Debugging Techniques

Learning Goals

  • Understand how to read a stack trace
  • Understand common error messages
  • Understand how to use pry to create breakpoints in code to help verify assumptions
  • Develop a debugging process

Tools & Repositories

To start, we need to make sure we have the appropriate tooling installed.

  • pry - gem install pry

We’ll also be using Erroneous Creatures which is in the Debugging Lesson directory of your mod-1-be-exercises repository.


  • What do you do when you don’t know what’s going wrong with your application?
  • What do you know about pry?
  • What questions do you still have about pry?

Debugging Process

There are two ways that programming can go wrong:

  1. Your program doesn’t run. You get an Error.
  2. Your program runs, but it doesn’t work the way you expect. You get a Failure.

Having a debugging process when things go wrong is crucial to being an effective developer. No matter how skilled you are at coding, you will always write bugs, so it is very important to know how to hunt them down and fix them.

The recommended series of steps you should take to Debug your program are:

  • Read your error (the WHOLE error)
  • Read your stack trace (find the error).
  • Verify your assumptions.
  • Try things.

You might add research to that list, but generally research is something that you do so that you can try things.

Stack Trace

A Stack Trace shows what line of code an error occurred on, and all the method calls that led to that error. It is like a treasure map of exactly where to find the cause of the error.

Reading a Stack Trace

Let’s look at an example. If we run the hobbit_spec.rb test in our erroneous_creatures directory with rspec spec/hobbit_spec.rb, we will see something like this (you may need to scroll to find this):

4) Hobbit can get tired if play 3times
   Failure/Error: @agee >= 32

     undefined method `>=' for nil:NilClass
   # ./lib/hobbit.rb:18:in `adult?'
   # ./lib/hobbit.rb:22:in `play'
   # ./spec/hobbit_spec.rb:79:in `block (3 levels) in <top (required)>'
   # ./spec/hobbit_spec.rb:78:in `times'
   # ./spec/hobbit_spec.rb:78:in `block (2 levels) in <top (required)>'

Let’s break this down line by line:

  • 4) Hobbit can get tired if play 3times: This is RSpec telling us what test was running when this error occurred.
  • NoMethodError: undefined method '>=' for nil:NilClass: This is the actual error that occurred
  • All of the following lines are part of the Stack Trace:
    • ./lib/hobbit.rb:18:in 'adult?': This is the first line of the stack trace, and is the line where the error actually happened. This is telling us that the error occurred in the hobbit.rb file on line 18. The next part, in 'adult?' tells us that this error happened in the adult? method. hobbit.rb:18 is the most important part of the whole stack trace. It tells us the exact location of the error.
    • ./lib/hobbit.rb:22:in 'play': The next line in the stack trace tells us where the adult? method was called from. Again, the most important part is the file and line number, hobbit.rb line 22. The last part, in 'play' is telling us that the play method was running when the adult? method was called.
    • ./spec/hobbit_spec.rb:79:in 'block (3 levels) in <top (required)>': The next line in the stack trace tells us where the play method was called from. It was called from the hobbit_spec.rb file on line 79 in a block.
    • ./spec/hobbit_spec.rb:78:in 'times' and ./spec/hobbit_spec.rb:78:in 'block (2 levels) in <top (required)>' are telling us that that block was part of a times loop that started on line 78.

If we chart this out as a series of method calls, it looks something like this:

it 'Hobbit can get tired if play 3times' -> times -> play -> adult?

Tracing back through our Program

When we use the stack trace, we start at the top and work our way down. In this case, we start at hobbit.rb:18 to see the line where the error occurred. The error was undefined method '>=' for nil:NilClass. Looking at that line of code, we can see that the variable @age was misspelled, causing it to be nil. Fixing the spelling resolves the error.

If we didn’t find an error in the play method, we could take another step back into the adult? method to see if we can find an error there.


