```ruby
module PyCall
  class PyError < StandardError
    def self.fetch
      ptrs = FFI::MemoryPointer.new(:pointer, 3)
      ptype      = ptrs + 0 * ptrs.type_size
      pvalue     = ptrs + 1 * ptrs.type_size
      ptraceback = ptrs + 2 * ptrs.type_size
      LibPython.PyErr_Fetch(ptype, pvalue, ptraceback)
      LibPython.PyErr_NormalizeException(ptype, pvalue, ptraceback)
      type = Conversions.to_ruby(ptype.read(:pointer))
      value = Conversions.to_ruby(pvalue.read(:pointer))
      traceback = Conversions.to_ruby(ptraceback.read(:pointer))
      new(type, value, traceback)
    end

    def initialize(type, value, traceback)
      @type = type
      @value = value
      @traceback = traceback
      super("Error occurred in Python")
    end

    attr_reader :type, :value, :traceback

    def to_s
      "#{type}: #{value}".tap do |msg|
        unless traceback.nil? || traceback.null?
          if (o = PyCall.format_traceback(traceback))
            msg.concat("\n", *o)
          end
        end
      end
    end
  end
end
```

Fix for ruby < 2.4
user
```ruby
module PyCall
  class PyError < StandardError
    def self.fetch
      ptrs = FFI::MemoryPointer.new(:pointer, 3)
      ptype      = ptrs + 0 * ptrs.type_size
      pvalue     = ptrs + 1 * ptrs.type_size
      ptraceback = ptrs + 2 * ptrs.type_size
      LibPython.PyErr_Fetch(ptype, pvalue, ptraceback)
      LibPython.PyErr_NormalizeException(ptype, pvalue, ptraceback)
      type = Conversions.to_ruby(ptype.read(:pointer))
      value = Conversions.to_ruby(pvalue.read(:pointer))
      traceback = Conversions.to_ruby(ptraceback.read(:pointer))
      new(type, value, traceback)
    end

    def initialize(type, value, traceback)
      @type = type
      @value = value
      @traceback = traceback
      super("Error occurred in Python")
    end

    attr_reader :type, :value, :traceback

    def to_s
      "#{type}: #{value}".tap do |msg|
        unless traceback.nil? || traceback.null?
          if (strs = PyCall.format_traceback(traceback))
            msg << "\n"
            strs.each {|s| msg << s }
          end
        end
      end
    end
  end
end
```
assistant