When does a Failing Pester Test return Green?

Words: 716
Time to read: ~ 3.5 minutes
TL;DR: Don't nest It blocks

I’ve been working more with different people in different departments more and more lately at work.

“Great” you might say and I would agree with you.

This has meant that any and all scripts that I write to give to these departments have to be tested so that they don’t fail when they are run.

“Great” you might say and, again, I would agree with you.

New-Fixture

For PowerShell scripts, I’ve been using Pester. When I want to write a new function, I run a Pester command to create a .ps1 and a .Tests.ps1 file.

It’s a simple enough setup – I checkout a new branch for the command and switch to it.

git checkout -b NewFunction

I know that Pester has a command that I can use to create a framework of a new function and a framwork of a Pester test file for that function.

Get-Command -Verb New -Module Pester |
    Where-Object Definition -like '*create*scripts*tests*'

This returns the command New-Fixture and a quick scan of the help shows that this is exactly what we are looking for.

(Get-Help New-Fixture).Synopsis

This function generates two scripts, one that defines a function
and another one that contains its tests.

(Get-Help New-Fixture).Synopsis

TDD

An effect of working with more and more departments is that I have less and less time to spend on these functions. That’s meant that I’ve had to take a hard look at what would be a “nice to have” and a “must have”.

Because of this I’ve spent more time on planning and writing the tests first. Then I write my scripts to pass these tests and only these tests.

If it is a “must have” then it gets a test. If it has a test then the script gets that functionality, or property, or parameter, etc.

If it’s a “nice to have” then it doesn’t get a test. If it doesn’t get a test then it’s not in the script. When I have time later then I’ll go back and see if I’ll add it

I have yet to have time to go back and add stuff but then I’ve yet to have a need to add any of the “nice to have”s.

You may think that thinking about what’s needed and writing the tests takes time and you’d be correct.

But anytime spent on the planning part is saved by not spending time writing code that isn’t needed and getting it to work. Overall, it’s a major time saving technique.

Back on Track

Here is what is in the .Tests.ps1 file created from the New-Fixture command.

$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path) -replace '.Tests.', '.'
. "$here\$sut"
Describe "<function name>" {
It "does something useful" {
$true | Should -Be $false
}
}

Contents of .Tests.ps1 file

It’s a nice template and all you need to do is modify that It block to have your test instead.

Here is where my Pester test failed and returned a green.

$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path) -replace '\.Tests\.', '.'
. "$here\$sut"

Describe "Test-FakeFunction" {
    It "does something useful" {
        It 'should pass by deafult' {
            $true | Should -Be $false
        }
    }
}
It has a helpful message for you…

Do you see it?

The Reason

In modifying the template, I forgot to take out the original It block and just put my It block inside it.

This lead to my block “pass by deafult” failed
(as it should cause of that typo) but the original, parent block it was in “does something useful” passed!

It surprised me that I had a passing and failing tests since I thought I had only written 1 test but the maintainers of Pester included a helpful little error message for us.

You are already in a test case.

In the error message…3 times

Further Action

I’m an advocate of “if you see an issue, raise it on Github”. Even if you don’t fix the issue yourself, somebody else more than likely will come along and fix it for everyone.

Am I going to raise an issue for this on Github though?

  • An issue where it was due to my bad typing?
  • An issue where they already have it raised in the error message?
  • An issue where I accidentally put an It block inside an It block when I shouldn’t have?

For this instance, I think we’re okay…

Author: Shane O'Neill

DBA, T-SQL and PowerShell admirer, Food, Coffee, Whiskey (not necessarily in that order)...

One thought on “When does a Failing Pester Test return Green?”

What's your opinion?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s