Mastering Python HTTP Error Handling: URLError and HTTPError Explained

This article discusses handling HTTP-related exceptions in Python using urllib2, focusing on URLError and HTTPError .

1. URLError

URLError occurs when there is no network connection or the target server cannot be reached. The exception includes a reason attribute, which is a tuple containing an error number and a message (e.g., [Errno 11001] getaddrinfo failed ).

2. HTTPError

HTTPError is a subclass of URLError and is raised for HTTP responses with error status codes (e.g., 404, 403, 401). Each HTTP response carries a numeric status code; codes 200–299 indicate success, while 400–599 indicate various client or server errors. The code attribute of an HTTPError instance holds the numeric error code.

Typical error codes include:

200 – Success

201 – Resource created

202 – Accepted, processing pending

204 – No content

301 – Permanent redirect

302 – Temporary redirect

400 – Bad request

401 – Unauthorized

403 – Forbidden

404 – Not found

5XX – Server error

3. Wrapping Exceptions

Two main strategies are recommended for handling these exceptions.

First approach

Create a script (e.g., urllib2_test08.py) that wraps the request in a try block, catching HTTPError first, then URLError. This order is crucial because HTTPError is a subclass of URLError and would be caught by the latter if placed first.

Second approach

Another script (e.g., urllib2_test09.py) demonstrates a different handling pattern, also catching HTTPError before URLError to ensure proper processing of HTTP-specific errors.