defined?(self.foo=())
#=> "assignment"
defined?(self.foo = "bar")
#=> "assignment"

Compare that to:

def foo
end

defined?(foo)
#=> "method"

To test if the foo= method is defined, you should use respond_to? instead:

respond_to?(:foo=)
#=> false

def foo=(bar)
end

respond_to?(:foo=)
#=> true