如何用Python在树莓派3上自动打印文件夹中新JPEG文件

Hey there! Let's get this auto-printing setup working for you step by step. You're already off to a great start with the folder monitoring script—we just need to tweak it for Python 3, add JPEG filtering, and hook up the CUPS printing functionality.

Step 1: Fix & Enhance the Folder Monitoring Script #

Your original script uses Python 2 syntax (like the unparenthesized print statements) which won’t work on Python 3.5. Let’s update it to only track JPEG files and handle incomplete file writes (so you don’t try printing a file that’s still being saved to the folder):

import os
import time

path_to_watch = '/home/pi/jebenter/'
# Cover all common JPEG extension variations
JPEG_EXTENSIONS = ('.jpg', '.jpeg', '.JPG', '.JPEG')

# Initialize the set of tracked JPEG files
before = set()
for f in os.listdir(path_to_watch):
    file_path = os.path.join(path_to_watch, f)
    if os.path.isfile(file_path) and f.endswith(JPEG_EXTENSIONS):
        before.add(f)

def is_file_ready(file_path):
    """Check if the file is fully written and accessible"""
    try:
        with open(file_path, 'rb'):
            return True
    except (PermissionError, IOError):
        return False

while True:
    time.sleep(2)  # Adjust interval based on how fast you need detection
    after = set()
    for f in os.listdir(path_to_watch):
        file_path = os.path.join(path_to_watch, f)
        if os.path.isfile(file_path) and f.endswith(JPEG_EXTENSIONS):
            after.add(f)
    
    # Find newly added JPEGs
    added_jpegs = after - before
    if added_jpegs:
        print(f"Detected new JPEGs: {', '.join(added_jpegs)}")
        for filename in added_jpegs:
            full_path = os.path.join(path_to_watch, filename)
            # Wait until the file is fully saved before printing
            while not is_file_ready(full_path):
                time.sleep(0.5)
            # Call our print function (defined next)
            print_jpeg(full_path)
    
    # Update tracked files for the next loop
    before = after

Step 2: Add the CUPS Printing Function #

Since your CUPS service is already running, you have two solid options to print from Python:

Option 1: Use the lp Command (No Extra Libraries Needed) #

The lp command is built into CUPS, so we can call it directly with Python’s subprocess module. First, find your printer’s name by running lpstat -p in the terminal (look for the line starting with device for <printer_name>).

Add this function to your script:

import subprocess

def print_jpeg(file_path):
    # Replace with your printer's name from lpstat -p
    PRINTER_NAME = "your_printer_name"
    try:
        # Send the file to the printer via CUPS
        result = subprocess.run(
            ['lp', '-d', PRINTER_NAME, file_path],
            check=True,
            capture_output=True,
            text=True
        )
        print(f"Successfully sent {file_path} to printer: {result.stdout}")
    except subprocess.CalledProcessError as e:
        print(f"Failed to print {file_path}: {e.stderr}")

Option 2: Use pycups (Pythonic CUPS Integration) #

If you prefer a native Python library instead of shell commands, install a Python 3.5-compatible version of pycups first:

pip install pycups==2.0.1

Then add this print function:

import cups

def print_jpeg(file_path):
    # Connect to the local CUPS server
    conn = cups.Connection()
    # Get available printers (use your printer's name from lpstat -p)
    PRINTER_NAME = "your_printer_name"
    
    if PRINTER_NAME not in conn.getPrinters():
        print(f"Printer {PRINTER_NAME} not found! Available printers: {list(conn.getPrinters().keys())}")
        return
    
    try:
        # Send the JPEG to the printer
        job_id = conn.printFile(PRINTER_NAME, file_path, "Auto-Print JPEG", {})
        print(f"Print job submitted successfully! Job ID: {job_id}")
    except cups.IPPError as e:
        print(f"Failed to send print job: {e}")

Step 3: Test & Tweak #

• Verify printer connectivity first: Manually print a test JPEG with lp -d your_printer_name /path/to/test.jpg to confirm CUPS is working.
• Adjust monitoring interval: The time.sleep(2) in the loop can be shortened or lengthened based on how quickly you need to detect new files.
• Optional: Add file archiving: To avoid re-printing files if the script restarts, add code to move printed JPEGs to an archive folder after printing.

内容的提问来源于stack exchange，提问作者Marije Miedema