Combine with livereload¶

A nice feature of PDF readers is when they detect changes to the .pdf files open and automatically reload them in the viewer. Adobe Acrobat Reader does not provide this feature but other viewers offer it, like the free & open source Sumatra PDF Reader under Windows.

When using such PDF reader, it can be very useful to use a "watch" mode, so that every change to the Python code will trigger the regeneration of the PDF file.

The following script is an example of using livereload with fpdf2 to do that. Launched without parameters, this script only generates a PDF document. But when launched with --watch as argument, it will detect changes to the Python script itself, and then reload itself with xreload, and finally regenerate the PDF document.

#!/usr/bin/env python3
# Script Dependencies:
#    fpdf2
#    livereload
#    xreload
import asyncio, logging, sys
from traceback import print_exc

from fpdf import FPDF
from livereload.watcher import get_watcher_class
from xreload import xreload


OUT_FILEPATH = "fpdf2-demo.pdf"


def build_pdf():
    pdf = FPDF()
    pdf.set_font("Helvetica", size=16)
    pdf.add_page()
    pdf.y += 50
    pdf.multi_cell(
        h=10,
        w=0,
        align="C",
        text="""Hello fpdf2 user!
Launch this script with --watch
and then try to modify this text while the script is running""",
    )
    pdf.output(OUT_FILEPATH)
    print(f"{OUT_FILEPATH} has been rebuilt")


async def start_watch_and_rebuild():
    logging.basicConfig(
        format="%(asctime)s %(name)s [%(levelname)s] %(message)s",
        datefmt="%H:%M:%S",
        level=logging.INFO,
    )
    logging.getLogger("livereload").setLevel(logging.INFO)
    watcher = get_watcher_class()()
    watcher.watch(__file__, build_pdf)
    print("Watcher started...")
    await watch_periodically(watcher)


async def watch_periodically(watcher, delay_secs=0.8):
    try:
        watcher.examine()
    except Exception:
        print_exc()
    await asyncio.sleep(delay_secs)
    xreload(sys.modules[__name__], new_annotations={"XRELOADED": True})
    await asyncio.create_task(watch_periodically(watcher))


# This conditional ensure that the code below
# does not get executed when calling xreload on this module:
if not __annotations__.get("XRELOADED"):
    build_pdf()
    # The --watch mode is very handy when using a PDF reader
    # that performs hot-reloading, like Sumatra PDF Reader:
    if "--watch" in sys.argv:
        asyncio.run(start_watch_and_rebuild())

Note that the module reloading mechanism provided by xreload has several limitations, cf. xreload.py.