Open classes (reopening classes) in Ruby - Step-by-Step Execution

Concept Flow - Open classes (reopening classes)

Define class

↓

Use class

↓

Reopen class

↓

Add/Change methods

↓

Use modified class

You first define a class, then later reopen it to add or change methods, and finally use the updated class.

Execution Sample

Ruby

class Dog
  def bark
    "Woof!"
  end
end

class Dog
  def wag_tail
    "Wagging tail"
  end
end

puts Dog.new.bark
puts Dog.new.wag_tail

This code defines a Dog class, then reopens it to add a new method wag_tail, and calls both methods.

Execution Table

Step	Action	Evaluation	Result
1	Define class Dog with method bark	Dog class created with bark method	Dog#bark returns "Woof!"
2	Reopen class Dog to add wag_tail method	Dog class updated	Dog#wag_tail returns "Wagging tail"
3	Create Dog instance and call bark	Dog.new.bark	"Woof!"
4	Create Dog instance and call wag_tail	Dog.new.wag_tail	"Wagging tail"
5	End of program	All methods accessible	Program ends successfully

💡 Program ends after calling both methods on Dog instances

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
Dog class	undefined	Defined with bark method	Reopened with wag_tail method added	Has bark and wag_tail methods

Key Moments - 2 Insights

Why can we define the Dog class twice without error?

In Ruby, reopening a class means the second definition adds to the existing class instead of replacing it, as shown in steps 1 and 2 of the execution_table.

Does reopening a class remove existing methods?

No, reopening a class adds or changes methods but keeps existing ones, so Dog has both bark and wag_tail after step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what does Dog#bark return at step 3?

A "Woof!"

B "Wagging tail"

C Error: method not found

D nil

💡 Hint

Check the Result column at step 3 in the execution_table.

At which step is the wag_tail method added to Dog?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the Action and Evaluation columns in the execution_table for when wag_tail is added.

If we removed the reopening of Dog class, what happens when calling wag_tail?

A Returns "Wagging tail"

B Raises NoMethodError

C Returns "Woof!"

D Returns nil

💡 Hint

Without reopening, wag_tail method does not exist; see variable_tracker and execution_table steps.

Concept Snapshot

Open classes in Ruby let you add or change methods anytime.
Define a class once, then reopen it later to add methods.
Existing methods stay unless explicitly changed.
Use reopened class normally with all methods available.

Full Transcript

This lesson shows how Ruby allows reopening classes to add or change methods after the initial definition. First, a Dog class is defined with a bark method. Then, the same Dog class is reopened to add a wag_tail method. Both methods can be called on Dog instances. The execution table traces each step: defining the class, reopening it, and calling methods. The variable tracker shows the Dog class gaining methods over time. Key moments clarify that reopening adds to the class without removing existing methods. The quiz tests understanding of method availability and behavior when reopening is omitted. This helps beginners see how Ruby classes are flexible and can grow over time.