Azure DevOps Pipeline to check if artifacts exists from a specific pipeline, if not found then trigger the pipeline

I have a main pipeline which is downloading pipeline artifact from a specific pipeline.

- task: DownloadPipelineArtifact@2
  inputs:
    source: specific
    project: ABC
    pipeline: 1
    runVersion: latestFromBranch
    runBranch: $(Build.SourceBranch)
    artifact: artifact-abc
    targetPath: $(Pipeline.Workspace)

I need to do a prerequisite check if artifact exists for the specific pipeline for the branch if not then run the specific pipeline. How to achieve this? Can we make use of a python script to invoke REST API to check and trigger the pipeline run?

You could use the Builds - List API to retrieve a list of builds, then loop through those results and check if the artifact exists.

- task: PowerShell@2
  displayName: Locate existing artifact
  inputs:
    targetType: inline
    script: |
      $project = "ABC"
      $pipeline = "1"
      $branch = "$(Build.SourceBranch)"
      $artifact = "artifact-abc"
      $baseUrl = "$(System.CollectionUri)/$project"
      $headers = @{ Authorization = "Bearer $(System.AccessToken)" }

      $url = $baseUrl + `
             "/_apis/build/builds?definitions=$pipeline" + `
             "&branchName=$branch" + `
             "&resultFilter=succeeded" + `
             "&statusFilter=completed" + `
             "&queryOrder=queueTimeDescending" + `
             "&api-version=7.1"

      # obtain list of build candidates
      $result = Invoke-RestMethod `
                  -Method Get `
                  -Uri $url `
                  -Headers $headers `
                  -UseBasicParsing

      # loop through builds to find artifact
      $builds = $result.value
      if $(builds) {
        $builds | ForEach-Object {
          $buildId = $_.id
          $artifactUrl = $baseUrl + `
                         "_apis/build/builds/$pipeline/artifacts" + `
                         "?artifactName=$artifact" + `
                         "&api-version=7.1"
 
          # check if artifact exists
          try {
            $result = Invoke-RestMethod `
                        -Method Get `
                        -Uri $artifactUrl `
                        -Headers $headers `
                        -UseBasicParsing
            if ($result.name -eq $artifact) {
              # artifact found!
              Write-Output "##vso[task.setvariable variable=artifactExists]true"
              break
            }
          }
          catch {}
        }
      }
      Write-Host "Artifact not found."

Then you can use the Pipeline - Run API to queue the pipeline. You'd then have to query the pipeline using Runs - Get until it's completed. This isn't an ideal, because you're using your build agent to monitor another build. If you only have a single build agent, this obviously won't work.

- task: PowerShell@2
  displayName: Invoke build
  condition: ne(variables['artifactExists'], 'true')
  inputs:
    targetType: inline
    script: |
      $branch = "$(Build.SourceBranch)"
      $project = "ABC"
      $pipeline = "1"
      $baseUrl = "$(System.CollectionUri)/$project"
      $headers = @{ Authorization = "Bearer $(System.AccessToken)" }

      $invokeUrl = $baseUrl + `
                     "/_apis/pipelines/$pipeline/runs" + `
                     "?api-version=7.1"
      $invokeBody = @{
        resources = @{
           repositories = @{
             self = @{
              refName = $branch
             }
           }
        }
      } | ConvertTo-Json -Depth 10

      # queue the build
      $result = Invoke-RestMethod `
                  -Method POST `
                  -Body $invokeBody `
                  -Uri $invokeUrl `
                  -Headers $headers
      
      $runId = $result.id

      # Monitor the build
      $monitorUrl = $baseUrl + `
             "/_apis/pipelines/$pipeline/runs/$runId?api-version=7.1"

      $done = false
      while (!$done) {
        Start-Delay -Seconds 60
        $result = Invoke-RestMethod `
                   -Method Get `
                   -Uri $monitorUrl `
                   -Headers $headers
        $done = $result.state -eq "completed"
      }