Flatten tex files by following all the \input{foobar} statements

flatten.py

#!/usr/bin/env python3.6
# -*- encoding: utf-8 -*-
"""
flatten.py

Flatten tex files by following all the \input{foobar}
Returns the flattened file on stdout
Usage: flatten.py < first.tex > flattened.tex
"""

import sys
import re

target = re.compile(r'^\\input{(.*)}')

def flatten(lf=[]):
    if lf:
        fsource = open(lf[-1],'rt')
    else:
        fsource = sys.stdin
    for line in fsource:
        mtarget = re.match(target, line)
        if mtarget:
            ftarget = mtarget.group(1)
            if not ftarget.endswith('.tex'):
                ftarget = f"{ftarget}.tex"
                if ftarget in lf:
                    yield f"Circular error in {line.split()}: {ftarget} already parsed in {lf}.\n"
                else:
                    yield from flatten(lf + [ftarget])
        else:
            yield line


for line in flatten():
    sys.stdout.write(line)