CODEWITHSUNDEEP
pythonerror-handling
Goutham
Goutham

Reputation: 2859

Can I get the exception from the finally block in python?

I have a try/finally clause in my script. Is it possible to get the exact error message from within the finally clause?

Upvotes: 56

Views: 55774

Answers (5)

Ian Clelland
Ian Clelland

Reputation: 44152

The finally block will be executed regardless of whether an exception was thrown or not, so as Josh points out, you very likely don't want to be handling it there.

If you really do need the value of an exception that was raised, then you should catch the exception in an except block, and either handle it appropriately or re-raise it, and then use that value in the finally block -- with the expectation that it may never have been set, if there was no exception raised during execution.

import sys

exception_name = exception_value = None

try:
    # do stuff
except Exception as e:
    exception_name, exception_value, _ = sys.exc_info()
    raise   # or don't -- it's up to you
finally:
    # do something with exception_name and exception_value
    # but remember that they might still be none

Upvotes: 19

JamesTheAwesomeDude
JamesTheAwesomeDude

Reputation: 1052

No, at finally time sys.exc_info is all-None, whether there has been an exception or not. Use [this instead]: …

The other answerer is correct in that you should be handling this inside the except clause.

However, for posterity / the record, here is an answer to the original question as stated:

import sys
try:
    int("not an integer LOL")
except:
    e = sys.exc_info()[1]
    # isinstance(e, ValueError) == True
    raise # this line is optional; I have it commented for the example output
else:
    e = None # you should do this to avoid a NameError
finally:
    print("I really wanted to access %s inside of a finally clause. And I'm doing so now."
      % repr(e))

This will print something like:

I really wanted to access ValueError("invalid literal for int() with base 10: 'not an integer LOL'") inside of a finally clause. And I'm doing so now.

Upvotes: 0

Mangu Singh Rajpurohit
Mangu Singh Rajpurohit

Reputation: 11420

Actually, other answers are bit vague. So, let me clarify it. You can always invoke sys.exc_info() from finally block. However, its output will vary depending whether exception has been actually raised. 

import sys

def f(i):

    try:
        if i == 1:
            raise Exception
    except Exception as e:
        print "except -> " + str(sys.exc_info())
    finally:
        print "finally -> " + str(sys.exc_info())

f(0)
f(1)

>>> 
finally -> (None, None, None)
except -> (<type 'exceptions.Exception'>, Exception(), <traceback object at 0x029438F0>)
finally -> (<type 'exceptions.Exception'>, Exception(), <traceback object at 0x029438F0>)

Thus, you can always know in finally block, whether exception was raised, if it's first level function. But sys.exc_info() will behave differently when length of call stack exceeds 1, as shown in below example. For more information, refer to How sys.exc_info() works?

import sys

def f(i):

    try:
        if i == 1:
            raise Exception
    except Exception as e:
        print "except -> " + str(sys.exc_info())
    finally:
        print "finally -> " + str(sys.exc_info())

def f1(i):
    if i == 0:
        try:
            raise Exception('abc')
        except Exception as e:
            pass

    f(i)

f1(0)
f1(1)

>>> 
finally -> (<type 'exceptions.Exception'>, Exception('abc',), <traceback object at 0x02A33940>)
except -> (<type 'exceptions.Exception'>, Exception(), <traceback object at 0x02A33990>)
finally -> (<type 'exceptions.Exception'>, Exception(), <traceback object at 0x02A33990>)

I hope, it makes things bit clearer.

Upvotes: 4

Alex Martelli
Alex Martelli

Reputation: 882171

No, at finally time sys.exc_info is all-None, whether there has been an exception or not. Use:

try:
  whatever
except:
  here sys.exc_info is valid
  to re-raise the exception, use a bare `raise`
else:
  here you know there was no exception
finally:
  and here you can do exception-independent finalization

Upvotes: 88

Josh
Josh

Reputation: 3596

You'll want to do that in the except clause, not the finally.

Refer to: http://www.doughellmann.com/articles/Python-Exception-Handling/

Upvotes: -4

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