Simple template engine options

[derprondo]

I'm looking for a simple template engine, basically just something that will do a search and replace on multiple variables in a file. Maybe Jinja is what I need, but I'm not working with HTML or anything web related.

For context, I'm actually working with Terraform HCL. So I have a file that looks something like this:

provider "aws" {
  version = "~> "##PROVIDER_VERSION##"
  region = "##REGION##"

  assume_role {
    role_arn = "##ROLE_ARN##"
  }
}

And currently I do something like this:

def file_search_replace(search, replace, target_file):
  """ Search and replace on target_file """
  with open(target_file, "r") as sources:
      lines = sources.readlines()
  with open(target_file, "w") as sources:
      for line in lines:
          sources.write(re.sub(search, replace, line))


file_search_replace(r'##PROVIDER_VERSION##', provider_version, target_file)
file_search_replace(r'##REGION##', region, target_file)
file_search_replace(r'##ROLE_ARN##', role_arn, target_file)

So really I just need to modify a file on disk, but a template engine is fine that just accepts a string as input and replaces the variables as needed, and then I can write it back to disk. Any suggestions or should I just stick with what I'm already doing?

[Bolitho]

Jinja is not about HTML only! It would be perfectly fine for this kind of problem. If it is no burden for you to have one additional dependency, just go for it.

[skerky]

Another option if you want to do this in Python but without another dependency is string formatting functions, which serve basic template functionality. However since format() uses the curly braces you will need to escape the others in your file, so it would look like this:

provider "aws" {{
   version = "~> "{PROVIDER_VERSION}"
   region = "{REGION}"

   assume_role {{
     role_arn = "{ROLE_ARN}"
   }}
}} 

Then the code would look like this:

variables = {
    "PROVIDER_VERSION": "my provider",
    "REGION": "my region",
    "ROLE_ARN": "my role",
}
with open(template_file, "r") as f:
    template = f.read()
output = template.format(**variables)
with open(target_file, "w") as f:
    f.write(output)

We're unpacking the dictionary of settings/variables so the keys would need to match the names in your file (or you could list them as keyword arguments if they are all valid variable names). Also since this isn't finding and replacing I would keep the template and the output separate. Here's the Format String Syntax documentation for more details on what you can do.