I'm writing a program that parses 10 websites, locates data files, saves the files, and then parses them to make data that can be readily used in the NumPy library. There are tons of errors this file encounters through bad links, poorly formed XML, missing entries, and other things I've yet to categorize. I initially made this program to handle errors like this:
try: do_stuff() except: pass
But now I want to log errors:
try: do_stuff() except Exception, err: print Exception, err
Note this is printing to a log file for later review. This usually prints very useless data. What I want is to print the exact same lines printed when the error triggers without the try-except intercepting the exception, but I don't want it to halt my program since it is nested in a series of for loops that I would like to see to completion.
Some other answer have already pointed out the traceback module.
Please notice that with
print_exc, in some corner cases, you will not obtain what you would expect. In Python 2.x:
import traceback try: raise TypeError("Oups!") except Exception, err: try: raise TypeError("Again !?!") except: pass traceback.print_exc()
...will display the traceback of the last exception:
Traceback (most recent call last): File "e.py", line 7, in <module> raise TypeError("Again !?!") TypeError: Again !?!
import traceback import sys try: raise TypeError("Oups!") except Exception, err: try: exc_info = sys.exc_info() # do you usefull stuff here # (potentially raising an exception) try: raise TypeError("Again !?!") except: pass # end of useful stuff finally: # Display the *original* exception traceback.print_exception(*exc_info) del exc_info
Traceback (most recent call last): File "t.py", line 6, in <module> raise TypeError("Oups!") TypeError: Oups!
Few pitfalls with this though:
From the doc of
Assigning the traceback return value to a local variable in a function that is handling an exception will cause a circular reference. This will prevent anything referenced by a local variable in the same function or by the traceback from being garbage collected. [...] If you do need the traceback, make sure to delete it after use (best done with a try ... finally statement)
but, from the same doc:
Beginning with Python 2.2, such cycles are automatically reclaimed when garbage collection is enabled and they become unreachable, but it remains more efficient to avoid creating cycles.
On the other hand, by allowing you to access the traceback associated with an exception, Python 3 produce a less surprising result:
import traceback try: raise TypeError("Oups!") except Exception as err: try: raise TypeError("Again !?!") except: pass traceback.print_tb(err.__traceback__)
... will display:
File "e3.py", line 4, in <module> raise TypeError("Oups!")