Question

I have got a project, where I have to do this very frequently:

if "something" in dict:
    some_var = dict["something"]
    del dict["something"]
else:
    error_handler_or_other_stuff()

However I have the idea to use this:

try:
    some_var = dict.pop("something")
except KeyError:
    error_handler_or_other_stuff()

My question is: in general, how “fast” try – except constructs are to handle exceptions? Is it OK to use it a lot, or it’s still faster to do the stuff “manually”. Also sometimes I have the situation where I have to convert value to integer:

try:
    some_var = int(dict.pop("something"))
except KeyError:
    error_handler_or_other_stuff("no such key")
except ValueError:
    error_handler_or_other_stuff("bad value for key")

Now the solution with exceptions seems to be quite nice, since I can do both of the checks in one step, also I removed the original key/value pair from dict, which is part of the problem. So I can tell at least: it looks like an elegant solution. However I am not sure it’s faster or if it has other disadvantages I should worry about.

Answer 1

These kind of performance questions are easily answered using the timeit module:

setup = '''
d = {'a': 1}
k = 'b'
'''

LBYL = '''
if k in d:
    pass
else:
    pass
'''

EAPF = '''
try:
    d[k]
except KeyError:
    pass
'''

from timeit import Timer

print min(Timer(LBYL, setup).repeat(7, 1000000))
print min(Timer(EAPF, setup).repeat(7, 1000000))

The results show 0.0546 for the if/else approach and 1.3370 for the try/except approach. The latter is about 25 times slower than the former.

That being said, you should generally use whatever expresses the clearest code.

Sidenote: the two approaches give different answers for subclasses of dict that define missing to return a value.