Mocking with Pester

Joel Bennett edited this page Feb 7, 2018 · 11 revisions

Pester provides a set of Mocking functions making it easy to fake dependencies and also to verify behavior. Using these mocking functions can allow you to "shim" a data layer or mock other complex functions that already have their own tests.

DESCRIPTION

With the set of Mocking functions that Pester exposes, one can:

  • Mock the behavior of ANY powershell command.
  • Verify that specific commands were (or were not) called.
  • Verify the number of times a command was called with a set of specified parameters.

MOCKING FUNCTIONS

Mock

Mocks the behavior of an existing command with an alternate implementation.

Assert-VerifiableMocks

Checks if any Verifiable Mock has not been invoked. If so, this will throw an exception.

Assert-MockCalled

Checks if a Mocked command has been called a certain number of times and throws an exception if it has not.

EXAMPLE

function Build ($version) {
    Write-Host "a build was run for version: $version"
}

function BuildIfChanged {
	$thisVersion = Get-Version
	$nextVersion = Get-NextVersion
	if ($thisVersion -ne $nextVersion) { Build $nextVersion }
	return $nextVersion
}

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

Describe "BuildIfChanged" {
    Context "When there are Changes" {
    	Mock Get-Version {return 1.1}
    	Mock Get-NextVersion {return 1.2}
    	Mock Build {} -Verifiable -ParameterFilter {$version -eq 1.2}

    	$result = BuildIfChanged

	    It "Builds the next version" {
	        Assert-VerifiableMocks
	    }
	    It "returns the next version number" {
	        $result | Should Be 1.2
	    }
    }
    Context "When there are no Changes" {
    	Mock Get-Version { return 1.1 }
    	Mock Get-NextVersion { return 1.1 }
    	Mock Build {}

    	$result = BuildIfChanged

	    It "Should not build the next version" {
	        Assert-MockCalled Build -Times 0 -ParameterFilter {$version -eq 1.1}
	    }
    }
}

If you need to mock calls to commands which are made from inside a Script Module, additional code is required. For details, refer to Unit Testing within Modules


MOCKING A FUNCTION THAT IS CALLED BY A METHOD IN A POWERSHELL CLASS

In PowerShell 6, functions called by classes can be mocked as above, with no known problems.

However previous versions of PowerShell, including all versions of Windows PowerShell up to 5.1 cache class definitions in such a way that they are never redefined, even if you remove the module and re-import, or modify the class. This breaks Pester's Mock command, as the scope where the mock must be injected cannot be found.

Dave Wyatt has provided this workaround:

Simply run your Pester tests in a fresh session every time; this is simple to do with Start-Job. I have this proxy function in my PowerShell profile to help with that:

function Invoke-PesterJob
{
[CmdletBinding(DefaultParameterSetName='LegacyOutputXml')]
    param(
        [Parameter(Position=0)]
        [Alias('Path','relative_path')]
        [System.Object[]]
        ${Script},

        [Parameter(Position=1)]
        [Alias('Name')]
        [string[]]
        ${TestName},

        [Parameter(Position=2)]
        [switch]
        ${EnableExit},

        [Parameter(ParameterSetName='LegacyOutputXml', Position=3)]
        [string]
        ${OutputXml},

        [Parameter(Position=4)]
        [Alias('Tags')]
        [string[]]
        ${Tag},

        [string[]]
        ${ExcludeTag},

        [switch]
        ${PassThru},

        [System.Object[]]
        ${CodeCoverage},

        [switch]
        ${Strict},

        [Parameter(ParameterSetName='NewOutputSet', Mandatory=$true)]
        [string]
        ${OutputFile},

        [Parameter(ParameterSetName='NewOutputSet', Mandatory=$true)]
        [ValidateSet('LegacyNUnitXml','NUnitXml')]
        [string]
        ${OutputFormat},

        [switch]
        ${Quiet}
    )

    $params = $PSBoundParameters

    Start-Job -ScriptBlock { Set-Location $using:pwd; Invoke-Pester @using:params } |
    Receive-Job -Wait -AutoRemoveJob
}
Set-Alias ipj Invoke-PesterJob

Source

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