The following exceptions are raised by urllib.error as appropriate:
The handlers raise this exception (or derived exceptions) when they run into a problem. It is a subclass of OSError.
The reason for this error. It can be a message string or another exception instance.
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.
This is usually a string explaining the reason for this error.
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.