We'll cover Pry, a type of REPL, and discuss how to install and use it to debug a program.
You've already been introduced to REPLs through using IRB (Interactive Ruby). REPL stands for Read, Evaluate, Print, Loop. It is an interactive programming environment that takes a user's input, evaluates it and returns the result to the user.
Ruby installs with its own REPL, which is IRB, that you've already been using.
Every time you type
irb into your terminal, you're entering into a REPL.
Pry is another Ruby REPL with some added functionality. When you enter IRB, you are entering a brand new interactive environment. Any code you want to play with in IRB, you have to write in IRB or copy and paste into IRB. Pry, on the other hand, is like a REPL that you can inject into your program.
Pry is far more flexible than IRB. Once you install the Pry library (via the Pry
gem—we'll walk through installation in a bit), you can use a
anywhere in your code.
Binding is a built-in ruby class whose objects can encapsulate the context of your current scope (variables, methods etc.), and retain them for use outside of that context.
binding.pry is essentially 'prying' into the current binding or
context of the code, from outside your file.
So when you place the line
binding.pry in your code, that line will get
interpreted at runtime (as your program is executed). When the interpreter hits
that line, your program will actually freeze and your terminal will turn into
a REPL that exists right in the middle of your program, wherever you added the
Let's take a look. In this repository, you'll see a file called
Fork and clone this repository.
Install Pry on your computer by navigating to your home directory (
cd ~ in
your terminal) and execute
gem install pry. (You don't need to do this if
you are working in the IDE.)
Look at the code in
You should see the following code:
require 'pry' def prying_into_the_method inside_the_method = "We're inside the method" puts inside_the_method puts "We're about to stop because of pry!" binding.pry this_variable_hasnt_been_interpreted_yet = "The program froze before it could read me!" puts this_variable_hasnt_been_interpreted_yet end prying_into_the_method
Here we are requiring
pry, which you must do to use pry, defining a method,
and then calling that method.
In the directory of this repo, in your terminal, run the file by typing
lib/pry_is_awesome.rb. Now, look at your terminal. You should see something
3: def prying_into_the_method 4: inside_the_method = "We're inside the method" 5: puts inside_the_method 6: puts "We're about to stop because of pry!" 7: binding.pry => 8: this_variable_hasnt_been_interpreted_yet = "The program froze before it could read me!" 9: puts this_variable_hasnt_been_interpreted_yet 10: end  pry(main)>
You have frozen your program as it executes and are now inside a REPL inside your program. You basically just stopped time! How cool is that?
In the terminal, in your pry console, type the variable name
and hit enter. You should see a return value of
"We're inside the method"
You are able to explore the data inside the method in which you've placed your binding.
Now, in the terminal, in your pry console, type the variable name
this_variable_hasnt_been_interpreted_yet. You should see a return value of
nil. That's because the binding you placed on line 7 actually froze the
program on line 7 and the variable you just called hasn't been interpreted yet.
Consequently, our REPL doesn't know about it.
Now, in the terminal, type
exit, and you'll leave your pry console and the
program will continue to execute.
In addition to exploring code inside Pry, you can also manipulate variables and try code out. This is where Pry really becomes helpful for debugging. If you have a method that isn't doing what it's supposed to do, instead of making changes in your text editor and running the tests over and over until you get it working, you can put a binding in your code and try things out. Once you've figured out how to fix the problem, you then update the code in your text editor accordingly.
Let's walk through an example together. In this repository that you've forked
and cloned down onto your computer, you'll see a
spec folder containing a file
pry_debugging_spec.rb. This is a test for the file
pry_debugging.rb, we have a broken method. Run
learn test to see the
failing test. You should see the following:
1) #plus_two takes in a number as an argument and returns the sum of that number and 2 Failure/Error: expect(plus_two(3)).to eq(5) expected: 5 got: 3 (compared using ==) # ./spec/pry_debugging_spec.rb:6:in `block (2 levels) in <top (required)>'
So what's happening? In the second line (the line starting with
Failure/Error), we can see that the test is calling the
plus_two method and
3 as an argument. Below that we can see that the test is expecting
5 to be returned, but that
3 is being returned instead. We remember that the
return value of a method in Ruby is generally the value of the last line of the
method, in this case,
def plus_two(num) num + 2 num end
So while our method is adding 2 to
num on the second line, it appears that it
is not updating
num. We have Pry required at the top of our
spec/pry_debugging_spec.rb file so we can use it to verify this. Let's place a
binding.pry in our code, right after that line:
def plus_two(num) num + 2 binding.pry num end
Now, run the test suite again and drop into your Pry console. Your terminal should look like this:
From: /Users/sophiedebenedetto/Desktop/Dev/Ruby-Methods_and_Variables/pry-readme/lib/pry_debugging.rb @ line 4 Object#plus_two: 1: def plus_two(num) 2: num + 2 3: binding.pry => 4: num 5: end  pry(#<RSpec::ExampleGroups::PlusTwo>)>
Let's check our current return value by typing
num at the Pry prompt. You
should see something like this:
 pry(#<RSpec::ExampleGroups::PlusTwo>)> num => 3  pry(#<RSpec::ExampleGroups::PlusTwo>)>
By checking the value of the variable inside our pry console, we can confirm
num is still equal to
3 and, as a result, the method is returning
How can we modify the code on line 2 so that the method behaves in the expected
way? We need to update the value of our
num variable so that it's equal to
the sum of itself and 2. Play around inside your Pry console: try code that you
think will update
num as needed, then check the value of
num to see if it
worked. Once you figure it out you can type
exit in your terminal to get out
of Pry, update the code in your text editor, and rerun the test to verify it's
passing. Be sure to remove the
It can take a little while to get the hang of using Pry so don't worry if it's still a little confusing. As you start working with more complex methods and data structures, you'll find it can be a very helpful tool.