Difference between except: and except Exception as e:
PythonPython 3.xPython Problem Overview
Both the following snippets of code do the same thing. They catch every exception and execute the code in the except:
block
Snippet 1 -
try:
#some code that may throw an exception
except:
#exception handling code
Snippet 2 -
try:
#some code that may throw an exception
except Exception as e:
#exception handling code
What is exactly the difference in both the constructs?
Python Solutions
Solution 1 - Python
In the second you can access the attributes of the exception object:
>>> def catch():
... try:
... asd()
... except Exception as e:
... print e.message, e.args
...
>>> catch()
global name 'asd' is not defined ("global name 'asd' is not defined",)
But it doesn't catch BaseException
or the system-exiting exceptions SystemExit
, KeyboardInterrupt
and GeneratorExit
:
>>> def catch():
... try:
... raise BaseException()
... except Exception as e:
... print e.message, e.args
...
>>> catch()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in catch
BaseException
Which a bare except does:
>>> def catch():
... try:
... raise BaseException()
... except:
... pass
...
>>> catch()
>>>
See the Built-in Exceptions section of the docs and the Errors and Exceptions section of the tutorial for more info.
Solution 2 - Python
except:
accepts all exceptions, whereas
except Exception as e:
only accepts exceptions that you're meant to catch.
Here's an example of one that you're not meant to catch:
>>> try:
... input()
... except:
... pass
...
>>> try:
... input()
... except Exception as e:
... pass
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
KeyboardInterrupt
The first one silenced the KeyboardInterrupt
!
Here's a quick list:
issubclass(BaseException, BaseException)
#>>> True
issubclass(BaseException, Exception)
#>>> False
issubclass(KeyboardInterrupt, BaseException)
#>>> True
issubclass(KeyboardInterrupt, Exception)
#>>> False
issubclass(SystemExit, BaseException)
#>>> True
issubclass(SystemExit, Exception)
#>>> False
If you want to catch any of those, it's best to do
except BaseException:
to point out that you know what you're doing.
All exceptions stem from BaseException
, and those you're meant to catch day-to-day (those that'll be thrown for the programmer) inherit too from Exception
.
Solution 3 - Python
There are differences with some exceptions, e.g. KeyboardInterrupt.
Reading PEP8:
> A bare except: clause will catch SystemExit and KeyboardInterrupt > exceptions, making it harder to interrupt a program with Control-C, > and can disguise other problems. If you want to catch all exceptions > that signal program errors, use except Exception: (bare except is > equivalent to except BaseException:).
Solution 4 - Python
Another way to look at this. Check out the details of the exception:
In [49]: try:
...: open('file.DNE.txt')
...: except Exception as e:
...: print(dir(e))
...:
['__cause__', '__class__', '__context__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', '__suppress_context__', '__traceback__', 'args', 'characters_written', 'errno', 'filename', 'filename2', 'strerror', 'with_traceback']
There are lots of "things" to access using the 'as e' syntax.
This code was solely meant to show the details of this instance.
Solution 5 - Python
Using the second snippet gives you a variable (named based upon the as
clause, in your example e
) in the except
block scope with the exception object bound to it so you can use the information in the exception (type, message, stack trace, etc) to handle the exception in a more specially tailored manor.