(May-01-2020, 12:36 AM)Larz60+ Wrote: you can use package pypdf (you'll need to install with pip install pypdf from command line)
there's an example here: https://stackoverflow.com/a/3444735

#!/usr/bin/env python3

"""
Merge PDF-Files

The order of supplied input files is preserved
The output_file is automatically removed from the list of input_files
"""
import sys
from argparse import ArgumentParser
#https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser
from contextlib import ExitStack
# https://docs.python.org/3/library/contextlib.html#contextlib.ExitStack
from pathlib import Path
# https://docs.python.org/3/library/pathlib.html#basic-use


from PyPDF2 import PdfFileMerger
# https://pythonhosted.org/PyPDF2/
# use PdfFileMerger instead of Writer


def pdf_cat(input_files, output_file):
    # remove the output_file from input_files, which
    # could happen if using globbing "*.pdf" for input files
    input_files = [file for file in input_files if file != output_file]
    with ExitStack() as stack, open(output_file, "wb") as fd_out:
        # stack and fd_out are handled by conextmanager
        # leaving the contextmanager calls the __exit__ methods
        # which closes all files (input_files, fd_out)
        # if an Exception occours, the files are still closed
        open_pdf_files = [
            stack.enter_context(open(fname, "rb"))
            for fname in input_files
        ]
        # adding the input_files to the stack of the conextmanager
        # which calls open, so the stack has open files in his collection
        merger = PdfFileMerger(strict=False)
        for file in open_pdf_files:
            print("Processing", file.name)
            merger.append(file)
        print("Writing to", fd_out.name)
        merger.write(fd_out)
        # some broken or big pdf files
        # seems to block merger.write
        # Look for PyPDF3 or PyPDF4, which are forks
        # maybe some bugs have been removed


if __name__ == '__main__':
    parser = ArgumentParser(description=__doc__)
    parser.add_argument("input_pdfs", nargs="+", type=Path, help="Input PDF Files")
    parser.add_argument("output_pdf", type=Path, help="Output PDF Filee")
    args = parser.parse_args()
    pdf_cat(args.input_pdfs, args.output_pdf)
    

def key_func(filename):
    try:
        month, year, id, name = filename.split("_", maxsplit=4)
        # maxsplit allows only 4 splits, so the filename
        # could contain also a dash, but it's not split
        month, year = int(month), int(year)
    except ValueError:
        # is raised, if too less _ are in the filename
        # is raised, if month or year is not a int
        # not sortable, wrong pattern of file_name?
        return (0,0)
    else:
        return (year, month)
        # sort first by year, then by month if year is equal


file_names = [
    "12_2020_54_sometext.pdf",
    "11_2019_14_sometext.pdf",
    "12_2021_41_sometext.pdf",
    "10_2020_12_sometext.pdf"
]
print(sorted(file_names, key=key_func))
print("Big first")
print(sorted(file_names, key=key_func, reverse=True))