Add Avoid Accidentally Disabling Pry as an rspec TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1435 TILs and counting..._
+_1436 TILs and counting..._
 
 ---
 
@@ -1099,6 +1099,7 @@ _1435 TILs and counting..._
 
 ### RSpec
 
+- [Avoid Accidentally Disabling Pry](rspec/avoid-accidentally-disabling-pry.md)
 - [Check Specific Arguments To Received Method](rspec/check-specific-arguments-to-received-method.md)
 - [Find Minimal Set Of Tests Causing A Flicker](rspec/find-minimal-set-of-tests-causing-a-flicker.md)
 - [Format Test Results As A JSON File](rspec/format-test-results-as-a-json-file.md)

rspec/avoid-accidentally-disabling-pry.md

@@ -0,0 +1,32 @@
+# Avoid Accidentally Disabling Pry
+
+I was recently working on a test that needed to mock an environment variable in
+order to observe the behavior under test. I initially ended up with the
+following lines in a `before` block.
+
+```ruby
+before do
+  allow(ENV).to receive(:[]).and_return("")
+  allow(ENV).to receive(:[]).with("API_SERVER_URL").and_return("localhost")
+end
+```
+
+The idea was to create a "clean" `ENV` for the test and then layer in any
+relevant environment variables from there.
+
+This is a misguided approach for a couple reasons. One particular and
+hard-to-debug issue is that this mocking of `ENV` accidentally disabled Pry
+(i.e. `binding.pry`). So once I was having issues with my test, I couldn't even
+inspect the code at runtime. It would skip right past any `binding.pry`
+statement I added.
+
+What happened is that [Pry (specifically `pry-rails`) looks for
+`ENV['DISABLE_PRY_RAILS']`](https://github.com/pry/pry-rails/blob/d8d0c6d87a5b8a3e570e0c80910fb80068f3553c/lib/pry-rails.rb#L6)
+and if that value is _truthy_ then it won't require the various `pry-rails`
+modules.
+
+The first `allow` has `ENV` responding with `""` which is truthy and will
+result in Pry being disabled.
+
+A more appropriate default would be `nil`. Also consider that there are many
+ways to access `ENV`, such as `#fetch`.