Debugging with Pry
Bugs = human errors in code
Debugging is the process of identifying and fixing those errors.
Essentially can be resumed in three steps:
- Identify the problem
- Understand the problem
- Implement a solution
Types of Error
- 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 end
- 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" else puts "Invalid option!!" end
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
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:
 pry(main)> arr = [1, 2, 3] => [1, 2, 3]  pry(main)> cd arr  pry(#<Array>):1> ls Enumerable#methods: 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 Array#methods: & 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:
 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 open1 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
require "pry" arr = [1, 2, 3].map do |n| n * 2 binding.pry # returns nil end p arr # [nil, nil, nil]
Debugging with Pry
require 'pry' and
binding.pry to our bugged code:
require "pry" puts "Please pick an option: 1 or 2" user_input = gets.chomp binding.pry if user_input == 1 puts "You picked option 1" elsif user_input == 2 puts "You picked option 2" else puts "Invalid option!!" end
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' 2: 3: puts "Please pick an option: 1 or 2" 4: user_input = gets.chomp 5: => 6: binding.pry 7: 8: if user_input == 1 9: puts "You picked option 1" 10: elsif user_input == 2 11: puts "You picked option 2"  pry(main)> user_input => "1"  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 with additional commands such as:
Other gems which extends
pry exist, such as
Just as with
pry, we need to add
require "pry-byebug" at the top of our code.
|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|