How to Monkey Patch in Ruby: Syntax, Example, and Tips

In Ruby, you monkey patch by reopening an existing class using class ClassName and then adding or changing methods inside it. This lets you modify or extend the behavior of built-in or third-party classes at runtime.

Syntax

To monkey patch in Ruby, you reopen the class you want to change by writing class ClassName. Inside this block, you can define new methods or override existing ones. When the class is reopened, Ruby merges your changes with the original class.

• class ClassName: Opens the class for modification.
• def method_name: Defines or overrides a method.
• end: Closes the method and class definitions.

ruby

class String
  def shout
    self.upcase + "!"
  end
end

Example

This example shows how to add a new method shout to Ruby's built-in String class. The method returns the string in uppercase followed by an exclamation mark.

ruby

class String
  def shout
    self.upcase + "!"
  end
end

puts "hello".shout

Output

HELLO!

Common Pitfalls

Monkey patching can cause unexpected behavior if not done carefully. Common mistakes include:

• Overriding methods without calling the original, which can break existing functionality.
• Conflicts when multiple patches modify the same method.
• Making changes that affect all instances globally, which can be hard to track.

To avoid issues, consider using alias_method to keep the original method and call it inside your override.

ruby

class String
  # Wrong: completely replaces the original method
  def reverse
    "reversed!"
  end
end

puts "abc".reverse  # Output: reversed!

# Better: keep original method and extend it
class String
  alias_method :original_reverse, :reverse
  def reverse
    original_reverse + " (reversed)"
  end
end

puts "abc".reverse  # Output: cba (reversed)

Output

reversed!
cba (reversed)

Quick Reference

Tips for safe monkey patching:

• Always reopen the class with class ClassName.
• Use alias_method to preserve original methods when overriding.
• Test patches thoroughly to avoid breaking code.
• Limit monkey patching to small, well-understood changes.
• Consider alternatives like refinements for safer scoped changes.

Key Takeaways

Monkey patching in Ruby means reopening a class to add or change methods.

Use alias_method to keep original methods when overriding to avoid breaking code.

Monkey patching affects all instances globally, so use it carefully.

Test your patches well to prevent unexpected side effects.

Consider safer alternatives like refinements for scoped changes.