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Version: v5

Mocking with Pester

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.


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#


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

Should -Invoke#

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

Should -InvokeVerifiable#

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


BeforeAll{    function Build ($version) {        Write-Host "a build was run for version: $version"    }        function Get-Version{        return 'Version'    }        function Get-NextVersion {        return 'NextVersion'    }        function BuildIfChanged {        $thisVersion = Get-Version        $nextVersion = Get-NextVersion        if ($thisVersion -ne $nextVersion) { Build $nextVersion }        return $nextVersion    }}
Describe "BuildIfChanged" {    Context "When there are Changes" {        BeforeEach{            Mock Get-Version {return 1.1}            Mock Get-NextVersion {return 1.2}            Mock Build {} -Verifiable -ParameterFilter {$version -eq 1.2}                $result = BuildIfChanged        }
        It "Builds the next version" {            Should -InvokeVerifiable        }                It "returns the next version number" {            $result | Should -Be 1.2        }    }    Context "When there are no Changes" {        BeforeEach{            Mock Get-Version { return 1.1 }            Mock Get-NextVersion { return 1.1 }            Mock Build {}                $result = BuildIfChanged        }
        It "Should not build the next version" {            Should -Invoke -CommandName Build -Times 0 -ParameterFilter {$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


Mocking a native application#

Mocking native commands can be done in a similar way as Powershell functions/cmdlets. The major difference is that native commands don't provide named parameters for us to use inside the mock scriptblock or in a ParameterFilter, so you'd have to rely on arguments made available using $args.

Describe 'Mocking native commands' {    It "Mock bash" {        function GetHTTPHeader ($url) {            & curl --url $url `            -I        }
        Mock curl { Write-Warning "$args" }
        GetHTTPHeader -url ""
        Should -Invoke -CommandName "curl" -Exactly -Times 1 -ParameterFilter { $args[0] -eq '--url' -and $args[1] -eq ''}
        # By converting args to string (will concat using space by default) you can match a pattern if order might change. remember linebreaks        Should -Invoke -CommandName "curl" -Exactly -Times 1 -ParameterFilter { "$args" -match "--url -I" }    }}

Changes from Pester v4#

Mocks are scoped based on their placement#

Mocks are no longer effective in the whole Describe / Context in which they were placed. Instead they will default to the block in which they were placed. Both of these work:

Describe "d" {    BeforeAll {        function f () { "real" }    }
    It "i" {        Mock f { "mock" }        f | Should -Be "mock"    }
    It "j" {        f | Should -Be "real"    }}
Describe "d" {    BeforeAll {        function f () { "real" }        Mock f { "mock" }    }
    It "i" {        f | Should -Be "mock"    }
    It "j" {        f | Should -Be "mock"    }}

Counting mocks depends on placement#

Counting mocks depends on where the assertion is placed. In It, BeforeEach and AfterEach it defaults to It scope. In Describe, Context, BeforeAll and AfterAll, it default to Describe or Context based on the command that contains them. The default can still be overriden by specifying -Scope parameter.

Describe "d" {
    BeforeAll {        function f () { "real" }        Mock f { "mock" }    }
    It "i" {        f        Should -Invoke f -Exactly 1    }
    It "j" {        f        Should -Invoke f -Exactly 1    }
    It "k" {        f        Should -Invoke f -Exactly 3 -Scope Describe    }
    AfterEach {        Should -Invoke f -Exactly 1    }
    AfterAll {        Should -Invoke f -Exactly 3    }}

Default parameters for ParameterFilter#

Parameter filters no longer require you to use param().

Describe "d" {
    BeforeAll {        function f ($a) { "real" }        Mock f { "mock" } -ParameterFilter { $a -eq 10 }    }
    It "i" {        f 10        Should -Invoke f -Exactly 1    }
    It "j" {        f 20        Should -Invoke f -Exactly 0    }}