Python Ftplib Resume Download [patched] [2026]

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()

Failing to switch file modes is the most common cause of corrupted files during resumption. python ftplib resume download

Using ab preserves existing sectors and cleanly appends new chunks to the end of the byte stream. Passive Mode ( set_pasv ) import os import sys from ftplib import FTP,

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 ) Local size: {local_size} bytes