Debugging with Pry

Quick Definition

Bugs = human errors in code

Debugging is the process of identifying and fixing those errors.

Debugging Steps

Essentially can be resumed in three steps:

  • Identify the problem
  • Understand the problem
  • Implement a solution

Types of Error

Syntax Errors

  • Missing characters
  • Misspelled keywords

Syntax errors generally stops code execution because the language doesn’t know how to interpret the code.


a = true

if a
  puts 'It's true!' # syntax error

Logical Errors

  • Errors in the logic of the code

Code can run but most probably produces unexpected results or miss some steps.


puts "Please pick an option: 1 or 2"
user_input = gets.chomp

if user_input == 1
  puts "You picked option 1"
elsif user_input == 2
  puts "You picked option 2"
  puts "Invalid option!!"

We could probably use a combination of puts to check results at each steps and debug our code, but let’s try with pry instead (see Debugging with Pry).

What is Pry?

Pry is a Ruby gem which can be installed with gem install pry.

At a basic level, Pry is a REPL: Read-Evaluate-Print Loop. It’s the name of an interactive environment that:

  • Read user input
  • Evaluates the input
  • Print the results to the user
  • Loops back to the start

IRB is also a REPL. Pry is like an advanced IRB for debugging.

Using Pry

Pry lets you move around different contexts and see which methods you can call on the current context.

For example, creating an array and cd into it, then calling ls will show every methods that can be used on the array:

[1] pry(main)> arr = [1, 2, 3]
=> [1, 2, 3]
[2] pry(main)> cd arr
[3] pry(#<Array>):1> ls
  chain        collect_concat  each_entry       each_with_object  find      grep      inject  member?    partition    slice_before  tally 
  chunk        detect          each_slice       entries           find_all  grep_v    lazy    min_by     reduce       slice_when    to_set
  chunk_while  each_cons       each_with_index  filter_map        flat_map  group_by  max_by  minmax_by  slice_after  sort_by     
  &    all?           collect!     delete_at   eql?        hash          length  permutation         reject!               rotate!    slice       to_ary     zip
  *    any?           combination  delete_if   fetch       include?      map     pop                 repeated_combination  sample     slice!      to_h       |  
  +    append         compact      difference  fill        index         map!    prepend             repeated_permutation  select     sort        to_s     
  -    assoc          compact!     dig         filter      insert        max     pretty_print        replace               select!    sort!       transpose
  <<   at             concat       drop        filter!     inspect       min     pretty_print_cycle  reverse               shelljoin  sort_by!    union    
  <=>  bsearch        count        drop_while  find_index  intersection  minmax  product             reverse!              shift      sum         uniq     
  ==   bsearch_index  cycle        each        first       join          none?   push                reverse_each          shuffle    take        uniq!    
  []   clear          deconstruct  each_index  flatten     keep_if       one?    rassoc              rindex                shuffle!   take_while  unshift  
  []=  collect        delete       empty?      flatten!    last          pack    reject              rotate                size       to_a        values_at
self.methods: __pry__
locals: _  __  _dir_  _ex_  _file_  _in_  _out_  pry_instance

Inside the context, calling one of the method above will give the result of the method:

[4] pry(#<Array>):1> first
=> 1

It is also possible to show the doc for a particular method with show-doc and the name of the method.

Invoking Pry at Runtime

First of all, we need to add require 'pry' at the top of our file.

Then, we need to add binding.pry somewhere in our code where we want it to stop in order to study it.

This is a good time to define binding: binding is the act of associating properties with names. The binding process might be determined at different phases of the life cycle of a program, but in our case we will scope it to the state of the program at time of execution.

When the program reaches binding.pry, Pry interrupts the program execution and pries open 1 the binding of our variables so that we can take a look around.


We can place binding.pry wherever we want in our code but this may have unforseen consequences on the output of our code. For example, placing binding.pry on the last line of some method like #map will have the side effect of returning nil.

require "pry"

arr = [1, 2, 3].map do |n|
  n * 2
  binding.pry # returns nil

p arr # [nil, nil, nil]

Debugging with Pry

Let’s add require 'pry' and binding.pry to our bugged code:

require "pry"

puts "Please pick an option: 1 or 2"
user_input = gets.chomp


if user_input == 1
  puts "You picked option 1"
elsif user_input == 2
  puts "You picked option 2"
  puts "Invalid option!!"

When running the code above, the execution will stop at binding.pry. From here we can call arguments and use methods at our convenience.

     1: require 'pry'
     3: puts "Please pick an option: 1 or 2"
     4: user_input = gets.chomp
 =>  6: binding.pry
     8: if user_input == 1
     9:   puts "You picked option 1"
    10: elsif user_input == 2
    11:   puts "You picked option 2"

[1] pry(main)> user_input
=> "1"
[2] pry(main)> user_input == 1
=> false

Obviously the problem here is we are comparing a string with an integer.

To solve this bug we can either convert user_input to an integer or replace integers with strings in the comparison statements.

Stepping through Code with pry-byebug

pry-byebug extends pry with additional commands such as:

  • next
  • step
  • continue

Other gems which extends pry exist, such as pry-nav and pry-debugger.

Just as with pry, we need to add require "pry-byebug" at the top of our code.


Command Effect
cd Change context
ls Show info
show-doc Show doc of the method it is called upon
whereami Show context. Can take an integer argument to show n lines above and below
exit-program Exit program (not just the loop)
Ctrl + D Jump to next line
This is awkward but it’s at this right moment I realize the name of the pry gem comes from the verb to pry.
Links to this page
#programming #debugging