In Python, what's the difference between 'except Exception as e' and 'except Exception, e' [duplicate]
This PEP introduces changes intended to help eliminate ambiguities in Python's grammar, simplify exception classes, simplify garbage collection for exceptions and reduce the size of the language in Python 3.0.
PEP 3110: "Catching Exceptions in Python 3000"
Short answer for the why: Exception, e and Exception, TypeError are hard to tell apart. Long answer: what Ignacio said.
The first proposal for using the "as" is here: http://mail.python.org/pipermail/python-dev/2006-March/062449.html. They thought it would be more intuitive to read the code