# 21.9. `urllib.error` — Exception classes raised by urllib.request

The `urllib.error` module defines the exception classes for exceptions raised by `urllib.request`. The base exception class is `URLError`.

The following exceptions are raised by `urllib.error` as appropriate:

exception `urllib.error.``URLError`

  The handlers raise this exception (or derived exceptions) when they run into a problem. It is a subclass of `OSError`.

  `reason`

    The reason for this error. It can be a message string or another exception instance.

  Changed in version 3.3: `URLError` has been made a subclass of `OSError` instead of `IOError`.

exception `urllib.error.``HTTPError`

  Though being an exception (a subclass of `URLError`), an `HTTPError` can also function as a non-exceptional file-like return value (the same thing that `urlopen()` returns). This is useful when handling exotic HTTP errors, such as requests for authentication.

  `code`

    An HTTP status code as defined in RFC 2616. This numeric value corresponds to a value found in the dictionary of codes as found in `http.server.BaseHTTPRequestHandler.responses`.

  `reason`

    This is usually a string explaining the reason for this error.

  `headers`

    The HTTP response headers for the HTTP request that caused the `HTTPError`.

    New in version 3.4.

exception `urllib.error.``ContentTooShortError`(msg, content)

  This exception is raised when the `urlretrieve()` function detects that the amount of the downloaded data is less than the expected amount (given by the Content-Length header). The `content` attribute stores the downloaded (and supposedly truncated) data.