Python Ftplib Resume Download Verified May 2026

In Python , you pass the starting byte position directly via the rest parameter inside the ftplib.FTP.retrbinary method. Under the hood, Python executes the low-level REST command on the control channel right before sending the RETR string down the pipe. Append Binary ( ab ) vs. Write Binary ( wb )

Using ab preserves existing sectors and cleanly appends new chunks to the end of the byte stream. Passive Mode ( set_pasv ) python ftplib resume download

import os import sys from ftplib import FTP, error_perm def resume_ftp_download(host, username, password, remote_filepath, local_filepath): """ Downloads a file from an FTP server with automatic resumption capabilities. """ # 1. Determine local file size if it exists if os.path.exists(local_filepath): local_size = os.path.getsize(local_filepath) file_mode = "ab" # Open in Append Binary mode print(f"Found partial file. Local size: {local_size} bytes. Resuming...") else: local_size = 0 file_mode = "wb" # Open in Write Binary mode print("No partial file found. Starting fresh download...") # 2. Establish connection to the FTP server ftp = FTP() try: print(f"Connecting to {host}...") ftp.connect(host, port=21, timeout=30) ftp.login(username, password) ftp.set_pasv(True) # Enable passive mode for firewall compatibility # 3. Check the size of the remote file to prevent over-downloading remote_size = ftp.size(remote_filepath) if remote_size is None: raise ValueError("Could not retrieve remote file size. Server may not support SIZE.") if local_size >= remote_size: print("Local file is already complete or larger than the remote file.") return True # 4. Open local file and trigger the resumable retrieval with open(local_filepath, file_mode) as local_file: # Prepare optional REST argument for ftplib # The 'rest' parameter takes a string or integer byte offset transfer_kwargs = {} if local_size > 0: transfer_kwargs['rest'] = local_size print(f"Downloading bytes {local_size} to {remote_size}...") # The callback receives chunks of binary data from the data channel command = f"RETR {remote_filepath}" ftp.retrbinary(command, local_file.write, blocksize=8192, **transfer_kwargs) print("Download completed successfully.") return True except error_perm as perm_err: print(f"FTP Permission/Protocol Error: {perm_err}", file=sys.stderr) return False except Exception as e: print(f"An unexpected error occurred: {e}", file=sys.stderr) return False finally: # Gracefully sever connection strings try: ftp.quit() except: ftp.close() # Example Usage if __name__ == "__main__": FTP_HOST = "example.com" FTP_USER = "anonymous" FTP_PASS = "guest@example.com" REMOTE_FILE = "large_dataset.zip" LOCAL_FILE = "large_dataset.zip" resume_ftp_download(FTP_HOST, FTP_USER, FTP_PASS, REMOTE_FILE, LOCAL_FILE) Use code with caution. Deep Dive into Key ftplib Mechanics The rest Parameter in retrbinary() In Python , you pass the starting byte