Python try…except comma vs ‘as’ in except

The definitive document is PEP-3110: Catching Exceptions

Summary:

• In Python 3.x, using as is required to assign an exception to a variable.
• In Python 2.6+, use the as syntax, since it is far less ambiguous and forward compatible with Python 3.x.
• In Python 2.5 and earlier, use the comma version, since as isn’t supported.