How do I pass a hash to a custom function in puppet?

Question

I have defined a custom function currently based on the very simple example here: https://docs.puppet.com/guides/custom_functions.html

module Puppet::Parser::Functions
  newfunction(:transform_service_hash) do |args|
    filename = args[0]
    hash_to_be_transformed = args[1]
    File.open(filename, 'a') {|fd| fd.puts hash_to_be_transformed }
  end
end

This kinda works. I can call it like this:

  $my_hash = { key => "value1" , key2 => "value2" }
  notify{ "new hash!! $my_hash" :}
  transform_service_hash('/var/tmp/blah',$my_hash)

and the file displays:

mgt21 ~ # cat /var/tmp/blah
keyvalue1key2value2

But, if I try to access elements of the hash, nothing changes:

module Puppet::Parser::Functions
  newfunction(:transform_service_hash) do |args|
    filename = args[0]
    hash_to_be_transformed = args[1]
    element1 = hash_to_be_transformed["key"] 
    File.open(filename, 'a') {|fd| fd.puts element1 }
  end
end

The above block outputs the exact same data to /var/tmp/blah.

And, interestingly, if I remove the filename pass and define it statically in the module:

  $my_hash = { key => "value1" , key2 => "value2" }
  notify{ "new hash!! $my_hash. element1 is: $my_hash.key" :}
  transform_service_hash($my_hash)

and

module Puppet::Parser::Functions
  newfunction(:transform_service_hash) do |args|
    hash_to_be_transformed = args[0]
    element1 = hash_to_be_transformed["key"] 
    File.open('/var/tmp/blah2', 'a') {|fd| fd.puts element1 }
  end
end

I get the following error: "Error 400 on SERVER: can't convert Hash into String" with a line reference pointing to "transform_service_hash($my_hash)"

I am new to both puppet and ruby...so I'm unsure I am not passing the element properly, if I am not receiving it properly, or if it something that puppet cannot handle. Please note that I am using version 3.8 of puppet and 1.8.7 of ruby.

Thanks for any help. I've been banging my head against this, and google hasn't been forthcoming yet.

---Edit to clarify my goals (I also edited my code a bit for specificity): I am attempting to pass a hash into a custom ruby function within puppet. The "test" hash has two elements: one string and one array. It is defined as such:

  $my_hash = { key => "value1" , key2 => ['array_value1', 'array_value2'] }
  $my_display_element=$my_hash["key2"][0]
  notify{ "new hash!! $my_hash. the first value of the array stored in element2 is: $my_display_element" :}
  transform_service_hash('/var/tmp/blah',$my_hash)

The function appears like so:

module Puppet::Parser::Functions
  newfunction(:transform_service_hash) do |args|
    filename = args[0]
    hash_to_be_transformed = args[1]
    element1 = args[1]["key"]
    element2 = args[1]["key2"][0]
    #element1 = hash_to_be_transformed["key"]
    #element2 = hash_to_be_transformed["key2"][0] 
    File.open(filename, 'a') {|fd| fd.puts "hash_to_be_transformed: #{hash_to_be_transformed}\n" }
    File.open(filename, 'a') {|fd| fd.puts "element1: #{element1}\n" }
    File.open(filename, 'a') {|fd| fd.puts "element2: #{element2}\n" }        
  end
end

For now, I just want to be able to see that I am able to access elements within the passed hash like a hash. So I'd love for the output file to look like:

hash_to_be_transformed: keyvalue1key2array_value1array_value2
element1: value1
element2: array_value1

However, in the output file, I see:

mgt21 ~ # cat /var/tmp/blah 
keyvalue1key2array_value1array_value2

Clearly, something is off here as my text is not being added and the full hash is just printed out just once and seemingly in string form.

I believe that this may be related to the error that I get when I don't pass in a file name (see above). I think that my hash is getting interpreted (or passed) as a string and, as such, I am unable to access the elements. Unfortunately, I still have been unable to verify this or figure out why it might be happening.

---Edit2 based on Matt's answer below.

I decided to simplify my code to isolate this "can't convert Hash into String error". I also made his suggested changes to remove the ambiguity from my key declarations.

  $my_hash = { 'key' => "value1" , 'key2' => ['array_value1', 'array_value2'] }
  $my_display_element=$my_hash["key2"][0]
  notify{ "new hash!! $my_hash. the first value of the array stored in element2 is: $my_display_element" :}
  transform_service_hash($my_hash)

and

module Puppet::Parser::Functions
  newfunction(:transform_service_hash) do |args|
    hash_to_be_transformed = args[0]
    element1 = args[0]['key']
    element2 = args[0]['key2'][0]
    File.open('/var/tmp/blah', 'a') {|fd| fd.puts "hash_to_be_transformed: #{hash_to_be_transformed}\n" }
    File.open('/var/tmp/blah', 'a') {|fd| fd.puts "element1: #{element1}\n" }
    File.open('/var/tmp/blah', 'a') {|fd| fd.puts "element2: #{element2}\n" }
  end
end

But, I still end up with the same "Hash to String error". It is worth noting that I also tried simplifying my hash to:

  $my_hash = { 'key' => "value1" , 'key2' => "value2" }

and I still get the "Hash to String error".

Can you make corrections to your code to accurately reflect what you have done? It cannot possibly be accurate when your beginning examples has a method that expects two arguments, you provide only one, and then you show the code not fatally erroring during execution. — Matt Schuchard
Sorry about that, I must have pasted the call from the second example (after defining filepath in the function itself). I've fixed it. — brbcoffee
Can you tell us what format you want the "/var/tmp/blah"? You seem to be passing a Hash but I don't know how you want that to look in the file. — kcdragon
Thank you for the follow up. I have added an edit section with clarifications and some updates to my investigations. I appreciate the continued help. — brbcoffee

Matt Schuchard Matt Schuchard · Accepted Answer · 2016-08-20T12:33:52

I quickly took your custom parser function and converted it into pure ruby like the following:

hash = { 'key' => 'value1', 'key2' => %w(array_value1 array_value2) }

def newfunction(filename, a_hash)
  element1 = a_hash['key']
  element2 = a_hash['key2'][0]
  File.open(filename, 'a') do |fd|
    fd.puts "hash_to_be_transformed: #{a_hash}"
    fd.puts "element1: #{element1}"
    fd.puts "element2: #{element2}"
  end
end

newfunction('foo.txt', hash)

This results in the output text file like the following:

hash_to_be_transformed: {"key"=>"value1", "key2"=>["array_value1", "array_value2"]}
element1: value1
element2: array_value1

This seems to confirm my initial suspicion about what is going wrong here. Your hash in Puppet of:

$my_hash = { key => "value1" , key2 => ['array_value1', 'array_value2'] }

has keys of implicit/ambiguous types. In the ruby code I used to test, I explicitly established them as strings. This also correlates strongly with these lines in your code failing:

element1 = args[1]["key"]
element2 = args[1]["key2"][0]

and your error message of:

Error 400 on SERVER: can't convert Hash into String

because you are specifying in your ruby code that you expect the keys to be string. Changing your hash in Puppet to:

$my_hash = { 'key' => "value1" , 'key2' => "value2" }

should fix this.

On an unrelated note, I recommend the use of linters to help you learn these languages. Puppet-Lint, Rubocop, and Reek will all help point out suboptimal and messy parts of your code to help you learn the new languages.

On a related note, you may want to put something like this at the top of your custom parser function:

raise(Puppet::ParseError, 'newfunction expects two arguments') if args.length != 2

How do I pass a hash to a custom function in puppet?

2 Answers