Format YAML by key name order with fmtyaml.py

fmtyaml.py

#!/usr/bin/env python3
# vim: et sw=4 ai
"""
This script takes in files named in presumed yaml format and OVERWRITES them
with all their keys sorted by the key name. It does not preserve comments. I've
not tested it with lists.

Using '-' as a filename opens and consumes stdin in addition to other files.
stdin is written to stdout with the same processing.

Use: fmtyaml.py <filename> ...             Overwrites all the filenames with
                                             contents sorted by keys
     fmtyaml.py <in.yaml >out.yaml         Writes out.yaml with in.yaml sorted
                                             by keys
     fmtyaml.py <filename> ... - <in >out  A combination of the above two

CAVETS:
    reqires pyyaml

    This may rely on the python version's internal dictionary key ordering.
    Tested with 3.10
"""
import sys
import fileinput
import yaml


def process(lines, is_stdin, filename):
    content = yaml.safe_load(lines)
    if not is_stdin:
        with open(filename, 'w') as dst: 
            yaml.safe_dump(content, dst)
    else:
        print(yaml.safe_dump(content))

if __name__ == "__main__":
    if '--help' in sys.argv or '-h' in sys.argv:
        print(__doc__)
        sys.exit(0)
    was_file_name = None
    was_stdin = False
    with fileinput.input(encoding='utf-8', mode='r') as input_files:
        lines = ''
        for line in input_files:
            if input_files.isfirstline():
                if was_file_name:
                    process(lines, was_stdin, was_file_name)
                was_file_name = input_files.filename()
                was_std_in = input_files.isstdin()
                lines = ''
            lines += line
        process(lines, was_std_in, was_file_name)

It's a little hacky working with fileinput which operates line by line when I really want to work file by file.
It unsafe as far as handling non-yaml. It might parse and then throw out something unexpected, or more likely it'll fail during safe_load() which is better, but not handled gracefully.