最佳实践：应该在Python代码中大量使用异常处理程序，还是应该避免使用异常处理程序？

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

Which construct makes your code easier to understand and maintain? Pick that one.

If the resulting code is too slow, then go back and consider whether a different error handling strategy is more efficient.

Answer 2

为什么不将dict.get或dict.pop与default参数一起使用？

Answer 3

Exceptions are not particularly slow. Obviously there is a lot more going on in exception handling than in a simple if, but unless you're doing it literally a million times per second, the speed difference is negligible.

Your example is so trivial that there is no reason to worry about speed to begin with. Micro-optimizations like that cost more time just thinking about them than you could ever save using them.

The code you've given is absolutely fine, that's what exceptions are there for.

Answer 4

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.