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?
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.
Exception is derived from BaseException, that's why except Exception does not catch BaseException. If you write except BaseException, it'll be caught too. Bare except just catches everything.
except must be last in a series of except blocks, while you won't get an error if you put except Exception before other except blocks: they'll just get ignored silently (if they handle Exception subclasses). Something to watch out for.
except and except Exception. "Something to watch out for" looks a little weird now, but at the time I expected Python to pick the most specific except block, regardless of where it was, and was a little disappointed to find out otherwise.
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.
except(Exception) never catches KeyboardInterrupt errors. as e doesn't have anything to do with it.
as e, because I assumed it's obvious what it does.
SystemExit or KeyboardInterrupt to escape from. Not a common case, but it does happen.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:).
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.
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.
except Exception, e:andexcept Exception as e:. This question asks what the difference is betweenexcept:andexcept Exception as e:.