How can I redefine a function that was exported by a powershell module

Question

Is there a way that I can re-define a function that was exported by a module and is used by another?

Given the following folder structure:

C:.
│   MyScript.ps1
│
└───MyModules
    ├───Deployment
    │       Deployment.psm1
    │
    └───Logging
            Logging.psm1

And the following content for each file:

MyScript.ps1:

#Add MyModules to the module path
$cleanedModulePath = @()
$env:PSModulePath -split ";" | %{
    if( !( $_ -Match "MyModules") ){
        $cleanedModulePath += $_
    }
}
$thisDir = (Split-Path -parent $script:MyInvocation.MyCommand.Definition)
$moduleDir = Join-Path $thisDir "MyModules"
$cleanedModulePath += $moduleDir
$env:PSModulePath = ($cleanedModulePath -Join ";")

Import-Module Deployment -DisableNameChecking

#Neither of these manage to redefine Log-Debug - even if put before the Import
function Log-Debug{}
Set-Item -Path function:Log-Debug -Value {} -Option "AllScope,ReadOnly" -Force

Do-Something

Deployment.psm1:

Import-Module Logging -DisableNameChecking

function Do-Something {
    Log-Debug "Do-Something outputting a message via Log-Debug"
    Write-Host "Do-Something outputting a message directly"
}


Export-ModuleMember -Function Do-Something

and Logging.psm1:

function Log-Debug {
    param([string] $message)
    Write-Host $message
}

Export-ModuleMember -Function Log-Debug

The output is:

C:\Temp\Deploy2>powershell .\MyScript.ps1
Do-Something outputting a message via Log-Debug
Do-Something outputting a message directly

but I would like it to be:

C:\Temp\Deploy2>powershell .\MyScript.ps1
Do-Something outputting a message directly

I realize that a better implementation of the Logging module would allow me to control the log level, but that's not the point of this question: is it possible to redefine/override Log-Debug so that it is a no-op?

As noted in the source of MyScript.ps1, I have tried to redefine the function using Set-Item and just redeclaring the function to no avail....

Answer 1

I would've guessed that simply defining an empty function with the same name should suffice. And it actually does, when both modules are imported directly in PowerShell:

PS C:\> $modules = "$env:USERPROFILE\Documents\WindowsPowerShell\Modules"
PS C:\> Get-Content "$modules\Logging\Logging.psm1"
function Log-Debug($msg) {
  Write-Host "DEBUG: $msg"
}
Export-ModuleMember -Function Log-Debug
PS C:\> Get-Content "$modules\Deployment\Deployment.psm1"
function Foo($msg) {
  Write-Output 'FOO: something'
  Log-Debug $msg
}
Export-ModuleMember Foo
PS C:\> Import-Module Logging -DisableNameChecking
PS C:\> Import-Module Deployment
PS C:\> Foo 'bar'
FOO: something
DEBUG: bar
PS C:\> function Log-Debug {}
PS C:\> Foo 'bar'
FOO: something
PS C:\> _

But for some reason you cannot re-define or remove a function that was imported from a module by another module (or at least I didn't find a way to do so).

However, you should be able to override the function by making use of command precedence. Since aliases are evaluated first, you can define an alias Log-Debug to a custom logging function:

PS C:\> $modules = "$env:USERPROFILE\Documents\WindowsPowerShell\Modules"
PS C:\> Get-Content "$modules\Logging\Logging.psm1"
function Log-Debug($msg) {
  Write-Host "DEBUG: $msg"
}
Export-ModuleMember -Function Log-Debug
PS C:\> Get-Content "$modules\Deployment\Deployment.psm1"
Import-Module Logging -DisableNameChecking
function Foo($msg) {
  Write-Output 'FOO: something'
  Log-Debug $msg
}
Export-ModuleMember Foo
PS C:\> Import-Module Deployment
PS C:\> Foo 'bar'
FOO: something
DEBUG: bar
PS C:\> function Log-DebugSuperseded {}
PS C:\> Set-Alias -Name Log-Debug -Value Log-DebugSuperseded
PS C:\> Foo 'bar'
FOO: something
PS C:\> _

That way you can even change the logging implementation on the fly, or switch back and forth between original and custom logging function:

PS C:\> Foo 'bar'
FOO: something
DEBUG: bar
PS C:\> function Log-DebugSuperseded {}
PS C:\> Set-Alias -Name Log-Debug -Value Log-DebugSuperseded
PS C:\> Foo 'bar'
FOO: something
PS C:\> function Log-DebugSuperseded {'baz'}
PS C:\> Foo 'bar'
FOO: something
baz
PS C:\> Remove-Item Alias:\Log-Debug
PS C:\> Foo 'bar'
FOO: something
DEBUG: bar
PS C:\> _

---

Edit:

To make this work in a script you need to define both function and alias as global objects:

PS C:\> Get-Content .\test.ps1
Import-Module Deployment
Write-Host '--- without alias ---'
Foo 'bar'
function global:Log-DebugSuperseded {}
Set-Alias -Name Log-Debug -Value Log-DebugSuperseded -Scope global
Write-Host '--- with alias ---'
Foo 'bar'
PS C:\> ./test.ps1
--- without alias ---
FOO: something
DEBUG: bar
--- with alias ---
FOO: something
PS C:\> _