Python PDF Merger Project

"""Merge PDF files in a chosen order with pypdf."""

from __future__ import annotations

import argparse
from pathlib import Path

from pypdf import PdfReader, PdfWriter
from pypdf.errors import PdfReadError


def validate_paths(inputs: list[Path], output: Path) -> None:
    if len(inputs) < 2:
        raise ValueError("Choose at least two input PDF files.")

    resolved_output = output.resolve()
    seen: set[Path] = set()
    for source in inputs:
        resolved_source = source.resolve()
        if source.suffix.lower() != ".pdf" or not source.is_file():
            raise ValueError(f"Not a readable PDF file: {source}")
        if resolved_source == resolved_output:
            raise ValueError("The output file cannot also be an input file.")
        if resolved_source in seen:
            raise ValueError(f"The same input was listed twice: {source}")
        seen.add(resolved_source)


def merge_pdfs(inputs: list[Path], output: Path) -> int:
    """Merge inputs, write output, and return the number of pages written."""
    validate_paths(inputs, output)
    output.parent.mkdir(parents=True, exist_ok=True)
    writer = PdfWriter()
    total_pages = 0

    try:
        for source in inputs:
            reader = PdfReader(source)
            if reader.is_encrypted:
                raise ValueError(f"Encrypted PDF requires a password and was skipped: {source}")
            total_pages += len(reader.pages)
            writer.append(reader, outline_item=source.stem)
        with output.open("wb") as output_file:
            writer.write(output_file)
    except PdfReadError as error:
        raise ValueError(f"One input is not a valid readable PDF: {error}") from error
    finally:
        writer.close()

    return total_pages


def parse_args() -> argparse.Namespace:
    parser = argparse.ArgumentParser(description="Merge PDF files in the order supplied.")
    parser.add_argument("inputs", nargs="+", type=Path, help="Two or more source PDF files")
    parser.add_argument("-o", "--output", type=Path, default=Path("merged.pdf"))
    return parser.parse_args()


def main() -> None:
    args = parse_args()
    try:
        pages = merge_pdfs(args.inputs, args.output)
    except (ValueError, OSError) as error:
        raise SystemExit(f"Merge failed: {error}") from error
    print(f"Created {args.output} with {pages} page(s).")


if __name__ == "__main__":
    main()