subtempl.py

#!/usr/bin/env python3

# Copyright (c) 2018-2022 Joergen Ibsen
#
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"""
Substitute template strings.

Read a mapping in JSON format and convert the input by substituting template
strings.

Example:
    If the map contains {'foo': 'bar'}, all instances of $foo in the input
    will be replaced with bar in the output. Use ${foo}s to produce bars,
    and $$ to insert a literal $.

See Also:
    https://www.python.org/dev/peps/pep-0292/
"""

import argparse
import json

from string import Template
from sys import exit, stderr

if __name__ == '__main__':
    parser = argparse.ArgumentParser(
        description='Substitute template strings.',
        epilog='Use `-\' to read from stdin and/or write to stdout.')
    parser.add_argument('mapfile', type=argparse.FileType('r', encoding='UTF-8'),
                        help='mapping in JSON format')
    parser.add_argument('infile', type=argparse.FileType('r', encoding='UTF-8'),
                        help='template file')
    parser.add_argument('outfile', type=argparse.FileType('w', encoding='UTF-8'),
                        help='output file')
    args = parser.parse_args()

    mapping = json.load(args.mapfile)
    lineno = 1
    try:
        for line in args.infile:
            args.outfile.write(Template(line).substitute(mapping))
            lineno += 1
    except KeyError as e:
        print('KeyError: {}: line {}'.format(e, lineno), file=stderr)
        exit(-1)
    except ValueError as e:
        print('ValueError: {}: line {}'.format(e, lineno), file=stderr)
        exit(-1)