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 the following line
binding.pry 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
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.
ruby lib/pry_is_awesome.rb. Now, look at your terminal. You should see something like this:
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?
inside_the_methodand hit enter. You should see a return value of
"We're inside the method"
You are actually able to explore and manipulate 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.
You can imagine how helpful it will be to use Pry to freeze programs and to "pry" methods open in order to solve tests and debug your code. 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
In that file, we have a broken method. Run
learn to see the failing test.
Oh no! A broken program! Luckily, we have Pry required at the top of our
spec/pry_debugging_spec.rb file, and we know how to use it. Let's place a
binding.pry right before the
end keyword like this.
def plus_two(num) num + 2 num binding.pry 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: num => 4: binding.pry 5: end  pry(#<RSpec::ExampleGroups::PlusTwo>)>
The test is calling our
plus_two method with the argument,
num, the value of
num set to
3, and the expected return value of
5. We remember that the return value of a method in Ruby is generally the value of the last line of the method. Let's check our current return value by typing
num into our Pry console. You should see something like this:
 pry(#<RSpec::ExampleGroups::PlusTwo>)> num => 3  pry(#<RSpec::ExampleGroups::PlusTwo>)>
By checking the value of the variable on the last line of our method inside our pry console, we can see that
num is set to
3 and therefore the method is returning
How can we fix this method so that it behaves in the expected way? This method is called
plus_two and the test is expecting a return value of
5, given a
3. Looks like our method should return the sum of the original number (
3) plus 2. But our method, as it currently stands, is returning the original number. Play around with it inside your Pry console and get the test to pass. Remember to type
exit in your terminal and then remove your
binding.pry when you think your test will pass.
Once you have your test passing, make sure the
binding.pry line has been removed and add commit and push your changes. Then open a pull request.
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