What I use to format personalised bulk mails. Works on any text though.

format_letter.py

#!/usr/bin/env python3
from string import Template
import sys
import collections
import pathlib


def from_yaml(file):
    import yaml
    return yaml.load(file)


def from_json(file):
    import json
    return json.load(file)


loader = {
    'yaml': from_yaml,
    'yml': from_yaml,
    'json': from_json
}


def format_letters(letter_template_raw, name_template_raw, data):

    has_def = 'defaults' in data and 'values' in data

    if has_def:
        defaults = data['defaults']
        values = data['values']
    else:
        values = data

    letter_template = Template(letter_template_raw)

    name_template = Template(name_template_raw)

    def format_one(instance):
        if has_def:
            instance = collections.ChainMap(instance, defaults)
        return name_template.substitute(instance), letter_template.substitute(instance)

    return map(format_one, values)


def main():

    _, letter_file, data_file, output_format = sys.argv

    with open(letter_file, 'r') as f:
        letter = f.read()

    with open(data_file) as f:
        _, ext = data_file.rsplit('.', 1)
        data = loader.get(ext, from_yaml)(f)

    for name, value in format_letters(letter, output_format, data):

        path = pathlib.Path(name)

        if not path.parent.exists():
            path.parent.mkdir(parents=True)

        with path.open('w+') as f:
            print(value, file=f)


# invoke with `./format_letter letter testdata.yaml output/letter-to-\${name}`
if __name__ == '__main__':
    main()

letter

Dear $greeting $title $name,

You are hereby invited to my party

$extra

Sincerely yours

Justus

testdata.yaml

# Example for how the data file might look

defaults:
  title: Dr.
  greeting: Mr.
  extra: ""

values:
  -
    title: ""
    name: Arthur
  -
    name: Maria
    greeting: Mrs.
    extra: Bring the kids as well.

# if you do not have defaults you can also omit the 'defaults' and 'values' key and just list the values