Debugging with pry and byebug in Ruby - Step-by-Step Execution

Concept Flow - Debugging with pry and byebug

Start Program

↓

Hit binding.pry or byebug

↓

Pause Execution

↓

Interact: Inspect Variables, Step, Continue

↓

Resume or Exit Debugger

↓

Program Continues or Ends

The program runs until it hits a debugger call, then pauses allowing you to inspect and control execution before continuing.

Execution Sample

Ruby

require 'pry'
def add(a, b)
  binding.pry
  a + b
end

add(2, 3)

This code pauses execution inside the add method so you can inspect variables before returning the sum.

Execution Table

Step	Action	Code Line	Variables State	Debugger Prompt	Output
1	Start program	def add(a, b)	a: undefined, b: undefined	No
2	Call add(2, 3)	add(2, 3)	a: 2, b: 3	No
3	Hit binding.pry	binding.pry	a: 2, b: 3	Yes - pry prompt
4	Inspect variables	binding.pry	a: 2, b: 3	Yes - pry prompt	a=2, b=3
5	Step over to next line	a + b	a: 2, b: 3	Yes - pry prompt
6	Continue execution	a + b	a: 2, b: 3	No	5
7	Program ends

💡 Program ends after returning the sum and no more code to run.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	Final
a	undefined	2	2	2	2
b	undefined	3	3	3	3

Key Moments - 3 Insights

Why does the program pause at binding.pry or byebug?

The debugger call tells Ruby to stop running and open an interactive prompt so you can inspect and control the program (see execution_table step 3).

Can I see variable values before the debugger line?

No, variables get their values when the method is called. Before that, they are undefined (see variable_tracker start values).

How do I continue running the program after stopping at the debugger?

You type 'continue' or 'c' at the debugger prompt to resume normal execution (see execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of variable 'a' at step 4?

A 2

B undefined

C 3

D nil

💡 Hint

Check the 'Variables State' column at step 4 in the execution_table.

At which step does the debugger prompt first appear?

A Step 2

B Step 5

C Step 3

D Step 6

💡 Hint

Look for 'Yes - pry prompt' in the 'Debugger Prompt' column.

If you remove 'binding.pry' from the code, what changes in the execution table?

A Variables become undefined

B Debugger prompt never appears

C Program never ends

D Output changes to nil

💡 Hint

Without the debugger call, the program runs straight through without pausing (see exit_note).

Concept Snapshot

Use binding.pry or byebug to pause Ruby code execution.
At the pause, interactively inspect variables and control flow.
Commands: 'next' to step, 'continue' to resume.
Remove debugger calls to run normally.
Great for finding bugs by watching code run step-by-step.

Full Transcript

This visual trace shows how Ruby's debugging tools pry and byebug pause program execution at a specific line. When the program hits binding.pry, it stops and opens a prompt where you can check variable values and step through code. Variables get their values when the method is called, so before that they are undefined. You continue running the program by typing 'continue' at the prompt. Removing the debugger call lets the program run without stopping. This helps find bugs by letting you watch what happens inside your code.

Practice

1. What is the main purpose of using binding.pry or byebug in Ruby code?

easy

A. To pause the program and inspect variables at a specific point

B. To speed up the execution of the program

C. To automatically fix syntax errors

D. To compile Ruby code into machine code

Solution

1. Step 1: Understand the role of debugging tools

   Both binding.pry and byebug are used to pause program execution.

2. Step 2: Inspect variables and program state

   When paused, you can check variable values and program flow to find issues.

3. Final Answer:

   To pause the program and inspect variables at a specific point -> Option A

4. Quick Check:

   Debugging tools pause and inspect = A [OK]

Hint: They pause code to check variables and flow [OK]

Common Mistakes:

• Thinking they speed up code
• Believing they fix errors automatically
• Confusing with code compilation

2. Which of the following is the correct way to insert a breakpoint using Pry in Ruby code?

easy

A. debug.pry

B. breakpoint.pry

C. pry.binding

D. binding.pry

Solution

1. Step 1: Recall Pry syntax for breakpoints

   The correct syntax to pause execution with Pry is binding.pry.

2. Step 2: Check other options for correctness

   Other options like breakpoint.pry, pry.binding, and debug.pry are invalid method calls.

3. Final Answer:

   binding.pry -> Option D

4. Quick Check:

   Correct Pry breakpoint syntax = D [OK]

Hint: Use exactly 'binding.pry' to pause with Pry [OK]

Common Mistakes:

• Swapping method and object order
• Using non-existent methods
• Forgetting to require 'pry'

3. What will be the output when running this Ruby code with byebug inserted?

def add(a, b)
  byebug
  a + b
end

puts add(2, 3)

medium

A. 5

B. byebug error

C. nil

D. SyntaxError

Solution

1. Step 1: Understand what byebug does

   It pauses execution allowing inspection but does not change the return value.

2. Step 2: Trace the function call and output

   The method add(2, 3) returns 5, which is printed by puts.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   byebug pauses but output is 5 [OK]

Hint: byebug pauses but returns normal output [OK]

Common Mistakes:

• Expecting byebug to change output
• Thinking byebug causes errors
• Confusing pause with program stop

4. You inserted binding.pry in your Ruby code, but when you run it, the program does not pause. What is the most likely reason?

medium

A. You used byebug instead of binding.pry

B. You forgot to require the 'pry' gem in your code

C. The Ruby version does not support debugging

D. You placed binding.pry after the return statement

Solution

1. Step 1: Check if 'pry' gem is loaded

   Without require 'pry', binding.pry is undefined and won't pause.

2. Step 2: Verify placement and usage

   Using binding.pry correctly requires the gem loaded; placing after return stops execution before breakpoint.

3. Final Answer:

   You forgot to require the 'pry' gem in your code -> Option B

4. Quick Check:

   Missing require 'pry' stops pause [OK]

Hint: Always require 'pry' before using binding.pry [OK]

Common Mistakes:

• Not requiring the gem
• Placing breakpoint after return
• Confusing pry with byebug usage

5. You want to debug a method that processes a list of numbers and stops only when a number is negative. Which debugging tool and approach is best to use?

hard

A. Use puts statements to print all numbers

B. Insert byebug at the start of the method without any condition

C. Insert binding.pry inside the loop with a condition to pause only if the number is negative

D. Run the program without breakpoints and guess the error

Solution

1. Step 1: Understand conditional breakpoints

   Using binding.pry with a condition lets you pause only when needed, saving time.

2. Step 2: Compare other options

   Using byebug without condition pauses every time, puts is less interactive, guessing is ineffective.

3. Final Answer:

   Insert binding.pry inside the loop with a condition to pause only if the number is negative -> Option C

4. Quick Check:

   Conditional breakpoint with binding.pry = A [OK]

Hint: Use conditional binding.pry to pause only on negative numbers [OK]

Common Mistakes:

• Pausing unconditionally causing many stops
• Relying only on print statements
• Ignoring debugging tools