How to know if urllib.urlretrieve succeeds?

When using the urllib.request.urlretrieve method to download files from the network, determining if it succeeds primarily relies on the method's return value and the presence of exceptions.

1. Checking the return value

The urllib.request.urlretrieve method returns a tuple containing two elements:

• The first element is the local file path (where the downloaded file is saved).
• The second element is an object containing HTTP headers.

For example:

python
import urllib.request

try:
    local_filename, headers = urllib.request.urlretrieve('http://www.example.com/somefile.zip', 'localfile.zip')
    print("Download successful!")
    print("Local file path:", local_filename)
    print("HTTP header information:", headers)
except Exception as e:
    print("Download failed:", e)

2. Checking for exceptions

If issues arise during the download process (such as network problems or URL issues), urllib.request.urlretrieve typically raises exceptions like URLError or HTTPError.

Therefore, using a try-except block to catch these exceptions is key to determining if the download succeeds.

In the above code example, if the download succeeds, it prints the local file path and HTTP header information; if it fails, it catches the exception and prints the error message.

3. Analyzing HTTP header information

By examining the returned HTTP header information, you can further verify if the file is complete or of the expected file type. For example, you can check the Content-Length in the headers against the actual file size to ensure the file is not truncated:

python
import os

file_size = os.path.getsize(local_filename)
header_size = headers.get("Content-Length")

if str(file_size) == header_size:
    print("File integrity verified.")
else:
    print("File may be corrupted.")

By following these steps, you can comprehensively determine if the urllib.request.urlretrieve download operation succeeds.