Cheap exception handling in Python?

Question

I read in an earlier answer that exception handling is cheap in Python so we shouldn\'t do pre-conditional checking.

I have not heard of this before, but I\'m relati

Answer 1

With Python, it is easy to check different possibilities for speed - get to know the timeit module :

... example session (using the command line) that compare the cost of using hasattr() vs. try/except to test for missing and present object attributes.

% timeit.py 'try:' '  str.__nonzero__' 'except AttributeError:' '  pass'
100000 loops, best of 3: 15.7 usec per loop
% timeit.py 'if hasattr(str, "__nonzero__"): pass'
100000 loops, best of 3: 4.26 usec per loop
% timeit.py 'try:' '  int.__nonzero__' 'except AttributeError:' '  pass'
1000000 loops, best of 3: 1.43 usec per loop
% timeit.py 'if hasattr(int, "__nonzero__"): pass'
100000 loops, best of 3: 2.23 usec per loop

These timing results show in the hasattr() case, raising an exception is slow, but performing a test is slower than not raising the exception. So, in terms of running time, using an exception for handling exceptional cases makes sense.

EDIT: The command line option -n will default to a large enough count so that the run time is meaningful. A quote from the manual:

If -n is not given, a suitable number of loops is calculated by trying successive powers of 10 until the total time is at least 0.2 seconds.