Elixir’s IEx.pry is a great debugging tool.
It allows you to stop time (“pry”) in the middle of your application in order to inspect the conditions in which your app is currently running.
However, if you’ve ever had to try and use IEx.pry while running your Elixir tests using
mix test, you’ve probably encountered a problem.
It won’t actually work.
You may have seen an error similar to this:
Cannot pry #PID<0.474.0> at Example.ProjectsTest ... Is an IEx shell running?
I’ll show you the way to use
IEx.pry/0 in your Elixir tests and a couple of quick tips to make using IEx.pry in your tests even easier.
The solution is straightforward. You have to run your mix tests in an interactive elixir session.
How do you do that?
Simply prepend your
mix test command with
iex -S mix test would run all of your available tests and anywhere you’ve put an
IEx.pry the shell will ask you:
Request to pry #PID<0.464.0> at Example.ExampleTest... .... Allow? [Yn]
Y will drop you into the pry prompt.
Avoiding Timeouts Using IEx.pry in ExUnit
If you are going to be debugging for a while in your pry shell, you should consider adding the the
--trace to the
test task, i.e.
iex -S mix test --trace to avoid timeouts while you are in
Otherwise, your test process may timeout and crash while you are still debugging using pry. It may raise an ExUnit.TimeoutError after 60 seconds:
** (ExUnit.TimeoutError) test timed out after 60000ms. You can change the timeout: ...
Running One Test File or Line Numbers
You can, of course, do the same thing when running a single test file or even a single test.
# Run single test file iex -S mix test --trace path/to/simple_test.exs # Run single test iex -S mix test --trace path/to/simple_test.exs:12
But here’s the deal:
If you are like me, you rarely run your Elixir tests in an interactive shell.
Because I want to write a test, run it quickly, watch it fail, and then make it pass. Over and over. Iterating quickly. I don’t have time to stop and run an interactive session every time.
Occasionally, however, I run into bugs when making my Elixir tests pass. When this occurs, my debugging workflow works like this:
I’ll try a couple of different things and if none of those reveals the bug, I’ll drop in a
IEx.pry somewhere in my test (don’t forget to
I do this enough that remembering the correct sequence of commands and typing them out becomes time-consuming and tedious. So, I’ve come up with a few tricks to speed this process up.
Here are two tips that will allow you to do this quickly.
VIM map for IEx Pry
Dropping in a
IEx.pry in your code requires that you add both
require IEx and
IEx.pry to your code.
That is too much typing.
So, to save my hands from carpal tunnel, I have leveraged mappings in Vim. I added this mapping to my
nmap <leader>r orequire IEx; IEx.pry<esc>
Now, all I have to do is hit “[spacebar] + r” to insert a
require IEx; IEx.pry into my test. But, that is only have the problem. I’ll need to run my test again with an IEx shell.
Shell alias for IEx shell tests
Let’s say you’ve added IEx.pry to your test, but now you want to run the same test you just ran again, but this time using pry.
You’ll probably have to go back through your history, find the test you ran, then move your cursor to the beginning of the line, then add
iex -S to your
mix test command.
Or if you are like me, I’ll often forget to prepend the
iex -S in front of my
Too. Much. Typing.
I came up with simple Bash/Zsh alias that I use all the time and now, you can too.
# Zsh users alias repry='fc -e - mix\ test=iex\ -S\ mix\ test\ --trace mix\ test' # Bash users alias repry='fc -s mix\ test=iex\ -S\ mix\ test\ --trace mix\ test'
What does this alias do? Well, it uses the ‘fc’ command in *nix that searches for the last
mix test in your command history and then replaces the
mix test with
iex -S mix test --trace. This alias works regardless of whether I ran my entire test suite using
mix test or ran one test using a specific line number.
Here’s a graphic to pull it all together:
Hope this information was helpful. If you have any questions or have another tip, feel free to leave a comment below.
This post is part of my series of Elixir Tutorials
You can also follow me on the Twitters at: @DeLongShot