Python try...except comma vs 'as' in except

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?

Turnover

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.

Laveen

Using as is the only way to assign the exception to a local in Python 3.x. But as is not required, since you don't have to specify it if you don't need it at all. – Capreolate

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

Rueful

Python versions before 2.6 allowed you to use a tuple to catch multiple exception types too. But the syntax was confusing; you'd use except NameError, ValueError: then wondered why the ValueError exception was never being caught. Or used except (NameError, e): and were left with a NameError for 'e'! – Scarf

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.

Antimonic

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)

Mendez

Note: There are some corner cases here to be aware of. In Py3, the as target (along with the rest of the exception info) is implicitly deled as soon as you exit the except block (this prevents some cyclic references involved in the stored traceback from delaying the release of memory until the cyclic GC gets around to running). So this equivalent code is slightly less equivalent, unless you use a try/finally within the except block to ensure del e is performed before exiting the except block. – Valera

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

Hammon