Python try...except comma vs 'as' in except
PythonPython 2.6Python Problem Overview
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?
Python Solutions
Solution 1 - Python
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.
Solution 2 - Python
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
Solution 3 - Python
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.
Solution 4 - Python
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])
Solution 5 - Python
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