$scriptblock = {
    param($wordToComplete, $commandAst, $cursorPosition)
        $words = $commandAst.ToString() 
        if ( $wordToComplete ) {
          $matches = (($words[0..$cursorPosition] -join '') | Select-String -Pattern "\s+" -AllMatches).Matches
          if ( $matches ) {
            $cursorPosition = $matches[-1].Index - 1
          }
        }
        $words = $words[0..$cursorPosition] -join '' -split "\s+"
        if ( $words.Count -ge 2 ) {
          pyenv completions $words[1] | where { $_ -match $wordToComplete }
        } else {
          pyenv commands | where { $_ -match $wordToComplete }
        }
}
Register-ArgumentCompleter -Native -CommandName pyenv -ScriptBlock $scriptblock