When you see an error in your terminal, it can be tempting to read it as “blah blah blah something isn’t working, let me open up my code and fix it”. Instead, you should read the error, the ENTIRE error, maybe even read it twice, and really try to understand your problem before you try to fix it. Here are some common errors and how we can interpret them:

NameError: uninitialized constant SomeClass::SomeConstant - Ruby doesn’t know what SomeConstant is.

undefined local variable or method 'x' for SomeObject (NameError) - Ruby doesn’t know what x is. It looked for a local variable x but couldn’t find one. It then looked for a method x and couldn’t find one for SomeObject

wrong number of arguments (given x, expected y) (ArgumentError) - You called a method with x number of arguments, but the method definition specifies it needs y number of arguments. This often happens when we call .new on something. Remember, when you call .new it also calls .initialize so you need to make sure the number of arguments you pass to .new match the number of arguments defined in .initialize

undefined method 'some_method' for SomeObject:SomeClass (NoMethodError) - you tried to call some_method on SomeObject, but SomeObject doesn’t respond to that method. This means that some_method is not defined in SomeClass. This error can take several forms:

  1. If you didn’t write SomeClass, you called a method that doesn’t exist i.e. "hello world".first.
  2. If you did write SomeClass, you misspelled the name of the method or you didn’t define some_method for SomeClass
  3. If SomeObject:SomeClass shows up as nil:NilClass, this means that something is nil that shouldn’t be.
  4. Sometimes SomeObject:SomeClass looks like #<SomeClass:0x00007f7fa21d5410>. You can read this as “you tried to call some_method on a SomeClass object”.

syntax error, unexpected end-of-input, expecting keyword_end - You are missing an end. Indenting your code properly will make it MUCH easier to hunt down the missing end.

syntax error, unexpected end-of-input, expecting keyword_end - You have an extra end or an end in the wrong place. Indenting your code properly will make it MUCH easier to hunt down the offensive end.

require': cannot load such file -- file_name (LoadError) - Ruby cannot load the file file_name. Make sure file_name is spelled correctly, the path is written correctly i.e. ./lib/file_name, and that you are running from the root directory of your project.

Verifying Your Assumptions

Not verifying your assumptions can be one of the costliest mistakes you make as a dev. It’s possible to be absolutely convinced that you know exactly what’s causing an error, spend hours working to resolve an issue that you’re sure exists, only to later find that the error occurred long before the piece of code that held your focus so tightly.

While it’s nice to drop into IRB or pry in terminal to see if there are methods that exist in Ruby that I can use to solve my problem, it’s even better to put a pry into my code to see exactly what I can do given the other methods and variables I’ve defined.

Let’s run the hippogriff_spec.rb, and review the errors that are generated there:

 2) Hippogriff when it flies it collects a unique moonrock
     Failure/Error: @moonrocks.push(rock)
       undefined method `push' for nil:NilClass
     # ./lib/hippogriff.rb:14:in `fly'
     # ./spec/hippogriff_spec.rb:38:in `block (2 levels) in <top (required)>'

Let’s start by reading that stack trace, and then answer the following questions with a partner:

  • What test is generating this error?
  • What line in that test is generating the error?
  • Is there any setup involved before we hit that line?
  • If so, can we use pry to confirm that the setup has been completed successfully? Do we have access to the variables that we think we do? Are they holding the objects we expect them to?
  • What about in the Hippogriff class itself? What line is generating an error?
  • Use pry to verify that the variables we are using in
  • that method are holding the objects we expect them to.
  • Can you identify the error?
  • Can you make the test pass?

Trying Things

One other thing we can do when we are trying to debug is to use pry to try something in our code before we actually commit to adding it to our class.

Let’s look at an error from our Wizard test suite:

2) Wizard is not always bearded
   Failure/Error: expect(wizard.bearded?).to eq(false)

     expected: false
          got: {:bearded=>false}

     (compared using ==)

     @@ -1 +1 @@
     +:bearded => false,

   # ./spec/wizard_spec.rb:25:in `block (2 levels) in <top (required)>'

With a partner:

  • Read the stack trace to determine where the error is occurring.
  • Use pry in the test file to verify any assumptions you may have about what's happening.
  • Use pry in the Wizard class to see if you can determine how to implement this method before you enter any code into the Wizard class. Ask yourself: how can I get the return value that I want?

Checks For Understanding

How does the stack trace read to tell you what’s going on?

What is one common error message?

What does it mean if you don’t hit your pry?

Exercise - Erroneous Creatures

See if you can finish updating the Erroneous Creatures to make the rest of the test suite pass.

Use the debugging techniques discussed above to diagnose and fix the bugs and get your creatures back to passing.

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