Question

What is the difference between , and as in except statements, eg:

try:
    pass
except Exception, exception:
    pass

and:

try:
    pass
except Exception as exception:
    pass

Is the second syntax legal in 2.6? It works in CPython 2.6 on Windows but the 2.5 interpreter in cygwin complains that it is invalid.

If they are both valid in 2.6 which should I use?

Answer 1

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.

Answer 2

Yes it s legal. I m running Python 2.6

try:
    [] + 3
except Exception as x:
    print "woo hoo"

>>> 
woo hoo

Update: There is another reason to use the as syntax. Using , makes things a lot more ambiguous, as others have pointed out; and here s what makes the difference. As of Python 2.6, there is multicatch which allows you to catch multiple exceptions in one except block. In such a situation, it s more expressive and pythonic to say

except (exception1, exception2) as e

rather than to say

except (exception1, exception2), e

which would still work

Answer 3

the "as" syntax is the preferred one going forward, however if your code needs to work with older Python versions (2.6 is the first to support the new one) then you ll need to use the comma syntax.

Answer 4

If you want to support all python versions you can use the sys.exc_info() function like this:

try:
    a = 1/ 0 
except (ZeroDivisionError, TypeError):
    e = sys.exc_info()[1]
    print(e.args[0])

(source:http://python3porting.com/noconv.html)

Answer 5

As of Python 3.7 (not sure about other versions) the comma syntax is not supported any more:

Source file exception_comma.py:

try:
    result = 1/0
except Exception, e:
    print("An error occurred")
    exit(1)

exit(0)

$ python --version --> Python 2.7.10

$ python exception_comma.py
An error occurred

$ python3 --version --> Python 3.7.2

$ python3 exception_comma.py
  File "exception_comma.py", line 3
    except Exception, e:
                    ^
SyntaxError: invalid syntax

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