Python - check if currently handling an exception

Question

How to check if the current code is being executed while processing an exception? By 'while processing an exception' I mean within an except block, functions called from it, or if catching a specific exception type, the optional issubclass override within the exception's metaclass.

def sth():
    handling_exception = # ???
    if handling_exception:
        print 'handling an exception'
    else:
        print 'not handling an exception'


try:
    1 / 0
except:
    sth()  # prints 'handling an exception'

sth()  # prints 'not handling an exception'

Checking sys.exc_info() is not enough because exceptions tend to remain there - they are not cleared automatically. And enforcing sys.exc_clear() is impractical.

sys.exc_info() is cleared when you exit the frame where it was caught. — Martijn Pieters
– Martijn Pieters, Commented Aug 10, 2015 at 13:55

Martijn Pieters · Accepted Answer · 2015-08-10 14:10:30Z

7

In Python 2, sys.exc_info() is cleared when you exit the frame where it was caught. So if you put the try..except in a function, exiting the function clears sys.exc_info(). In Python 3, the information is cleared when you exit the exception handler.

As such it can be used to detect if an exception was raised in this frame or any calling frame. If you need to somehow detect if an exception is being handled within the same frame as the try..except you can call sys.exc_clear() earlier to facilitate that.

There are no other methods of detecting if an exception was raised and handled.

Demo in Python 2:

>>> import sys
>>> from __future__ import print_function
>>> def baz():
...     print('Child frame', sys.exc_info())
... 
>>> def foo(clear=False):
...     bar(clear)
...     print('Calling frame', sys.exc_info())
... 
>>> def bar(clear=False):
...     try:
...         raise ValueError('bar')
...     except ValueError:
...         print('Handling an exception', sys.exc_info())
...         baz()
...     if clear: sys.exc_clear()
...     print('Frame that handled exception', sys.exc_info())
...     baz()
... 
>>> def baz():
...     print('Child frame', sys.exc_info())
... 
>>> foo()
Handling an exception (<type 'exceptions.ValueError'>, ValueError('bar',), <traceback object at 0x10075f6c8>)
Child frame (<type 'exceptions.ValueError'>, ValueError('bar',), <traceback object at 0x10075f6c8>)
Frame that handled exception (<type 'exceptions.ValueError'>, ValueError('bar',), <traceback object at 0x10075f6c8>)
Child frame (<type 'exceptions.ValueError'>, ValueError('bar',), <traceback object at 0x10075f6c8>)
Calling frame (None, None, None)
>>> foo(True)
Handling an exception (<type 'exceptions.ValueError'>, ValueError('bar',), <traceback object at 0x10075f290>)
Child frame (<type 'exceptions.ValueError'>, ValueError('bar',), <traceback object at 0x10075f290>)
Frame that handled exception (None, None, None)
Child frame (None, None, None)
Calling frame (None, None, None)

and in Python 3 (same function definitions):

>>> foo()
Handling an exception (<class 'ValueError'>, ValueError('bar',), <traceback object at 0x10f740e88>)
Child frame (<class 'ValueError'>, ValueError('bar',), <traceback object at 0x10f740e88>)
Frame that handled exception (None, None, None)
Child frame (None, None, None)
Calling frame (None, None, None)

edited Aug 10, 2015 at 14:10

answered Aug 10, 2015 at 13:56

Martijn Pieters

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3 Comments

Michał Pawłowski Over a year ago

Unfortunately, doesn't seem to be cleared when exiting the except block and that's what I wanted: gist.github.com/pawlowskimichal/d58886a2ebe11a4b0416

Martijn Pieters Over a year ago

@MichałPawłowski: yes, that illustrates what I am saying.

Martijn Pieters Over a year ago

@MichałPawłowski: it is safe to call sys.exc_clear() when no exception was raised, why not just call that right after a try:..except:, perhaps even in a finally block if you are using a loop with break or continue statements.

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